Second Convective Waves Experiment (COWEX-II)



Second Convective Waves Experiment (COWEX-II)

Scientific Overview Document

Executive Summary

1. Introduction

1.1 Key objectives

1.2 Relation to previous field studies

2. Project rationale

3. Scientific questions

4. COWEX-II

4.1 Location and timing

4.2 Flight plans

4.3 Data management

4.4 Educational component

Appendix A: list of participants

Appendix B: reviewers’ comments on an earlier submission

References

Second Convective Waves Experiment (COWEX-II)

Scientific Overview Document

Executive Summary

The Second Convective Waves Experiment (COWEX-II) is designed to resolve key scientific uncertainties regarding the dynamic interaction between horizontal roll circulations in the boundary layer and gravity waves. Such roll circulations commonly develop in the near-coastal marine convective boundary layer (MBL) during cold-air outbreaks (CAO). This mainly aircraft-based project is scheduled for the months of January-February 2004 (??) off the mid-Atlantic coast of the United States. The location and timing are chosen because of the relative frequency and intensity of cold-air outbreaks and associated cloud streets, the proximity to the NASA Wallops aviation facility, and the availability of air space.

Participants in this project include investigators from NASA Goddard Space Flight Center, the National Center for Atmospheric Research, xx universities in the United States and xx universities in Europe. The field campaign will be tightly coupled with numerical modeling efforts simulating the atmospheric boundary layer and its interactions with the underlying ocean surface and the free atmosphere.

The key question of COWEX-II regards the role of free-tropospheric gravity waves in the organization of convection in the MBL, in particular in case of cloud streets within a CAO over water. The kinematic and thermodynamic structure of horizontal roll vortices, cloud streets, and overlying gravity waves will be described in great vertical detail. An understanding of the dynamic coupling between horizontal roll vortices and gravity waves will yield a better insight of the large and highly variable energy fluxes between the surface and the free atmosphere, resulting in a better parameterization of this interaction in operational prediction models.

COWEX-II will deploy the University of Wyoming King Air (K/A) and a P-3 aircraft along coordinated multi-level flight legs. In situ measurements will be made of atmospheric variables and fluxes, and the P-3 will carry a GPS dropsonde system. An essential objective of COWEX-II is to use novel remote-sensing technologies to measure the wind, thermodynamic, and cloud structure of the lower troposphere in regions of cloud streets. Proposed instruments include the …. ??? possibilities:

o NOAA HRDL (High-resolution Doppler Lidar, to estimate air motion in lidar backscattering regions) or the DLR Wind Infrared Doppler Lidar

o WCR (Wyoming Cloud Radar, a 94 GHz dual-beam dual-Doppler down- or side-looking radar)

o HARLIE (Holographic Airborne Rotating Lidar Instrument, to map aerosol backscatter profiles above and within the MBL)

o LASAL (Large Aperture Scanning Airborne Lidar, similar to HARLIE??)

o LEANDRE-II (a differential absorption lidar, to measure water vapor profiles)

o ROWS (Radar Ocean Wave Spectrometer , to infer sea-surface stress and near-surface wind vectors

o THIS LIST IS ONLY TENTATIVE AND INCOMPLETE

1. Introduction

1.1 Key objectives

The central objective of COWEX-II (second COnvective Waves EXperiment), to be conducted off the mid-Atlantic coast of the United States during January-February 2004, regards the role of free-tropospheric gravity waves in the organization of convection in the marine boundary layer (MBL), in particular in the case of horizontal roll vortices that for during cold-air outbreaks (CAO) over water. Both horizontal roll vortices and overlying gravity waves are essential in the vertical transfer of momentum, moisture, and heat from the sea surface into the troposphere. These fluxes can be quite large and an accurate parameterization of their magnitude is essential both in general circulation models and in operational weather prediction models. For instance, the energy transfer from the ocean surface can be a key component in the rapid intensification of extratropical lows. The field campaign will be tightly coupled with numerical modeling efforts simulating the atmospheric boundary layer and its interactions with the underlying ocean surface and the free atmosphere. These efforts will involve the evaluation of subgridscale energy transfer parameterizations in NWP models and GCMs.

This proposal is part of a multi-agency effort to fund COWEX-II, which will deploy the University of Wyoming King Air (K/A) and a P-3 along coordinated multi-level flight legs. The K/A will be used mainly to measure fluxes and atmospheric variables at various levels in and above the MBL. The P-3 will be equipped with a GPS dropsonde system. An important objective of COWEX-II is to use and evaluate novel remote-sensing technologies to measure the wind, thermodynamic, and cloud structure of the lower troposphere in regions of cloud streets. These technologies have proven (or will soon have proven) their capabilities, for instance in IHOP which focuses on the CBL over land.

The following scientific questions will be addressed by the joint effort:

OTHER INVESTIGATORS TO ADD MORE BROAD SCIENTIFIC QUESTIONS HERE

➢ To what extent can remote-sensing technologies, esp. aerosol backscatter lidars and cloud radar, be used to map and describe the detailed vertical structure of horizontal roll vortices, cloud streets, and overlying gravity waves in the MBL during CAOs over water.

➢ Under what static stability and wind shear conditions are gravity waves present on top of and above the MBL during CAOs over relatively warm water?

➢ For given atmospheric conditions, are cloud streets part of deeper roll circulations that may occur in the absence of gravity waves above the MBL top? Or are cloud streets embedded in the undulations of the MBL inversion and in phase with overlying gravity waves? Can the two cloud street mechanisms (MBL helical rolls and gravity waves) be active simultaneously, and if so, what conditions optimize synergy ?

➢ Does the latent-heat release in cloud streets affect the structure, depth, and spacing of horizontal roll vortices and their interaction with gravity waves?

➢ How do the shear and stability profiles affect the vertical propagation of convectively generated gravity waves and their momentum transfer to the free troposphere?

➢ How can the observed MBL flux enhancement and gravity wave momentum transfer be parameterized?

The following observations are needed to answer these questions:

• profiles of ambient wind, temperature, and humidity;

• tomography of large organized eddies in the MBL, the MBL top, and aerosol layers aloft;

• the horizontal and mainly vertical airflow in horizontal roll vortices, cloud streets, and overlying gravity waves;

• wind estimates just above the sea surface;

• momentum, heat, and moisture flux measurements at levels ranging from close to the sea surface to high above the MBL inversion.

• others ??

2. Relation to previous field studies

Several field campaigns have been staged to gain a better understanding of PBL dynamics during CAOs offshore. The Genesis of Atlantic Lows Experiment (GALE), conducted slightly south off the proposed COWEX-II area around the same time of the year, included two CAO intensive observing periods. Chou and Ferguson (1991) show that roll circulations enhance sensible and latent heat fluxes from the surface to the upper CBL during one of these periods. They also show that entrainment at the CBL top might affect the budgets of temperature and humidity fluxes throughout the CBL, but not in the unstable surface layer.

