LT Pierre-Yves Daré - NPS



LT Pierre-Yves Daré

San Francisco Bay Mesoscale Atmospheric Circulation in Summer

A case study

OC 3570

Operational Oceanography and Meteorology

Summer 2006

LIST OF CONTENTS

LIST OF FIGURES AND TABLES 3

1. Introduction 4

2. Data analysis 5

2.1. Cruise data 5

2.2. Synoptic and mesoscale analysis 12

2.3. Wind observations 17

2.4. Oakland soundings 18

3. Discussion 19

4. Conclusion 23

ACKNOWLEDGEMENTS 25

REFERENCES 26

LIST OF FIGURES AND TABLES

Fig. 1 Topography of San Francisco Bay 4

Fig. 2 Ship track, strong wind periods and rawinsonde launch positions on 23 July 6

2006

Fig. 3 Ship track, strong wind periods and rawinsonde launch positions on 24 July 7

2006

Fig. 4 One-minute averaged time-series of wind speed and direction for 23 July 2006 8

Fig. 5 One-minute averaged time-series of wind speed and direction for 24 July 2006 8

Fig. 6 Rawinsonde profiles (temperature, dew point, wind direction and speed) 10-11

Fig. 7 Sea-level pressure at 0000 UTC on 24 July 2006 (GFS analysis) 13

Fig. 8 1000-500mb thickness at 0000 UTC on 24 July 2006 (GFS analysis) 14

Fig. 9 Sea-level pressure, surface geostrophic wind and surface wind at 1200 UTC 15-16

on 23 July, 0000 UTC and 1200 UTC on 24 July 2006 (Eta-12km analysis)

Fig. 10 Surface wind at 0500 UTC on 23 July 2006 (Nuss San Francisco Bay model) 16

Fig. 11 Wind observations by onshore stations, buoys and ships at 1200 UTC on 23 17

July, 0000 UTC and 1200 UTC on 24 July 2006

Fig. 12 Oakland soundings at 1200 UTC on 23 July, 0000 UTC and 1200 UTC on 18

24 July 2006

Fig. 13 Cross-section of the San Francisco Bay entrance channel at 0000 UTC 22

on 24 July 2006 (Eta-12km analysis)

Fig. 14 San Francisco Bay gap flows in summer 23

Table 1 Rawinsonde launch summary 12

1. Introduction

The San Francisco Bay topography (Fig. 1) offers a unique configuration for mesoscale atmospheric circulation: the coastal mountain chain and the narrow openings between the inner-bay and the Pacific Ocean force the flow to be strongly affected by the topography. In addition, the large coastal upwelling along the central California coast leads to a significant cross-shore thermal gradient at the surface, enhanced by a strong ground heating in the Central Valley especially in summer.

During the summer 2006 OC 3570 cruise, research vessel Point Sur has investigated the San Francisco Bay sea circulation. This investigation was carried out on 23 and 24 July along tracks covering a large part of the bay area, from its southern region up to its northern one (Fig. 2 and 3). Thanks to ship sensors, sea-surface pressure, wind speed and direction have been recorded every second, providing high temporal resolution time-series of these parameters. Moreover, six rawinsondes launched within the bay showed some aspects of the three-dimensional wind distribution.

[pic]

Fig. 1 Topography of San Francisco Bay (heights are given in feet)

The studied period (23-24 July 2006) was characterized by relatively weak winds and particularly high temperatures in San Francisco area. Also, at some places inside the bay, strong wind events have been recorded. At first sight, these events may be related to land/sea-breeze circulation and/or topography-forced flow (gap flow).

The purpose of this study is to determine the origin of the strong wind at these places and thus to figure out the main characteristics of the surface air flow in San Francisco Bay during this summertime period.

In section 2, the cruise datasets will be presented, analyzed and compared to model outputs (GFS, Eta-12 km-resolution and Nuss’ San Francisco Bay model), buoy and shore observations. Then, the results and the surface atmospheric circulation in San Francisco Bay will be discussed in section 3 before concluding in section 4.

2. Data analysis

2.1. Cruise data

R/V Point Sur entered San Francisco Bay at about 0100 UTC on Sunday 23 July 2006 and left the area at about 1815 UTC on Monday 24 July 2006. The ship tracks are given in Fig. 2 and Fig. 3, respectively for the first day and the second one. The times and locations of strong wind records and rawinsonde launches are also shown on these figures.

