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Marine Stratus Cloud Climatology

Product Information:


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The Cooperative Institute for Research in the Atmosphere (CIRA) in Fort Collins, Colorado, is developing the Marine Stratus Cloud Climatologies together with the Eureka, CA National Weather Service (NWS) Office.

The Eureka, CA NWS office is receiving this product from CIRA through an ftp site.

The size of one netCDF file, centered over Eureka, CA is 1.4 MB. This is a static database, so updates are not required.

Product Description:


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A major forecast challenge for the Eureka NWS office during the summer is the marine stratus and fog layer that forms along the California coast. The formation and dissipation of this layer affects ceiling and visibility forecasts for local airports as well as visibility and temperature forecasts for the general public. This product will be used to help forecast the duration and burn off rate of the marine stratus deck based on the thickness of this layer at 1200 UTC.

The terrain of the northern California coast is complex with sharp changes in elevation. The higher temporal and spatial resolution as well as the larger number of bands will offer improved Marine Stratus Cloud Climatology products during the GOES-R era.

For this project, images for channels 1(visible), 2 (3.9 µm) and 4 (10.7 µm) from GOES West at 4 km resolution were collected every hour for May through July, 1999-2009 covering the western U.S., and sectorized to cover most of northern California, including the County Warning Area (CWA) for the Eureka office. Then the images were grouped by calendar month and hour for further processing.

The algorithm used is based off the Jedlovec and Laws (2003) method. This method was chosen due to its ability to detect low clouds and fog throughout a twenty-four hour period. The 3.9 µm and 10.7 µm satellite images are matched according to time and subtracted at each pixel location. Once completed for all image pairs, the smallest negative difference and the smallest positive difference is determined for the period and hour to form two composite images. In addition, the warmest 10.7 µm temperature value for each pixel location in the set is also determined. These composite images were used to perform tests on the images within the period to determine if each pixel is cloudy or clear. During daylight hours, an additional check is performed using the matching visible image. Once these tests have been completed for a given image pair, a cloud/no cloud image is produced. The same procedure is performed on the rest of the image pairs.

The cloud/no cloud images at 1200 UTC were then sent to the Eureka office to determine the marine layer depth (MLD) for each day. The 1200 UTC was chosen because this is when the cloud layer is typically most stable, does not include the day/night terminator, and should represent the furthest extent of the marine stratus inland for that day. The Geographic Information Systems (GIS) platform ArcGIS was used to analyze the satellite imagery superimposed on topography. Specifically, the elevation of topography under the outermost pixels on the eastern edge of the stratus layer was extracted and used to calculate the MLD. Then, an average was taken from all the elevations along the cloud/no cloud boundary, and this average was designated the MLD for that particular day. The dataset was then stratified according to the MLD assigned for each day (see Table 1). Each MLD level is called a regime. The data was then further divided into four time periods: May15-June15, June16-July15, July16-August15, and August 16-September 15.

The MLD information for each day was then sent to CIRA, where it was used to stratify the data to form cloud composites for each regime at each hour for each time period. The final results are cloud composites for each hour of the day, in sets according to the MLD at 1200 UTC.

Product Examples and Interpretation


Here are some examples of the Marine stratus climatology. Figure 1 shows a map of the Eureka area.

Fig 1: Map of Eureka CWA

Fig 1: Map of Eureka CWA

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Fig 2: Cloud percent climatology for Jul 16-Aug 15, 1999 to 2009 for 1200 UTC, when MLD is 751-1250 ft at 12 UTC. 121 cases

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Fig 3: Cloud percent climatology for Jul 16-Aug 15, 1999 to 2009 for 1800 UTC, when MLD is 751-1250 ft at 12 UTC. 119 cases

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Fig 4: Cloud percent climatology for Jul 16-Aug 15, 1999 to 2009 for 0000 UTC, when MLD is 751-1250 ft at 12 UTC. 118 cases

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Fig 5: Cloud percent climatology for Jul 16-Aug 15, 1999 to 2009 for 0600 UTC, when MLD is 751-1250 ft at 12 UTC. 120 cases

 

Figure 2 is the climatology for 1200 UTC (5 am local) when the MLD is between 751-1250 feet thick (regime 3) in the Eureka area during the July 16 to August 15 period. This is also the time the MLD is measured. The cloud percent is in the 90% range over Eureka, with a tongue of higher percentage cloud cover penetrating inland southeast of the station, along the Eel river valley. There is also a large amount of fog/marine stratus that has penetrated inland around the San Francisco and Monterey areas to the south.

Figure 4 is the climatology for 0000 UTC (5 pm local), an evening time when the marine stratus should be mostly burned off. A strong gradient between high and low cloud percentage runs very close to the coastline. Figures 3 and 5 are for 1800 and 0600 UTC, showing stages between these two extremes.

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Fig 6: Cloud percent climatology for Jul 16-Aug 15, 1999 to 2009 for 1200 UTC, when MLD is 1751-2250 ft at 12 UTC. 14 cases

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Fig 7: Cloud percent climatology for Jul 16-Aug 15, 1999 to 2009 for 1800 UTC, when MLD is 1751-2250 ft at 12 UTC. 14 cases

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Fig 8: Cloud percent climatology for Jul 16-Aug 15, 1999 to 2009 for 0000 UTC, when MLD is 1751-2250 ft at 12 UTC. 14 cases

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Fig 9: Cloud percent climatology for Jul 16-Aug 15, 1999 to 2009 for 0600 UTC, when MLD is 1751-2250 ft at 12 UTC. 14 cases

Figure 6 is the climatology for the same time and period as Figure 2 (1200 UTC), except that the MLD is between 1751-2250 feet (regime 5). This composite shows that the greatest cloud percents have penetrated much further inland, not only down the Eel river valley but also into the Klamath river valley to the north. The time series for Figures 7-9 also shows the deeper penetration of the marine stratus compared to Figures 3-5.

It should also be noted that this regime only had 14 cases during the 11 years of the July 16-August 15 period, while regime 3 is more common with 119 cases. This affects the smoothness of the composites. During the other periods, regime 5 does not have enough cases to produce a viable composite. Regime 8 combines regimes 5-7 in these periods, in order to boost these numbers to produce useable products.

By knowing the depth of the marine stratus layer at 12 UTC, a Eureka forecaster will be able to chose a regime that best matches the situation, using the time series as a starting point for the marine stratus burnoff forecast during that day.

 

Advantages and Limitations


The advantage of this product is that it provides the forecaster with a visual series of time steps of how the marine stratus layer could burn off during the day under a given set of conditions (MLD). It also aids a new forecaster in understanding a common weather phenomenon of the area.

The algorithm used for this product does have problems around the day/night terminator. Thus, composites around the time of sunrise and sunset should be viewed with caution. In addition, some of the regimes do not have enough cases to be representative of conditions.

Table 1

Regime 1 MLD < 100 ft
Regime 2 MLD 101-750 ft
Regime 3 MLD 751-1250 ft
Regime 4 MLD 1251-1750 ft
Regime 5 MLD 1751-2250 ft
Regime 6 MLD 2251-2750 ft
Regime 7 MLD > 2751 ft
Regime 8 MLD > 1751 ft (regimes 5, 6 and 7)

Jedlovec, G.J., and K. Laws: GOES cloud detection at the global hydrology and climate center. Preprints CD, 12th Conf. On Satellite Meteorology and Oceanography, Long Beach, CA, Feb 2003, Amer. Meteor. Soc., P1.21.