Cloud streets over Lake Michigan have been studied in various campaigns including MASEX (Mesoscale Air-Sea Exchange Experiment, 1985) and Lake-ICE (Lake-Induced Convection Experiment, 1998)(Kristovich et al, 2000). Boers and Melfi (1987) used an airborne nadir lidar to document roll circulations at increasing offshore fetch during MASEX. The same lidar was used by Melfi et al (1985) to document the horizontal variations of the CBL top and aerosol layers aloft during a CAO over the Atlantic. They showed a uniformly undulating CBL top, whose crests appeared to be in phase with cells (or streets) embedded in the CBL. Corresponding undulations were found in the backscattering layers aloft. The scales of these undulations were shown to be related to the wind shear strength across the CBL top. This led to a speculation of a dynamic coupling between CBL convection and internal gravity waves aloft.

One of the objectives of the first COWEX, a joint NASA-NCAR project conducted off the mid-Atlantic US coast in the winter of 1990, was to test this hypothesis. The primary objectives of COWEX-I were to verify that the waves were indeed present over cloud streets, to understand the dynamic coupling between waves and streets, and to estimate the vertical flux of energy and momentum carried by the waves. The COWEX-I flight plans were designed to over-fly areas of cloud street formation. Such areas occur quite frequently during CAOs off the east coast of the USA. Unfortunately, during the 6 weeks allotted for COWEX-I, only one weak CAO occurred on Feb 17, 1990. On this day the wind shear across the CBL top was weak and a blend of cellular and linear convection occurred. The co-existence of cells and rolls is not uncommon, esp. in case of strong surface heat flux (LeMone, 1973; Grossman, 1982; Kristovich, 1993; Lohou et al., 1998).

The analysis of the 17 Feb 1990 event yielded two surprising finding. Firstly, the thermals were not grouped underneath the wave crests, as documented by aerosol backscatter lidar. It is possible that a more distinct linear organization of PBL convective overturning only forms when the condensation level is reached: latent heat release in the taller thermals may enhance undulations in the CBL top and thereby suppress convection in the wave troughs. Secondly, gust probe measurements did not indicate a strong signal in the vertical velocity above the gravity waves at levels above 2.0 km. There is some evidence that waves were trapped by an almost neutral layer between 2.0 and 2.4 km. Above this layer, the wave energy decayed exponentially and at 3.7 km the vertical velocity amplitude was undetectable. Here the early development of gravity wave-convection interaction may have been captured, providing an example of how the process evolves with time. Unfortunately, with the fixed-beam lidar system, it was impossible to ascertain how far the waves extended into and out of the flight plane.

The major differences of COWEX-II compared to previous experiments include the following:

o COWEX-II will use novel remote-sensing probes to document the fine-scale horizontal and vertical structure of aerosol layers, the MBL top, and cloud streets. These probes will also describe the kinematic and thermodynamic structure of the lower troposphere in regions of cloud streets.

o COWEX-II will combine flux and state measurements with remotely-sensed profiles along coordinated multi-level flight legs involving two aircraft

o OTHER DIFFERENCES ?? COMPARISON WITH LAKE-ICE ??

2. Project Rationale

THIS RATIONALE MAY NEED TO BE DEFINED MORE BROADLY

Cloud streets have received ample attention, both during cold-air outbreaks over water, and in the CBL over land, yet their size and spacing is still poorly understood. The possibility that gravity waves above the CBL (as suggested by the experience of glider pilots and the soaring patterns of birds [Woodcock, 1940]) may be generated by shallow convection within the CBL has important implications for understanding the global circulation. We know that gravity waves generated by orography or jet streams have a profound influence on atmospheric energetics (Palmer et al., 1986). If gravity waves are shown to exist commonly over areas of linearly organized shallow convection, they too might have important implications for NWP models and GCMs.

Surface heat and momentum fluxes are quite large and variable during CAOs. It is not known to what extent these fluxes are modulated by horizontal roll vortices. An improvement in the parameterization of fluxes at the surface and across the MBL can have important impacts, for instance on the prediction of east coast cyclogenesis (e.g. Reddy and Raman, 1994) or on the ocean circulation (e.g. Xue et al, 2000). Recent modeling studies by Kershaw (1995), Sang (1993) and Chun and Baik (1998) strongly indicate that convectively generated gravity waves can have a profound influence on atmospheric circulation dynamics. In fact, Kershaw (1995) found that under conditions of strong surface flux and vertical wind shear, the momentum flux of convectively generated gravity waves can approach that typical of mountain waves. To our knowledge no comprehensive observations of this potentially important phenomenon exist. Without such observations, it is almost impossible to develop accurate parameterizations for use in NWP and climate models. The COWEX-II field mission can provide these much-needed measurements and provide a way to develop and test parameterizations.

The development, mainly in the last decade, of novel remote sensing platforms enables detailed description of the structure and kinematics of gravity waves, cloud streets, horizontal roll vortices, and of the wind field near the ocean surface. The combination of this remotely sensed kinematic, thermodynamic and structural information with aircraft-measured fluxes and dropsonde data provides a unique opportunity to verify high-resolution model simulations and to test hypothesis on the interaction between OLE in the CBL and overlying gravity waves.

3. Scientific questions

BELOW ARE 4 SECTIONS (3.1-3.4) THAT DESCRIBE SOME SPECIFIC QUESTIONS WE HOPE TO ADDRESS IN COWEX-II. THERE ARE NO DOUBT OTHER RELATED ISSUES THAT CAN BE ADDRESSED IN COWEX-II. FEEL FREE TO CONTRIBUTE TEXT DESCRIBING ADDITIONAL ISSUES OR TO BUILD ON THIS TEXT. THIS IS ONLY A VERY FIRST DRAFT DOCUMENT

3.1 Cloud streets and organization of CBL turbulence

Organized convection within the MCBL has been studied since at least the late 1920's. From the soaring patterns of birds offshore Woodcock (1940) inferred the existence, under conditions of unstable stratification and sufficiently high surface wind, of a roll circulation whose axis was approximately aligned with the mean wind. Visible satellite imagery often exhibits a cloud street pattern in post-frontal airmasses over relatively warm water, for instance off the US Atlantic and Gulf coasts, over the Bering Sea (Walter and Overland, 1983) and over the Great Lakes (Kristovich et al, 2000). Cloud streets also occur over the continental CBL, where they tend to be shorter-lived (e.g. Weckwerth et al, 1999).

Cloud streets are a manifestation of shear organization of BL convection, with some energy coupled to the mean wind through dynamic instability (Brown, 1972). Under low wind conditions the MCBL is characterized by cellular convection, while cloud streets form when the wind is sufficiently strong (Kuettner, 1971; Brown, 1980; Etling and Brown, 1993). The cellular convection is sometimes referred to as Rayleigh-Bénard convection and is characterized by aspect ratios (horizontal scale/vertical scale) that are O(1). Such convection is commonly observed in the MCBL, however there also exist (often simultaneously) cellular convection with aspect ratios an order of magnitude larger which cannot be explained by Rayleigh-Bénard convection. Cloud streets are commonly believed to form in the parallel updrafts between helical roll circulations within the MCBL and these circulations may occur in the absence of clouds (i.e. in a more shallow or drier CBL). Latent heat release may be important in the organization BL convection, especially in deeper, precipitating clouds (Rao and Agee, 1996; Chlond, 1992). Observations, simulations, laboratory experiments and theory find cloud streets to be oriented in a direction within ~30 of the CBL mean wind (Brown, 1972 and many others), however the CBL wind shear (which is usually mainly speed shear) (Ferrare et al, 1991) or its gradient (Shirer, 1980) may play a role in determining the street orientation. Foster and Levy (1999) showed that BL baroclinity can have a significant effect on the orientation of BL rolls.