Fig. 4 and Fig. 5 are the plots of one-minute averaged wind speed and direction time-series relative to 23 July and 24 July records respectively. As shown on these two figures (by the red lines), the mean wind speed is low over the whole period: 3.4 m/s (with a standard deviation of 3.3 m/s) for the first day and 5.4 m/s (with a standard deviation of 2.4 m/s) for the second one. Thus, regarding these basic statistical values, a strong wind event has been defined by a wind speed equal or greater than 10 m/s (20 knots).

Firstly, we observe that most of the strong wind records correspond to the bay entrance region. We may also note that the wind in the northern part of the bay may be strong, whereas it remains mostly weak in the southern part.

[pic]

Fig. 2 Ship track, strong wind periods and rawinsonde launch positions on 23 July 2006

Moreover, regarding the time of these events, we infer that they may occur at every time, without any preferred period of the day (afternoon, evening, night…). Particularly, it is noticeable that each ship station along the bay entrance axis led to strong wind observations, whatever the time was.

By matching the ship position and the wind direction time-series, we firstly note that the wind at the bay entrance was always southwesterly-westerly. This lets us infer that the wind remains steady (strong and southwesterly-westerly) along the bay entrance axis.

As the ship cruised in the southern part of the bay (from 1430 to 2215 UTC on 23 July and from 0535 to 1030 UTC on 24 July), the recorded wind was relatively constant in the westerly-northwesterly direction.

[pic]

Fig. 3 Ship track, strong wind periods and rawinsonde launch positions on 24 July 2006

[pic]

Fig. 4 One-minute averaged time-series of wind speed and direction for 23 July 2006. The vertical dashed line shows the time when R/V Point Sur entered the bay. The red solid line indicates the mean wind speed and the red dashed lines point out the wind speed standard deviation around the mean value.

[pic]

Fig. 5 One-minute averaged time-series of wind speed and direction for 24 July 2006. The vertical dashed line shows the time when R/V Point Sur left the bay. The red solid line indicates the mean wind speed and the red dashed lines point out the wind speed standard deviation around the mean value. Red circles highlight the two passages in the northern bay.

For the northern bay records (from 0015 to 0500 UTC and from 1250 to 1730 UTC on 24 July), the winds kept a southerly-southwesterly direction, with being rather southerly to the south of Richmond-San Rafael Bridge and southwesterly to its north.

Concerning the wind speed in the northern bay, we note a significant change between the two ship passages (pointed out in Fig. 5 by red circles): during the first passage (0015 to 0500 UTC) occurring in the late afternoon, the winds were much stronger than during the second one (1250 to 1730 UTC), which took place in the morning.

Six rawinsondes have been launched in San Francisco Bay. Their launch positions are displayed in Fig. 2 and Fig. 3. The temperature, dew point, wind speed and direction profiles measured by the rawinsondes are shown in Fig. 6 and Table 1 summarizes the main sounding characteristics. Note that three of the six soundings describe strong wind events.

First of all, the six soundings are characterized by a shallow (≤ 100 m) surface layer caped by a significant temperature inversion. In some cases, the marine layer is only a few tens of meters thick.

During the strong wind observations (2307 UTC on 7/23, 0521 UTC and 1758 UTC on 7/24), we also note that the layer of high wind speed is very thin (no more than 30 m), which depicts these wind events as near-surface phenomena.

There is not any obvious correlation between surface wind speeds and inversion heights. Indeed, the surface layer was thin in every observation, however strong the surface wind was. On the contrary, the highest surface wind speeds correspond to the strongest inversions. However, on the 1054 UTC sounding, we observe a strong inversion but not a high surface wind speed. This case is particular, since the launch position is near the bay entrance axis and also seven minutes after the launch time, strong surface winds have been recorded (Fig. 3).

Note also that some values of wind speed and direction are sometimes missing along the profiles: this is due to rawinsonde malfunctioning.

|[pic] |[pic] |

|[pic] |[pic] |

|[pic] |[pic] |

Fig. 6 Rawinsonde profiles (temperature, dew point, wind direction and speed) at

a) 1629 UTC (7/23/2006); b) 2307 UTC (7/23/2006); c) 0247 UTC (7/24/2006)

|[pic] |[pic] |

|[pic] |[pic] |

|[pic] |[pic] |

Fig. 6 (continued) Rawinsonde profiles (temperature, dew point, wind direction and speed) at

d) 0521 UTC (7/24/2006); e) 1054 UTC (7/24/2006); f) 1758 UTC (7/24/2006)

|Launching time |Position |Strong wind observation|

|1629 UTC on 7/23 |Southern bay |NO |

|2307 UTC on 7/23 |Golden Gate area |YES |

|0247 UTC on 7/24 |Northern bay |NO |

|0521 UTC on 7/24 |Golden Gate area |YES |

|1054 UTC on 7/24 |South Golden Gate area |NO |

|1758 UTC on 7/24 |Golden Gate area |YES |

Table 1 Rawinsonde launch summary

2.2. Synoptic and mesoscale analysis

For the analysis, three different models have been used: GFS (Global Forecasting System) for the synoptic scale, Eta 12km-resolution and Professor Nuss’ (NPS, Meteorology Department) San Francisco Bay model, which has a resolution of 1.25 km.

