Fog patterns along the coast of Monterey Bay



Fog patterns along the coast of Monterey Bay

Christy G. Cowan

September 3, 2005

Department of Meteorology, Naval Postgraduate School, Monterey, California

1. Introduction and Background

The National Weather Service Climate Station in Monterey, in their summary of July 2005 weather on the Monterey Peninsula, said: “A look at the remarks section of the observation page along with the X’s in the fog column highlight the feature of the month: fog and lots of it! 28 fog days were counted at the NWSCS. That is, 28 days with one or more hours of horizontal visibility limited to six miles or less. The July 2005 fog days featured fog for many, many hours/day, with visibilities less than three miles (and often a quarter mile or less) on more than 60% of the fog days. The number of fog days is not a July record. There were 31 fog days in July 1996. Although local records on extent and duration, coupled with intensity, do not exist, this observer rates July 2005 as one of the foggiest—duration, extent, intensity combined —in recent memory. The Marina observer reported sixteen days with dense fog (< ¼ mile visibility) – a near record for some 20 years of July observing.”

Summers are notoriously foggy along Monterey Bay, but this July was particularly foggy. Additionally, there were also persistent patterns evident in the fog/low cloud. A casual observer could easily notice that an area running from Del Monte Beach to Sand City and inland beyond the airport, was almost consistently clear while surrounding areas were ‘socked-in.’ Figure 1 shows a clears example of this pattern.

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Figure 1- MODIS image of Monterey Bay 23 July 2005 1335PST

So, it became the goals of this project to 1) determine if this fog pattern can be linked to wind or temperature patterns found in the observations and 2) establish if the causes of the fog pattern can be resolved by 3km COAMPS.

2. Data and Methods

The first step taken in this research was to choose a particular date and time-period to study. In order to do this, MODIS imagery from the Naval Research Laboratory (NRL), Monterey, CA was used. These images are available once or twice daily. Fortunately, the time of the satellite pass was around 2100GMT throughout 18-23 July which fit well with a time of interest around 1400PST. The relevant images were downloaded from the NRL website.

Additionally, images from cameras set up by Professor Ed Thornton and maintained by Mark Orzech, both of the Naval Postgraduate School (NPS), were viewed. Cameras are present in four locations along the coast- Del Monte Beach (1 camera), Sand City (2 cameras), Fort Ord (2 cameras), and Marina (5 cameras). Pictures are taken at varying intervals anywhere from 2 to 30 minutes during daylight hours. These images are stored on a drive in the NPS UNIX system and were easily downloaded. Due to good availability of MODIS and camera images to verify that the subject pattern was present, the afternoon of 23 July was chosen as the period of interest for this study. An overview of the camera locations and the camera images for the period of interested are presented in Appendix A. From the #1 Sand City camera (looking south), it is obvious that Del Monte Beach is clear and sunny. But looking north from the Del Monte Beach and the #2 Sand City cameras, one can see that the fog has rolled into just to the north. And the cameras at Fort Ord and in Marina are clearly in the fog.

Next, observational data was collected. This data is available in real-time from the weather.nps.nav.mil website. A dataset containing the observations for the period of interest was prepared by Richard Lynd of the NPS Meteorology Department. The stations considered included the M0, M1 and M2 buoys, Monterey Bay Aquarium (MBA), Del Monte Beach (DMB), Monterey Airport (MRY), R30, R07, Fort Ord (ORD), and Salinas Airport (SNS). Figure 2 shows the distribution of these stations.

In the end, R07 and Monterey Airport were chosen as representative stations in the clear and Fort Ord was chosen as a representative station in the fog. Graphs of the wind and temperature at these locations from 1100-1500PST on 23 July are available in Appendix B.

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Figure 2- Observation Locations

Next, GARP was used to extract 3km COAMPS analysis/forecast data for the period of interest. Three potential conditions behind the fog patterns were considered- 1) a slope in the boundary layer, 2) small scale subsidence, and 3) a split in the flow. So the following fields were gathered for the times 05, 08, 11 and 1400PST on 23 July: 10m Temperature, 10m Winds, Vertical Velocity at 1000mb, and a Vertical Cross Section of Potential Temperature running along a line from the SE portion of the Bay inland, beyond the airport. These output images are available in Appendix C.

3. Results

The Del Monte Beach observations show no sustained seabreeze-related increase in wind speed in the afternoon. The direction of the wind varied from Northwest (from over water) to North-Northeast (from over land). The temperature increased steadily throughout the afternoon. A similar pattern is seen at the Monterey Airport. However, the wind veered to the North-Northeast or North by 1300PST and remained there. This would have brought in a steady flow from over land. The Fort Ord observations are very different. The wind increased from 1kt to 11kntos over the period as the seabreeze became stronger. The wind direction, although variable, was always generally from the west bringing in a flow from over water. And the temperatures, decreased throughout the afternoon in fog.