Two theories have been advanced to explain the 2-D organization of BL convection. The most commonly held view is that the linear updrafts in the CBL are generated by organized large eddies (OLE) in the form of roll vortices aligned along the mean wind (Etling and Brown, 1993). Organized motions, also called coherent structures, embedded in turbulent flows form in order to transport momentum and/or heat more efficiently than is possible by local down-gradient transport. Observations (Miura, 1986) and simulations (Sykes and Henn, 1989) concur with theory (Asai, 1972) that an increase in low-level wind shear causes OLEs to transform from 3-D cellular to primarily 2-D linear convection. In favorable thermodynamic conditions, shallow clouds will form in the updraft regions and cloudiness will be suppressed in the downdrafts. Many numerical and observational studies have confirmed that this mechanism does generate cloud streets (e.g. LeMone and Pennell, 1976; Kelly, 1982; Muller et al., 1985; Chlond, 1992; Atkins et al., 1995; Mourad and Walter, 1996; Brooks and Rogers, 1997). The primary instability responsible for the rolls will tend to scale with CBL depth and has an aspect ratio in the range of 2.4 to 6. The larger values are believed to be associated with larger thermal instability (Kelly, 1984) or with larger wind shear (Melfi et al, 1985). Rolls with aspect ratios exceeding 6 have been observed (e.g. Walter and Overland, 1983) and, as discussed below, nonlinear interactions can energize such modes (Mourad and Brown, 1990). Documenting the presence or absence of such large aspect ratio rolls in the presence of large wavelength cloud streets is a key component of the proposed experiment.

Alternatively, several researchers have proposed that large aspect ratio cloud streets are associated with gravity waves in the stably stratified troposphere above the CBL (e.g. Clark et al., 1986; Kuettner et al., 1987; LeMone, 1990; and Sang, 1993). In this model,boundary layer convection and/or the mean shear across the CBL inversion (or stable layer) can generate gravity waves. It is well-known that the MCBL top above cloud streets undulates and that gravity waves within the MCBL inversion are common, but little is known about MCBL convection-generated gravity waves above the inversion. Gravity waves generated by upstream orography may also be present in the troposphere above the MCBL in a CAO off the east coast. Any of the these vertically propagating waves may interact with the CBL convection, preferentially amplifying scales of CBL convection in a manner that reinforces the waves. Clouds will preferentially form below wave ridges, leading to the formation of cloud streets. This mechanism suggests that the primary scaling length for the cloud street spacing is no longer the CBL depth (~0.5 - 2.0 km) but is instead determined by the wavelength of the tropospheric gravity waves which is determined by the stratification within this stable layer, and, the shear between the CBL top and the stable layer. This wavelength can be ~7 - 15 km which will result in large aspect ratio cloud streets. Indeed, cloud streets with such spacings but without sub-cloud rolls have been observed (LeMone and Meitin, 1984). Cause and effect remains nebulous since gravity waves are likely to be generated whenever cloud streets form.

Thus, there is evidence that two fundamentally different yet possibly synergistic processes are responsible for cloud streets. The large-eddy dynamics of the CBL may have been quite different between documented cases and quite likely the fluxes at the surface and across the CBL inversion also differed. Several other questions remain about cloud streets, including whether gravity waves exist above the CBL inversion and how they propagate and transfer momentum, whether latent heat release is important in the organization and energy budget of BL convection, and how OLEs influence surface fluxes.

3.2 Characteristics of horizontal rolls in the CBL

The atmospheric boundary layer is home to a wide variety of coherent structures. Among the most commonly observed and most important are the persistent roll vortices that align themselves near to the mean wind direction and span the depth of the boundary layer in near-neutral to moderately unstable stratification (Etling and Brown, 1991). The secondary circulation induced by the perturbations organizes the turbulence by sweeping up the smaller-scale eddies, collecting them in the near-surface convergence zones and transporting them vertically (LeMone, 1976). It is generally accepted that rolls are generated by a combined dynamic/thermal instability of the CBL mean flow that reaches a finite amplitude equilibrium in a modified mean flow (Brown, 1970; 1972; Foster, 1996; 1997a). CAOs are ideal for the formation of CBL rolls because of the combination of large shear and large buoyancy fluxes. Hence, linear organization of CBL convection is common during CAOs over relatively warm oceans waters.

Various criteria have been proposed for the formation of linear convection, including a minimum wind speed of 5 m s-1 (Christian and Wakimoto, 1989), a shear between surface and CBL top between 10-3 - 10-2 s-1 (Miura, 1986), a shear gradient of a least 10-5 m-1 s-1, (Kuettner, 1971), and h/L > -21 (Grossman, 1982). In the latter h/L is a standard CBL stability parameter in which the CBL height, h, (representing the typical scale of convective eddies) is normalized by the Monin-Obukhov length L, which depends on the surface momentum and buoyancy fluxes. The more negative L is, the more convectively unstable the CBL is. These criteria are satisfied in most cases reported in the literature, but for each there are exceptions (Weckwerth et al, 1997). Each of these criteria can be related to different aspects of the convective-dynamic instability problem and such empirically obtained criteria, to be tested in COWEX-II cases, are useful confirmations of the basic theory.

While the fundamental instability mode for roll vortices results in a circulation with an aspect ratio around 2.4-6, roll circulations of much larger scale can be generated through nonlinear interactions between resonant triads of unstable modes. In most convective conditions with shear, normal modes with a large range of aspect ratios are unstable. The modes of significantly larger or smaller horizontal extent have lower growth rates than the fundamental mode. However, the existence of weakly unstable large aspect ratio modes is the basis for an explanation for the generation of large aspect ratio cloud streets. For example, Mourad and Brown (1990) investigated the weakly nonlinear interactions between resonant triads of 2-D Ekman layer normal modes. In this system energy is exchanged between the instabilities until a quasi-equilibrium state is reached. This system generates multiscale CBL rolls with primary organization at the wavelengths corresponding to the three normal modes. They found that initially rapidly growing lower aspect ratio normal modes can couple energy into slowly growing modes that could be of much larger scale than the most unstable normal mode for the given mean state. The fundamental (i.e. most unstable) instability acts as a conduit to couple perturbation energy these otherwise weakly growing modes. Through this mechanism the PBL final state would be determined by a combination of the naturally growing instabilities and those that are externally forced, as, for example, by tropospheric gravity waves. This model provides an explanation for the observations of large aspect ratio rolls where linear or simple nonlinear analyses would predict only the fundamental, smaller aspect ratio cells. It could also explain why simultaneous roll organization at multiple horizontal wavelengths have been observed (e.g. Walter and Overland, 1983). Detailed observations of such organization are lacking.