As shown on Fig. 7 (GFS sea-level pressure analysis for 0000 UTC on 7/24/2006) and Fig. 8 (GFS 1000-500mb thickness analysis for 0000 UTC on 7/24/2006), the studied period was characterized by a stationary synoptic-scale thermal low over California. This large thermal low is consistent with the high surface temperatures and low pressures recorded in San Francisco Bay (not shown in this paper) on 23 and 24 July.

Fig. 9 depicts the evolution of the sea-level pressure, geostrophic and surface wind fields over the studied period, shown by the Eta-12 km model. Generally speaking, the wind directions given by Eta-12 km analysis are consistent with the ship records. However, concerning the wind speed, the analysis from the model provides underestimated values.

On Fig. 9, we note a diurnal variation of the cross-shore pressure gradient, with the strongest pressure gradient during the afternoon (0000 UTC on 7/24). However, we do not observe any reversal of this pressure gradient during daytime versus nighttime, which would be noticeable in a land/sea-breeze circulation system. This absence of pressure gradient reversal is coherent with the low level cross-shore thermal gradient (isentrope maps not shown here) which maintains its direction towards land over the whole studied period. The strongest pressure gradient during the late afternoon (0000 UTC on 7/24) is also consistent with the ship records (Fig. 4 and 5), in which we observe the maximum surface wind speed around 2300 UTC on 23 July (Golden Gate Bridge area).

[pic]

Fig. 7 Sea-level pressure at 0000 UTC on 24 July 2006 (GFS analysis)

Based on Fig. 9, the surface winds remain almost geostrophic offshore, whereas they tend to be parallel to the pressure gradient in the bay area. The surface winds seem also to be strongly affected by the topography in San Francisco Bay.

Another interesting point is that no diurnal wind reversal is noticeable: this lets us infer that the land/sea-breeze circulation is not predominant in the bay, as proved above by the absence of sea-level pressure gradient reversal.

[pic]

Fig. 8 1000-500mb thickness at 0000 UTC on 24 July 2006 (GFS analysis)

The Nuss model analysis provides the same results as Eta-12 km. Fig. 10 is an example of this model output which points out the strong influence of the topography on the surface wind circulation inside the bay. Concerning Nuss model outputs, the wind direction is always consistent with the ship observations in the southern bay and mostly consistent with the observations in the northern bay and in the bay entrance region. However, like for the Eta-12 km model, the output wind speed is significantly underestimated versus the observed values, especially in the Golden Gate Bridge area.

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Fig. 9 Sea-level pressure, surface geostrophic wind (blue) and surface wind (red) at a) 1200 UTC on 23 July 2006; b) 0000 UTC on 24 July 2006 (Eta-12km analysis)

|[pic] |

Fig. 9 (continued) Sea-level pressure, surface geostrophic wind (blue) and surface wind (red) at c) 1200 UTC on 24 July 2006 (Eta-12km analysis)

[pic]

Fig. 10 Surface wind at 0500 UTC on 23 July 2006 (Nuss San Francisco Bay model)

2.3. Wind observations

Fig. 11 shows the wind observations made by buoys, ships and stations on shore, for the synoptic times (1200 UTC on 23 July, 0000 UTC and 1200 UTC on 24 July 2006). These observations are consistent with the three model analysis and especially point out the absence of wind reversal. Again, this lets us infer that the land/sea-breeze circulation is not predominant in San Francisco Bay during the studied period.

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Fig. 11 Wind observations by onshore stations (red), buoys and ships (blue) at a) 1200 UTC on 23 July 2006 (0500 in local time); b) 0000 UTC on 24 July 2006 (1700 in local time); c) 1200 UTC on 24 July 2006 (0500 in local time)

2.4. Oakland soundings

The soundings carried out at Oakland airport, given in Fig. 12, depict a very thin surface layer as in rawinsonde profiles. The vertical distribution of temperature, dew point, wind speed and direction does not change significantly over the 3 soundings (1200 UTC on 23 July, 0000 UTC and 1200 UTC on 24 July 2006). In particular, we again do not observe any wind reversal between the morning soundings (1200 UTC on 23 and 24 July) and the late-afternoon one (0000 UTC on 24 July).