The most significant findings from the observations are 1) the fact that there does not appear to be much of a seabreeze in the ‘clear pocket’ and 2) that the flow in this areas appears to be (at least intermittently) from over land. Although these two conditions go some distance in explaining the clear area, it has yet to be determined what causes the conditions themselves. 3km COAMPS was applied to this problem.

The COAMPS temperature contours show no significant along-coast gradient. In fact, they do not at all capture the significant differences between the ‘clear pocket’ (rising temperatures) and Fort Ord (falling temperatures) that are seen in the observations. COAMPS wind data does not show the significant increase in wind speed at Fort Ord and lack of such in the ‘clear pocket.’ Additionally, COAMPS does not display a split in the flow as proposed. The vertical velocity contours show no significant along-coast differences and more importantly, show no areas of subsidence to account for the clear area. Lastly, the cross-section of potential temperature appears practically flat throughout the period, ruling out a slope in the boundary layer as a possible explanation.

4. Discussion

The San Francisco and Monterey Bay areas are notorious for foggy and cloudy summers. The development of this fog and low cloud relies upon the inversion that is created during the summer months by the cool upwelled water off the coast underlying the subsidence from the Pacific High. The patterns of the fog and cloud are highly dependent upon the complex terrain features including bays and coastal mountains (Combs, Blier, Stratch, & DeMaria, pg 1). In the southern Monterey Bay area in particular, it is worth noting that, although it is not clearly evident on a topographic map (such as Figure 2), the small ‘hill’ in the center of Monterey upon which the Presidio sits, may provide some topographic blocking for the clear area.

In this study, the observations and 3km model data were considered. While the observations did provide some insight into the wind patterns that might be responsible for the clear area, the model data did not. It is very likely that this small ‘microclimate’ is too small to be resolved by a 3km model. However, the fact that the model did not even capture the significant wind and temperature differences between the airport and Fort Ord was very surprising.

A clear pocket of this small size could easily seem insignificant. However, it could have significant impacts on local economics, such as tourism and real estate prices, but even more importantly on airport operations. It would seem to be no surprise that the Monterey airport was built in the place that it was. It is somewhat disappointing for tourists, however, who may exit the airport under clear skies only to find the areas they want to visit are shrouded in fog and/or cloud.

5. Conclusion and Future Work

One of the original objectives of this research was to determine if the persistent fog and low cloud pattern observed during the period of interest was real or only perceived. After consulting both satellite and beachfront camera imagery, it became obvious that it was indeed real. At that point, ‘why’ became the question. With only surface observations available for comparison, it was acknowledged that their value would be limited. However, there was wind pattern evident that could go some way towards explaining the clear area. In the foggy areas, there was a steady wind from the west (bringing in the clouds from over the Bay). But in the sunny areas, as the afternoon progressed, the wind shifted to a North or North-northeast direction which brought in a flow from over clear land.

These observations were interesting, but did not go far towards explaining the meteorological conditions that lead to this pattern. For this purpose, 3km COAMPS model data was consulted. Several potential meteorological conditions were considered – a slope in the boundary layer, small scale subsidence in the clear area, or split flow. Unfortunately, none of these conditions were apparent in the COAMPS data. It was concluded that if these, or other meteorological conditions are present and cause this clear pocket, they are too small to be resolved by a 3km model

It would seem that future work on this topic using the 3km model data would be fruitless. Perhaps a more comprehensive comparison of the surface observations (or better yet, rawindsonde data) coupled with a detailed study of the small-scale topographic effects would go some way towards explaining the wind pattern and resulting fog/low cloud pattern.

References:

(1) Renard, R.J., Summary of the July 2005 Weather on the Monterey Peninsula, National Weather Service Climate Station, Monterey, CA.

(2) Combs, C. L., Blier, W., Stratch, W., DeMaria, M., Exploring the Timing of Fog Formation and Dissipation over San Francisco Bay Area using Satellite Cloud Composites. (From internet, no publishing information available).

Acknowledgements

Richard Lynd for assistance in obtaining surface observations

Mark Orzech for assistance in obtaining beach camera images

Professor Wash for assistance in obtaining NRL MODIS imagery

Bob Creasey for making 3km COAMPS model data available

Professor Nuss for making 3km COAMPS model data available and for advice

Professor Rosenfeld and Davidson for general guidance

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Appendix A

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Appendix A

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Appendix B

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Appendix B

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Appendix B

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Appendix C

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Appendix C

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Appendix C

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Appendix C

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Appendix C

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Appendix C

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Appendix C

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Appendix C

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