It is well known that CBL-scale coherent structures such as rolls transport momentum, moisture and heat across the depth of the CBL much more efficiently than simple diffusive transport or even randomly organized thermals. This is why roll circulations form in sheared CBLs sustaining large surface fluxes. Observational (e.g. Brooks and Rogers, 1997) and numerical (e.g. Glendening, 1996) studies show that the inherently non-local roll-induced fluxes play a major role in the transport of momentum and heat across the CBL. For example, in a large eddy simulation (LES) study, Glendening (1996) found that the ascending roll motions accounted for approximately 27% and 24% of the fluxes of longitudinal momentum and moisture respectively at CBL mid-levels. Very similar results are found in nonlinear stability analyses of CBL rolls. There is also a significant atmospheric feedback to the ocean: the sea surface is cooled slightly, a significant amount of momentum is imparted on the ocean, and the roll circulation may leave an imprint on sea surface temperature (SST) (Xue et al., 2000). The SST is important as a higher SST implies a higher moisture and heat transfer to the atmosphere, and SST gradients can modify momentum transfer at the surface. Large horizontal SST gradients visibly affect CBL roll dynamics and turbulent fluxes in the CBL (Foster and Levy, 1998; Foster et al,1999).

Several observational studies highlight the importance of roll circulations to the flux of momentum at the sea surface. Hein and Brown (1988) used aircraft data to document flux enhancement on both sides of cloud streets. Mourad and Walter (1996) and Mourad (1996) combined nearly coincident AVHRR visible and ERS-1 synthetic aperture radar (SAR) imagery to study clouds streets in a CAO. They found streaks in the SAR backscatter off the sea surface that had the same spacing as the cloud streets in the AVHRR image. These streaks were associated with local increases in the sea surface stress, which they associated with the surface imprint of CBL rolls. The variability of the backscatter within the streaks of enhanced backscatter is consistent with LeMone's (1976) hypothesis that the roll circulations organize the smaller-scale turbulent eddies. Savchenko (1999) used airflow, radar return, and sea surface elevations from a coastal platform to establish a connection between CBL OLEs and cm-scale sea surface roughness during a CAO. Mourad et al. (2000) and Vandemark et al. (2000) obtained turbulent flux and radar measurements from low-level (15 m) flights over the ocean during a CAO. They found that the roll-scale modulation in turbulent fluxes was well-correlated with variations in the sea surface slope. This flight coincided with a SAR image of the sea surface from the RadarSAT satellite which exhibited significant streaking at the roll scale. The cross-streak modulation of the SAR-inferred surface wind speeds was consistent with the low-level flux data. Detailed measurements are needed to describe roll circulations and to quantify their impact on fluxes at the surface and across the CBL.

The CBL gradually deepens with increasing distance offshore, and roll circulations usually change into large aspect ratio cellular convection. This transition depends on the magnitudes of both surface heating and wind shear (Agee and Gilbert, 1989). At the same time cumulus clouds tend to merge into a continuous deck of stratocumulus, and deeper clouds may produce precipitation (Bechtold et al, 1992). Latent heat release also becomes increasingly important in the organization and maintenance of rolls further offshore. Latent heating and precipitation may have a significant impact on the vertical velocity spectrum and therefore the structure, depth, and spacing of CBL convection, however this issue is poorly understood. Rao and Agee (1996) find that the inclusion of the ice phase alone in a LES can change the convective geometry from cellular to linear.

3.3 Gravity waves and cloud streets

The development and ducting of gravity waves by convection (shallow or deep) was first discussed by Harman (1962). His theory was expanded by Townsend (1965, 1966, 1968) who studied the effects of moving convection on wave development in the free troposphere. Stull (1976) took the perturbation approach a step further investigating the effects of the penetration of a relatively large convective plume into the stable atmosphere aloft. Mason and Sykes (1980, 1982) used a numerical model of limited domain to study wave development and found waves in the stable troposphere that were generated by organized convection in the modeled CBL. Clark et al. (1986) used a high-resolution 2-D numerical model to demonstrate that a feedback between CBL clouds and tropospheric gravity waves is possible. This feedback acts to organize and maintain the CBL convection. Balaji and Clark (1988) extended this work with a 3-D model which also showed a strong interaction between convection in the CBL and waves in the free troposphere.

The spatial organization and perhaps the existence of large aspect ratio cloud streets may depend on the interaction of convection in the CBL with overlying gravity waves in much the same way as is seen in the theoretical studies of Clark et al. (1986) and Sang (1993). Convection in the CBL initiates wave activity and waves will modulate the top of the CBL and organize shallow convection into longer scales. Experimental research by Kuettner et al. (1987) and Hauf (1993) suggests that gravity waves are likely in the free troposphere when conditions in the CBL are conducive to cloud street formation. Their wavelengths range from 6 to 15 km and they extend from the top of the CBL to as high as the tropopause on occasion. Hauf (1993) found that gravity waves were initiated in the presence of a large gradient between the mean CBL wind and the wind immediately above the CBL inversion. Both Kuettner et al. and Hauf found that wind shear across the CBL inversion was necessary for the development of waves, with stronger wind shear producing larger amplitude waves. Directional shear at the top of the CBL was found to be especially effective in generating gravity waves. During an arctic CAO near Spitsbergen, Muller et al. (1999) found cloud streets aligned with and normal to the mean wind. The former occupied much of the CBL depth and were associated with roll circulations, as evidenced by SAR imagery. The latter were associated with waves in the CBL inversion, and in depth confined to the wave amplitude. The waves (and associated shallow rolls with updrafts under the wave crests) were attributed to inflection point instability due to strong wind shear. Below the CBL inversion shear-driven roll circulation was suppressed by the buoyancy-driven rolls.

In summary, theory and observations indicate that gravity waves may exist above the stable layer capping linearly organized BL convection, however their role in cloud street dynamics and energy transfer is not well understood. The depth and the length scale of cloud streets appears to depend on whether or not ambient tropospheric conditions support gravity wave propagation. Structure, intensity and spacing may be modulated further by latent heat release within the clouds. Gravity waves may effectively communicate surface conditions into the free atmosphere, through vertical energy transfer. This implies that the breaking of CBL-generated gravity waves may deposit significant quantities of momentum in the free atmosphere.

3.4 Numerical simulation and parameterization questions

THE EMPHASIS HERE IS ON THE UNIVERSITY OF WASHINGTON PBL MODEL. EFFORTS BY OTHER GROUPS CAN BE INCLUDED HERE

Foster and Brown at the University of Washington (UW or UWash) have a strong interest in the effects of coherent structures (especially rolls) on the flow in the CBL. Two foci of this research have been to (1) improve PBL parameterizations so that the effects of rolls are captured; and, (2) improve the basic theoretical understanding of the convective/dynamic instability mechanisms associated with rolls.

Physically, it is expected that the net effect of CBL rolls averaged over many roll wavelengths is to enhance the average surface buoyancy and momentum fluxes. Nonlinear theories for roll dynamics consistently show an enhancement of the near-surface wind and a reduction of the shear across the CBL. This inherently non-local effect is not parameterized in standard PBL models. However, the effect has been parameterized in the UW PBL model (Brown and Liu, 1982). This PBL model is a theoretical match between a Monin-Obukhov similarity surface layer model and a stratification- and baroclinic-dependent outer layer that has been modified by be the presence of rolls. The surface layer model employs state-of-the-art air-sea flux parameterizations of variable surface roughness, viscous sub-layer effects and moisture-dependent stability calculations.