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Fig. 12 Oakland soundings at a) 1200 UTC on 23 July 2006 (0500 in local time); b) 0000 UTC on 24 July 2006 (1700 in local time); c) 1200 UTC on 24 July 2006 (0500 in local time).

The blue profile represents air temperature and the pink one dew point.

Moreover, we note some similarities between the 1629 UTC rawinsonde profiles and the 1200 UTC Oakland sounding on 23 July, and between the 1054 UTC rawinsonde profiles and the 1200 UTC Oakland sounding on 24 July : this is true for every variable (temperature, dew point, wind speed and direction).

3. Discussion

The absence of surface wind reversal inside the bay and along the coast, pointed out by all the observations (R/V Point Sur, buoys, ships and onshore stations) and by the model analysis, clearly leads to the conclusion that the land/sea-breeze circulations are not the mesoscale predominant wind components in this summertime period, even if the thermal forcing is large. This result is also confirmed by the pressure gradient direction, which remained constant in the offshore direction over the whole studied period.

In addition, regarding the wind direction inside the bay (all observations and model analysis), the surface wind circulation seems strongly affected by the bay topography. In particular, all the observations and model analysis describe a funneling effect along the axis of the bay entrance (Golden Gate Bridge area) and in the northern bay, and the cruise data point out some wind accelerations in the narrow areas. Thus the strong winds events, observed during the OC 3570 summer 2006 cruise, are due to narrow gaps within high topography (Fig. 1).

First of all, we observe a synoptic pattern favorable for gap flows through the bay entrance channel over the whole studied period: as shown on Fig. 7, a stationary sea-level cold high- pressure area to the west (offshore) and a surface low-pressure zone to the east (inland), creating a pressure gradient along the channel axis. In addition to this, based on Fig. 9b, there is a large mesoscale thermal forcing induced by the land heating, which enhances the cross-shore pressure gradient during the late afternoon.

Secondly, the observed surface layer depths (Fig. 6) allow us to compute the Rossby radius of deformation, which for topographically forced flows may be given by equation (1):

[pic] (1)

with [pic] and [pic]

[pic]

This value is much greater than the width of the bay entrance channel (Golden Gate Bridge area) and that of the gap between Richmond and San Rafael, which are respectively about 2800 m and 9300 m. Thus, these comparatively low widths inhibit any geostrophic adjustment and force the strongly stratified (cold air at the surface caped by a strong temperature inversion) surface flow to be controlled by the topography.

In addition, we may explain the strong winds observed during the first passage in the northern bay (from 0015 to 0500 UTC on 24 July 2006) by the large pressure gradient through the bay entrance channel, which was enhanced by the afternoon thermal forcing (Fig. 9b) and which led to a strong wind at the channel exit. Then this strong wind was blocked by the bay topography and had to be channeled through the northern gap in which there was again some acceleration because of the pressure gradient along the gap axis (Fig. 9b).

If we use a simple two-dimensional model for the flow dynamics within the gap and only consider the dominant balance (between the pressure gradient through the gap and the inertial acceleration of the wind in the gap), we may write the following equation:

[pic] (2)

where the x-axis is the along-gap direction, u the wind speed component along this axis, p the pressure and [pic]the density.

By integrating Eq. (2) with respect to x, we obtain a simple expression for the wind speed in the gap:

[pic] (3)

or [pic] (4)

Here we assume [pic] and that [pic] corresponds to the surface wind at the gap entrance.

Based on Fig. 9b, for the bay entrance channel case (Golden Gate Bridge area), we have at 0000 UTC on 24 July [pic] and [pic] (between the channel entrance and exit). Therefore, at the channel exit, the surface wind speed is:

[pic]

This value is consistent with the mean wind speed measured by R/V Point Sur between 2300 UTC on 23 July and 0000 UTC on 24 July (Fig. 4), when she was to the east of the Golden Gate Bridge.