The UW PBL model has a 30-year heritage. It has been widely used and is available at as UWPBL 3.0. A document describing the complete derivation of the model as well a users guide may be downloaded at the same site. There has been considerable verification of the model (e.g. Brown, 1999; Etling and Brown, 1991; Brown and Liu, 1982). It was completely described in papers by Foster and Brown (1994a,1994b) and Brown and Foster (1994). Using this model, Brown and Foster (1994) and Foster and Brown (1994a) have shown that omission of the effects of rolls will introduce a typical 10% (20% maximum) low bias in surface stress over the oceans in typical general circulation model (GCM) simulations. Hence it is important that roll effects be included in PBL parameterizations. Brown and Foster (1994b) analyzed the physics behind standard PBL parameterization and discussed the reasons why this effect cannot be represented properly in such models.

The basic theory for rolls is based on the nonlinear evolution of initially linear instabilities in a sheared/convective PBL. It includes variable 3-D mean wind profiles, variable eddy viscosities, variable mean temperature profiles, horizontal temperature gradient effects and surface-based convective forcing. For a very wide range of relevant PBL conditions the solutions readily converge. The analyses have been carried out to very high order and the agreement with laboratory flows and direct numerical simulation of rotating boundary layer flows is excellent. These solutions also lend insight in the details of the non-local flux transports. For example, we find that the vertical flux of longitudinal momentum is highly inhomogeneous and is concentrated within the warm updrafts between the rolls. This result explains the observations by LeMone (1976) that showed that rolls act to collect the smaller-scale eddies and transport them vertically. The theoretically-derived percentage of the vertical transport due to these inherently non-local effects is consistent with that found by Glendening (1996).

Using these analyses as a guide, Foster is has developed a level 2.5'-type PBL parameterization that correctly includes the nonlocal transport effects of rolls (and potentially other coherent structures) on the third-order covariances. Because the pure roll effects can be correctly extracted via the theory, it is relatively straightforward to modify the existing parameterizations which are necessarily purely local and also well-known to be far too weak (e.g. Moeng and Wyngaard, 1986; Foster and Brown, 1994). COWEX-II data will be used to test the new model's performance.

The nonlinear model for PBL rolls can be used to extend the 2-D, neutrally-stratified, barotropic triad interaction model of Mourad and Brown (1990) to 3-D including variable stratification and surface convection. This will allow testing whether this is the primary mechanism through which PBL rolls may form large aspect ratio cloud streets. Other aspects of CBL flow need to be simulated, such as stratification-modified OLE, baroclinically-modified OLE, multi-scale OLE, subgrid variability and transient near-surface streaks (Brown, 1972; Foster, 1996; Foster and Levy, 1998; Mourad and Brown, 1990; Foster, 1997a; Drobinski and Foster, 2000). The effects of these coherent structures on the PBL flow and surface fluxes are important and yet are generally not included in numerical weather prediction (NWP) models. These theoretical aspects need to be tested against COWEX-II observations, with the objective to extend the PBL model to include any effects of gravity waves on organized circulations in the CBL, and on surface fluxes.

4. COWEX-II : the field campaign

4.1 Location and timing

The Atlantic region off the Virginia coast is the prime candidate for COWEX-II because of the relative frequency of CAOs, the availability of relatively unrestricted airspace (is this still true?), and the proximity to NASA’s Wallops Island flight facility. The months of December and January experience the most numerous and most intense CAOs in this region, according to a two-year climatology by Chibe et al (2002). The sea-air temperature difference (Ts-Ta) during CAOs tends to be largest at this time. CAO anticyclones that move south-eastward into the eastern USA occur every 2-10 days in winter (Davis et al, 1991). Persistent, low-frequency (>10 day) modes of the general circulation play an important role in controlling the intensity and frequency of CAOs over the eastern US (Konrad, 1998), but the behavior of these modes is not predictable beyond their period (i.e. >10 days). Ten years of hourly data from two buoys off the Virginia coast suggest that 3-5 significant CAOs can be expected during almost any five-week period comprising October to February. Here a CAO event is defined as an offshore wind event with a large difference between air and water temperature.

It is fortunate that large-scale patterns leading to a CAO are rather predictable, up to 5-7 days in advance, with more certainty about the exact timing of the start of a CAO (usually the passage of a cold front) at about 48 hours in advance. Factors affecting the exact timing of the flights are: (1) hour-of-day, (2) overpass times of relevant satellites (DMSP F13-15, ERS-2, Radarsat, Envisat). A flight just after sunset has the advantages that lidar systems can operate over a longer range and that the visible satellite imagery, which is used to determine flight tracks, is relatively fresh. It is not clear how well cloud street orientations can be determined from the aircraft.

We hope to sample at least 4 separate cloud street episodes, which will require having the airplanes at our disposal for a period of roughly 5-6 weeks.

4.2 Flight plans

WHAT FOLLOWS BELOW ARE FLIGHT PATTERNS AS PROPOSED A FEW YEARS AGO. CLEARLY THE PATTERNS AND COORDINATION ARE OPEN FOR DISCUSSION

The presence and orientation of cloud streets will first be determined from GOES and polar orbiting satellite imagery. This obvious point is important because experience testifies that it is difficult to determine cloud street orientation from an aircraft, even if it flies ~5 km higher. Next, in order to establish the presence and orientation of gravity waves, the P-3 will take off first to fly a square reconnaissance pattern at 6 km MSL over the selected experiment area extending about 100 km off the coast and about 75 km wide (Fig 1). The orientation of the square is of little importance since both LASAL/HARLIE and the HRDL/WIND lidar are scanning and since they cover a fairly wide swath from an altitude of 6 km. Following reconnaissance, various coordinated dual-aircraft scenarios are possible:

1. gravity waves are observed and are aligned with cloud streets;

2. gravity waves are observed but not aligned with cloud streets, or multiple cloud street orientations exist (Fig 3);

3. no gravity waves can be detected;

4. the presence cloud streets (evident in the satellite imagery) cannot be verified visually or instrumentally, e.g. BL convection is primarily cellular.

The last two cases can be considered 'control cases'. In these cases the aircraft will fly both along and across the cloud streets (as seen from the air or only evident in satellite imagery), to detect linear OLEs in the CBL in orthogonal directions, to identify sufficient conditions for gravity waves not to form, and to contrast cloud streets with gravity waves aloft to cloud streets without them. In most cases the P-3 and K/A aircraft fly in coordination [i.e. matching (x,y,t)] at three levels, chosen to optimize spatial coverage and resolution by both the remote sensing and the in situ probes:

P-3 at 6 km, K/A at 0.1H (H= MCBL depth) and not below 100 m; LASAL/HARLIE and HRDL/WIND in survey mode; WCR in single-beam zenith mode; dropsondes may be released

P-3 at 4.5 km, K/A just above cloud base or mid-way in the clear CBL; WCR in side-looking dual-Doppler mode

P-3 at 3 km, K/A just above the cloud tops; LASAL/HARLIE and HRDL/WIND collecting detailed information (sector scans); WCR nadir dual-Doppler mode

|[pic] | |

| |Fig 1: Proposed COWEX-II aircraft flight |

| |pattern and dropsonde locations in case the |

| |cloud streets and gravity waves are not |

| |aligned (or in case multiple cloud street |

| |orientations are found). H is the depth of the|

| |MCBL. |

| | |

| | |

| | |

| |Other possible flight scenarios are discussed |

| |at http:// |

| |weather.uwyo.edu/~geerts/cowex/flight.html |

The straight & level legs are generally 65 km long. Sufficient flux and wave/roll statistics can be gathered from such legs oriented across the cloud streets or gravity waves. Even when the gravity waves are aligned with the cloud streets, the aircraft will fly in orthogonal directions to verify that no wave/roll structure is present in other directions. The lowest safe flight level for the K/A is ~100 m MSL. This is above the height of most steam devils (which may contaminate temperature and humidity measurements), except perhaps during very strong CAOs (Bluestein, 1990).

Radiometer and flux measurements at this level are essential to estimate SST, surface wind, and surface fluxes. The WCR would collect velocity and reflectivity data of the clouds aloft at this time. The second K/A flight level is near cloud base (or in the middle of the CBL if clouds are absent). Flux and lateral WCR measurements are expected to be most accurate here. Finally the K/A would fly at a third level, in the MBL inversion, where wave-like variations of humidity and vertical air motion should be most distinct, and from where the vertical cloud and MBL airflow structure can be documented.

As shown in Fig 1, the comb elements will be displaced by 4-10 km in the downstream direction. This allows for a change in altitude during the turn, and quasi-Langrangian air sampling (i.e. the downstream displacement is a function of the cross-track wind). Possible flight maneuvers other than the tri-level combs include stacked tri-level coordination flights at fixed positions along well-defined cloud streets (in order to assess the impact of roll circulations on surface stress, wind and flux profiles) and K/A spirals (in case of well-defined cellular convection). The total distance covered per successful mission ranges between 1,400-1,600 km (1,150-1,300 km) and the total flight duration will be between 4.5-5.0 hours (3.7-4.1 hrs) for the P-3B (K/A) respectively, given an airspeed of 98 m/s. Both aircraft will be operating a differential GPS system (relative to a fixed station at Wallops) to accurately coordinate flight legs in space and time, and will have a dedicated radio communication frequency.

4.3 Field phase management

4.4 Data processing and central facility

4.5 Educational component

Appendix A: list of participants

Appendix B

Select comments from reviewers of the NSF proposal “Aircraft Investigation of Cloud Street-Gravity Wave Interaction and Energy Transfer”, Fall 2000

o The modelling work is not well integrated with the field data, nor does it clearly link to operational NWP needs.

o One of the areas COWEX should address is as to how fluxes will be measured in a strongly baroclinic zone, close to the Gulf Stream.

o It is unclear how the phase speeds and momentum flux divergence will be determined by the measurements.

o It is unclear how the feedbacks between the waves and cloud streets, and their possible modulation by latent heat will be accomplished. In addition, little discussion is given as to how the vertical energy transfer within the CBL and the waves will be quantified

o The proposed observations in COWEX-II will not be able to quantify the vertical energy transfer within the PBL and by the waves. Because of the large surface fluxes normally observed during cold air outbreaks, there is a substantial gradient near the surface that cannot be interpolated from the aircraft measurements. Surface wind measurements using microwave scatterometer will not be accurate with current-wave interactions present in this region due to the presence of the Gulf Stream.

o One of the objectives of COEX-II is to test an existing parameterization and develop new ones for NWP models and GCMs. This has not been adequately addressed. There is no discussion on the approach to be taken to develop new PBL parameterizations for NWP models and GCMs. This objective is a major one and it is not clear whether this study would have enough data or results to achieve it.

o It appears that 1990 (COWEX-I) must have been an anomalous year in terms of cold-air outbreaks. The proposal did not explicitly address this. It would be a shame to have another minimal year.

o There are no direct measurements of the state of the sea surface, such as water temperature and air-sea differences. Continuous measurements from a buoy(s) located within the study area would provide continuous data within which to frame the aircraft and sattelite observations

o The number of key questions is too long and needs to be focused.

o Few details are given about the analysis of gravity waves. Such analyses are not straightforward. Often gravity wave signals are confounded by turbulence, noise, and secondary circulations. Seldom are they monochromatic or long-lasting, and this further complicates the problem.

o I would have like to have seen alternatives to the program in the event of instrument failure or absence of the desired events.

References

Agee, E.M., and S.R. Gilbert, 1989: An aircraft investigation of mesoscale convection over Lake Michigan during the 10 January 1984 cold-air outbreak. J. Atmos. Sci., 46, 1877-1897.

Asai, T., 1972: Thermal instability of shear flow turning the direction with height. J. Meteor. Soc. Japan, 50, 525-532.

Balaji, V. and T.L. Clark, 1988: Scale selection in locally forced convective fields and the initiation of deep cumulus. J. Atmos. Sci., 45, 3188.

_____, Redelsperger, J.-L. and Klaasten, G.P., 1993: Mechanisms for the mesoscale organization of tropical clusters in GATE Phase III. Part I: Shallow cloud bands, J. Atmos. Sci. 50, 2281-2301.

Bechtold, P., C. Fravolo, and J.P. Pinty, 1992: A study of a 2-dimensional cloudiness transition during a cold-air outbreak event. Bound. Lay. Meteor., 60, 243-270.

Bluestein, H.B., 1990: Observations of steam devils over a lake during a cold-air outbreak. Mon. Wea. Rev., 118, 2244-2247.

Boers R., and S.H. Melfi, 1987: Cold air outbreak during MASEX: Lidar observations and boundary layer model test, Bound. Lay. Meteor., 39, 41-51.

Brooks, I.M, and D.P. Rogers, 1997: Aircraft observations of boundary layer rolls off the coast of California. J. Atmos. Sci., 54, 1834-1849.

Brown, R.A., 1970: A secondary flow model for the planetary boundary layer. J. Atmos. Sci. 27, 742-757.

_____, 1972: On the inflection point instability of the stratified Ekman boundary-layer, J. Atmos. Sci., 29, 850-859.

_____, 1980: Longitudinal instabilities and secondary flows in the planetary boundary layer: a review. Rev. Geophys. Space Phys., 18, 683-697.

_____, 2000: Serendipity in the use of Satellite Scatterometer, SAR and other sensor data. Johns Hopkins APL Technical Digest, 21, 21-26.

_____, and W. T. Liu, 1982: An operational large-scale marine planetary boundary layer model. J. Appl. Meteor., 21, 261-269.

_____, and R.C. Foster, 1994: On PBL models for general circulation models. Global Atmos.- Ocean Syst., 2, 163-183.

Chlond, 1992: 3D simulation of cloud street development during a cold-air outbreak. Bound.- Layer Meteor., 58, 161-200.

Chou, S-H, and M.P. Ferguson, 1991: Estimates of sensible and latent heat fluxes over the western Atlantic Ocean after a cold-air outbreak. Bound. Layer Meteor., 56, 51-81.

Christian, T.W., and R.M. Wakimoto, 1989: The relationship between radar reflectivities and clouds associated with horizontal roll convection on 8 August 1982. Mon. Wea. Rev., 117, 1530-1544.

Chun, H-Y and J-J Baik, 1998: Momentum flux by thermally induced gravity waves and its approximation for large-scale models. J. Atmos. Sci., 55, 3299-3310.

Clark, T.L., T. Hauf and J.P. Kuettner, 1986: Convectively forced internal gravity waves: Results from two-dimensional numerical experiments. Quart. J. Roy. Met. Soc., 112, 899-925.