Another fundamental point is that, in the entrance bay channel, strong winds were only observed to the east of the Golden Gate Bridge (Fig. 2, 3, 4 and 5) and especially when R/V Point Sur entered San Francisco Bay around 0100 UTC on 23 July, the wind significantly increased as the ship went eastward. Similarly, as R/V Point Sur exited San Francisco Bay around 1800 UTC on 24 July, a rapid decrease in wind speed was observed as the ship steered to the west. These observations are coherent with a main characteristic of gap flows: the strongest winds are always recorded at the exit of the gaps. This is also clearly shown by Eq. (3) and (4).

Fig. 13 is a cross-section of the bay entrance channel at 0000 UTC on 24 July (Eta-12 km analysis), where the Golden Gate Bridge is located in the middle of the section. As mentioned above, the surface layer is thin and very stable: the vertical gradient of potential temperature is positive and large just above the cold near-surface air. This corresponds to the strong temperature inversion at low height. We also note a slightly negative slope of isentropes (and hence a slightly negative slope of inversion height) which is consistent with the flow acceleration in the channel.

On the cross-section are also plotted the tangential and normal winds: these show that the surface wind is parallel to the channel axis near the surface at the Golden Gate Bridge location. However, the model analysis does not point out any high wind speed.

In general, even if the models well analyze the pressure gradient strength and its variations, they only provide good solutions for wind directions and underestimate the wind speeds in the gaps: it is due to the fact that the models cannot represent accurately the topography (sharp slopes) because of a too coarse resolution. Thus they cannot predict accurately the flow intensification generated by the gaps.

Note that a short-term forecast for winds in San Francisco Bay is available at . This website has not been used for the analysis because the archives were not accessible. Since it seems to provide relatively accurate wind speed, it would be worth using it in future studies.

[pic]

Fig. 13 Cross-section of the San Francisco Bay entrance channel at 0000 UTC on 24 July 2006

(Eta-12km analysis) showing the isentropes, tangential winds (red barbs) and normal winds (blue barbs)

4. Conclusion

During the 23-24 July period, the high wind speeds recorded in some places within San Francisco Bay were not generated by the land/sea-breeze circulations. Indeed they were due to the topography of the bay: gap flows were generated in narrow openings between coastal mountains all around the bay. This phenomenon has already been described by a few studies.

Fig. 14 summarizes the most common gap flows observed during summer in San Francisco area (from Gilliam, Weather of the San Francisco Bay Region). Also, Neiman et al. provided a complete description of the channeled flows through the gaps in the surroundings of San Francisco during winter, when there is a cold air pool over the Central Valley: the wintertime gap flows take place at the same locations as those in summer, but are in the opposite direction.

[pic]

Fig. 14 San Francisco Bay gap flows in summer

(from Gilliam, Weather of the San Francisco Bay Region)

Although the land/sea-breeze circulation does not seem to play any major role in the flow within San Francisco Bay during the studied period, we must consider that in addition to being hydraulically controlled the surface circulation is also thermally driven. Indeed, to generate a gap flow, a pressure gradient in the cross-shore direction is necessary: during the 23-24 July period, the pressure gradient was induced by the strong mid-latitude high-pressure zone over the Pacific Ocean and the synoptic thermal low over Northern California. Moreover, the cross-shore pressure gradient was intensified by the daytime differential heating between the land and the ocean. On the contrary, it weakened during the night as the surface cooling was stronger in land than over the sea.

Thus, this case illustrates the combined effects of coastal topography and differential heating between land and ocean (characteristic of coastal regions) on the low-level atmospheric circulation. Due to the San Francisco Bay topography and the commonly strong surface heating in the Central Valley during summers, we may think that the significant gap flows in San Francisco area, described in this paper, are typical of the summertime surface circulation in this region. Since, as shown in this study, all the models do not still describe accurately the wind strength in such a complex topography, other studies, based on in-situ measurements, will be necessary to confirm this last point.

ACKNOWLEDGEMENTS

Special thanks to Pr. Peter Guest who has supervised this project and provided all the means to realize it (rawinsonde launches…).

Thanks to Pr. Curt Collins, R/V Point Sur officers and crew members, who have made this study possible.

Thanks also to Bob Creasey for archiving all the observed data and model outputs, to Richard Lind for providing cruise data in a convenient format, and to Mary Jordan for her mapping matlab files.

REFERENCES

Neiman, P. J., F. M. Ralph, A. B. White, D. D. Parrish, J. S. Holloway and D. L. Bartels, 2006: A Multiwinter Analysis of Channeled Flow through a Prominent Gap along the Northern California Coast during CALJET and PACJET. Mon. Wea. Rev., 134, 1815-1841.

Gilliam, H., 2002: Weather of the San Francisco Bay Region. University of California Press, 106 pp.

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