Davis, J.M., F.L. Estis, P. Bloomfield, and J.F. Monahan, 1991: Complex principal component analysis of sea-level pressure over the eastern USA. Int. J. Climatology, 11, 27-54.

Dobosy, R.J., T.L. Crawford, J.I. MacPherson, R.L. Desjardins, R.D. Kelly, S.P. Oncley, and D.H. Lenshow, 1997: Intercomparison among four flux aircraft at Boreas in 1994. J. Geophys. Res., 102, 29101-111.

Drobinski, P., et al., 1998: Evidence of organized large eddies by ground-based Doppler lidar, sonic anemometer and sodar. Bound.- Layer Meteor., 88, 343-361.

_____, and R.C Foster, 2000: On the dynamics of near-surface streaks in the neutrally-stratified boundary layer. Submitted to Bound.-Layer Meteor.

Etling, D. and Brown, R.A., 1993: Roll vortices in the planetary boundary layer: a review. Bound.-Layer Meteor. 65, 215-248.

Ferrare, R.A., J.L. Schols, E.W. Eloranta, and R. Coulter, 1991: Lidar observations of banded convection during BLX83. J. Appl. Meteor., 30, 312-326.

Flamant, C et al., 1999: Evidence of a Mixed-Layer Dynamics Contribution to the Entrainment Process, Bound.-Layer Meteor., 93, 47-73.

Foster, R.C., 1996: An analytic model for PBL roll vortices. PhD Thesis, University of Washington, 200 pp.

_____, 1997a: Structure and energetics of optimal Ekman layer perturbations. J. Fluid Mech., 333, 97-123.

_____, 1997b: Nonlinear equilibration of Ekman layer normal modes. 11th Conference on Atmospheric and Oceanic Fluid Dynamics, 23-27, June, 1997, Tacoma, WA.

_____, 1999, Optimal perturbations associated with PBL streaks, 12 Conference on Atmospheric and Oceanic Fluid Dynamics, 7-11, June, 1999, New York, NY.

_____, and R. A. Brown, 1994a: On large-scale PBL modelling: Surface wind and latent heat flux comparisons. Atmos. Ocean Syst., 2, 199-219.

_____, and R. A. Brown, 1994b: On large-scale PBL modelling: The surface layer models. Atmos. Ocean Syst., 2, 185-198.

_____, A. Enloe and R.A. Brown, 1999: Baroclinic modification of the marine surface wind vectors observed by the NASA scatterometer. J. Geophys. Res., 104, 31,225 -36.

_____, and G. Levy, 1998: The contribution of organized roll vortices to the surface wind vector in baroclinic conditions. J. Atmos. Sci, 55, 1466-1472.

French, J. R., G. Vali, and R. D. Kelly, 1999: Evolution of small cumulus clouds in Florida: Observations of pulsating growth. Atmos. Res., in press.

French, J. R., G. Vali, and R. D. Kelly, 2000: Observations of microphysics pertaining to the development of drizzle in warm, shallow cumulus clouds. Q. J. R. Meteorol. Soc., submitted.

Glendening, J.W., 1996: Linear eddy features under strong shear conditions. J. Atmos. Sci., 53, 3430-3449.

Grossman, R.L., 1982: An analysis of vertical velocity spectra obtained in the BOMEX fair-weather, trade-wind boundary layer. Bound.-Layer Meteor., 23, 323-357.

Hauf, T., 1993: Aircraft observation of the convection waves over southern Germany-a case study. Mon. Wea. Rev. 121, 3282-3290.

Hein, P.F. and Brown, R.A., 1988: Observation of longitudinal roll vortices during Arctic cold-air outbreaks over open water, Bound.-Layer Meteor. 45, 177-199.

Hock, T. and H. Cole, 1991: A new aircraft universal lightweight digital dropsonde. Seventh AMS Symposium on Meteorological Observations and Instrumentation, New Orleans, LA, Amer. Meteor. Soc., 291-296.

Karyampudi, V.M., J. Simpson, S. Palm, and H. Pierce, 1997: Lidar observations of Saharan dust layer and its influence on tropical cyclogenesis. 22nd Conference on Hurricanes and Tropical Meteorology. Ft. Collins, Colorado. 59.

Karyampudi, V.M., S. Palm, J. Reagan, H. Fang, W. Grant, R. Hoff, C. Moulin, H. Pierce, O. Torres, E. Browell and S. Melfi, 1999: Validation of the Saharan Dust Plume Conceptual Model Using Lidar, Meteosat and ECMWF Data. Bull. Amer. Meteor. Soc., 80, 1045-1075

Kelly, R.D., 1982: A single Doppler radar study of horizontal roll convection in a lake-effect snow storm. J. Atmos. Sci., 39, 1521-1531.

_____, 1984: Horizontal roll and boundary-layer interrelationships observed over Lake Michigan. J. Atmos. Sci., 41, 1816-1826.

Kershaw, R., 1995: Parameterization of momentum transport by convectively-generated gravity waves. Q. J. R. Meteorol. Soc., 121, 1023-1040.

Konrad, C.E., 1998: Persistent planetary scale circulation patterns and their relationship with cold air outbreak activity over the eastern United States. Int. J. Climatology, 18, 1209-1221.

Kristovich, D.A.R., and co-authors, 2000: The lake-induced convection experiment and the snowband dynamics project. Bull. Amer. Meteor. Soc., 81, 519-542.

Kropfli, R. A., and R. D. Kelly, 1996: Meteorological research applications of mm-wave radar. Meteor. and Atmos. Phys., 59, 105-121.

Kuettner, J.P., 1959: The band structure of the atmosphere, Tellus 11, 267-294.

_____, 1971: Cloud bands in the Earth's atmosphere: observation and theory, Tellus, 23, 404-425.

_____, P.A. Hildebrand and T.L. Clark, 1987: Convection waves: observations of gravity wave systems over convectively active boundary layers. Quart. J. Royal Meteor. Soc.,113, 445-467.

LeMone, M.A., 1973: The structure and dynamics of the horizontal roll vortices in the planetary boundary layer, J. Atmos. Sci. 30, 1077-1091.

_____, 1976: Modulation of turbulence energy by longitudinal rolls in an unstable planetary boundary layer. J.Atmos Sci., 33, 1308-1320.

_____, 1990: Some observations of vertical velocity skewness in the convective planetary boundary-layer. J.Atmos Sci., 47, 1163-1169.

_____, and A. Pennell, 1976: Relationship of trade wind cumulus distribution to subcloud layer fluxes and structure. Mon. Wea. Rev., 104, 524-53.

_____, and R.J. Meitin, 1984: Three examples of fair-weather mesoscale boundary-layer convection in the tropics. Mon. Wea. Rev., 112, 1985-1997.

Leon, D. C., and G. Vali, 1998: Retrieval of three-dimensional particle velocities from airborne Doppler radar data. J. Atmos. and Oceanic Tech., 15, 860-870.

_____, A. Guyot, P. Laborie, A. Pazmany, J. Pelon, J. Testud, and G. Vali, 1999: Vertical plane velocity fields retrieved from dual-beam airborne Doppler radar data. 29th Internat. Conf. on Radar Meteor., Montreal, Canada, July 1999, 472-475.

Lilly, D.K., 1966: On the stability of the Ekman boundary flow, J. Atmos. Sci. 23, 481-494.

Lohou, F., A. Druilhet, and B. Campistron, 1998: Spatial and temporal characteristics of horizontal rolls and cells in the atmospheric boundary layer based on radar and in situ observations. Bound.-Layer Meteor., 89, 407-444.

Mason, P.J. and R.I. Sykes, 1982: A two-dimensional numerical study of horizontal roll vortices in an inversion-capped planetary boundary layer. Quart. J. R. Met. Soc., 108, 801-823.

McFarlane, N.A., 1987: The effect of orographically excited gravity wave drag on the general circulation of the lower stratosphere and troposphere. J. Atmos. Sci. 44, 1775-1800.

Melfi, S.H., J.D. Spinhirne, S.H. Chou and S. Palm, 1985: Lidar observations of vertically organized convection in the planetary boundary layer over the ocean. J. Clim. Appl. Meteor. 24, 806-821.

Miura, Y. 1986: Aspect ratio of longitudinal rolls and convection cells observed during cold-air outbreaks, J. Atmos. Sci. 43, 26-39.

Mourad, P.D., and R.A. Brown, 1990: Multiscale large eddy states in weakly stratified planetary boundary layers, J. Atmos. Sci. 47, 414-438.

_____, 1996: Inferring multiscale structure in atmospheric turbulence using satellite-based synthetic aperture radar imagery. J. Geophys. Res. 101, 18433-18449.

_____, and B.A. Walter, 1996: Satellite scatterometer evidence of roll circulations in the marine atmospheric boundary layer . J. Geophys. Res., 101, 16391-400.

Muller,G., B. Brummer, and W. Alpers, 1999: Roll convection within an Arctic cold-air outbreak: interpretation of in situ aircraft measurements and spaceborne SAR imagery by a three-dimensional atmospheric model. Mon. Wea. Rev., 127, 363-380.

Palm, S. P., and S. H. Melfi, 1992: On the existence of convectively produced gravity waves. Sixteenth International Laser Radar Conference, Cambridge, Massachusetts. NASA Conference Pub. 3158. 389-392.

_____, S.H. Melfi, and D.L. Carter, 1994: New airborne scanning lidar system: applications for atmospheric remote sensing. Appl. Opt., 33, 5674-5681.

_____, D. Vandemark, D. Hines, G. Schwemmer and S.H. Melfi, 1996: Remote sensing of the marine atmospheric boundary layer and ocean surface winds: observations from the LITE correlative flights. 1996 International Geoscience and Remote Sensing Symposium, Lincoln, Nebraska, 1257.

_____, D. Hagan, G. Schwemmer and S.H. Melfi, 1998: Inference of marine atmospheric boundary layer moisture and temperature structure using airborne lidar and infrared radiometer data. J. Appl. Meteor., 37, 308-324.

Palmer, T.N., G.J. Shutts and R. Swinbank, 1986: Alleviation of a systematic bias in general circulation and numerical weather prediction models through an orographic gravity wave drag parameterization. Quart. J. Roy. Meteor. Soc., 112, 1001-1039.

Pasquier, J.R.M., and P.R. Jonas, 1998: Turbulent transport in fields of warm cumulus clouds. Quart. J. Roy. Meteor. Soc., 124, 363-387.

Pazmany, A., R. McIntosh, R. Kelly, and G. Vali, 1994: An airborne 95 GHz dual-polarized radar for cloud studies. IEEE Trans. Geosci. and Remote Sensing, 32, 731-739.

Rao, G.S., and E.M. Agee, 1996: Large eddy simulation of turbulent flow in a marine convective boundary layer with snow. J. Atmos. Sci., 53, 86-100.

Reddy, N.C., and S. Raman, 1994: Scales and spectra of turbulence over the Gulf Stream during offshore cyclogenesis. Bound. Lay. Meteor , 68, 387-417.

Sang, J.G., 1993: On the dynamics of convection waves. Q. J. R. Meteorol. Soc., 119, 715-732.

Savchenko, 1999: Effect of large eddies on atmospheric surface layer turbulence and the underlying wave field. J. Geophys. Res. - Oceans, 104, 3149-3157.

Schwemmer, G., S. P. Palm, S.H. Melfi, and K.D. Evans, 1996: Retrieval of atmospheric boundary layer parameters from LITE and LASAL. Advances in Atmospheric Remote Sensing With Lidar : selected papers of the 18th international laser radar conference, Berlin Germany. 161-164.

Shirer, H.N., 1980: Bifurcation and stability in a model of moist convection in a shearing environment. J. Atmos. Sci., 37, 1586-1602.

Sykes, R.I., and D.S. Henn, 1989: Large-eddy simulation of turbulent sheared convection. J. Atmos. Sci., 46, 1106-1118.

Stull, R.B, 1976: Internal gravity waves generated by penetrative convection. J. Atmos. Sci., 33, 1279-1286.

Townsend, A.A., 1965: Excitation of internal waves by a turbulent boundary layer. J. Fluid Mech., 22, 241-252.

_____, 1966: Internal waves produced by a shear layer. J. Fluid Mech., 24, 307-319.

_____, 1968: Excitation of internal waves in a stably-stratified atmosphere with considerable wind shear. J. Fluid Mech., 32, 145-171.

Turner, J.S., 1973: Buoyancy effects in fluids. Cambridge University Press. 368 pp.

Vali, G., R. D. Kelly, J. French, S. Haimov, D. Leon, A. Pazmany, and R. E. McIntosh, 1998: Finescale structure and microphysics of coastal stratus. J. Atmos. Sci., 55, 3540-3564.

Vandemark, D., F. Jackson, E. Walsh and B. Chapron, 1994: Airborne radar measurements of ocean wave spectra and wind speed during the grand banks ERS-1 SAR wave experiment, Atmos.-Ocean, 32, 143-178.

_____, et al., 2000: Measured changes in ocean surface roughness due to atmospheric boundary layer rolls. Submitted to J. Geophys. Res.

Walter, B.A., 1980: Wintertime Observations of Roll Clouds over the Bering Sea, Mon. Wea. Rev. 108, 2025-2031.

_____, and Overland, J.E., 1984: Observations of longitudinal rolls in a near neutral atmosphere, Mon. Wea. Rev. 112, 200-208.

Weckwerth, T.M., J.M. Wilson, and R.M. Wakimoto, 1996: Thermodynamic variability within the convective boundary layer due to horizontal convective rolls. Mon Wea. Rev., 124, 769-784.

_____, J.W. Wilson, R.M. Wakimoto, and N.A. Crook, 1997: Horizontal convective rolls: Determining the environmental conditions supporting their existence and characteristics. Mon. Wea. Rev., 125, 505-526.

_____, T.W. Horst, and J.W. Wilson, 1999: An observational study of the evolution of horizontal convective rolls. Mon. Wea. Rev., 127, 2160-2179.

Woodcock, A.H., 1940: Convection and soaring over the open sea. J. Marine Res., 3, 248-253.

Xue, H., Z. Pan, and J.M. Bane, 2000: A 2D coupled atmosphere-ocean model study of air-sea interactions during a cold-air outbreak over the Gulf Stream. Mon. Wea. Rev., 128, 973-996.

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download