Upstream conditions at Cape Mendicino

On June 7, 12, and 26, the C130 aircraft recorded meterological data in the Cape Mendicino region. Along a line perpendicular to the Cape, the aircraft flew vertical profiles of the marine atmospheric boundary layer, descending from about 2000 m to 30 m above the sea surface. The data from each profile along this line was used to estimate the layer's depth, and to determine the average wind speed of the layer.

Because the marine atmospheric boundary layer looks only approximately like a two layer fluid, its "depth" is defined as midway between the bottom and top of the inversion layer.

Layer averages

Along the northern June 7 survey line, the layer depth and average speed show a sloping MABL and moderate winds. This estimate was obtained by combining height from the lidar and profiles, and speed from the profiles, SSMI satellite, and 30 m winds. The SSMI satellite winds and 30 m winds were fudged to best match the profile layer averaged speeds. A second degree polynomial was fit to a smooth version of the composited observations to get the estimate.
Along the southern June 7 survey line, the layer depth and average speed show that speeds were high, especially close to the Cape, and layer depth was moderate.
Along the June 12 survey line, the layer depth and average speed show that speeds increased offshore, with a deeper layer.
Along the June 26 survey line, the layer depth and average speed show low speeds and a very deep layer.

Pressure forcing of the layer

850 mb height maps

To find the pressure forcing experienced by the marine atmospheric boundary layer on a given day, the output of the National Weather Service's "Eta" model was used. A good explaination of upper air maps is found here. The horizontal gradient of the 850 mb geopotential height field can be converted to the pressure gradient field through the barometeric formula (found in Saucier, 1955 p. 47):

dP = - (g*p/R)*dZ/T_v

where dP = pressure gradient,
g = average global value of g at sea level,
p = 850 mb,
R = gas constant for dry air,
dZ = geopotential height gradient. Parcels on a geopotential level have the same potential energy. Due to local changes in "g", the geopotential deviates from elevation above sea level.
T_v = virtual temperature, which accounts for the warming of air due to water vapor.

To obtain the synoptic pressure forcing of the atmopheric boundary layer, the pressure at a level above the layer must be used or else it will include pressure forcing due to dynamic changes in the layer's depth. The 850 mb pressure surface is about 1500 m high over the water andis an appropriate level to consider synoptic systems on since surface effects are minimal. However, additional pressure signals could be present between 1500 m and the layer's top.

Using the aircraft data to obtain a large-scale pressure gradient proved problematic, since the aircraft flies along a pressure surface rather than at a single altitude when it is above about 600 m. Also the presence of gravity waves and a strong diurnal signal made it difficult to get a believable pressure gradient.

The Eta output used here to derive pressure forcing is from its "analysis" stage, which compiles all measurements that report to the World Meteorological Organization. The data used here is not a forecast, but rather a blending of data in a sensible way to give the 850 mb field. Upper air data from raob balloon soundings go into the 850 mb data. These stations are pretty widely spaced, and there are almost none over the water. The location of raob release sites in California can be seen here. However the WMO upper air station network represents the best picture available of what is going on above the surface.

The Eta model output, which covers the continetal US, is available in 2 resolutions, the 80 km and the 48 km resolutions. Although the 80 km output is more appropriate for calculating a large-scale atmospheric forcing, the 48 km output had to be used on 6/13 since the 80 km file was flawed.

Pressure gradients derived from 850 mb geopotential height gradient

The data on this page is unpublished. If it is used please cite the author Kathleen Edwards, the Center for Coastal Studies, and the Coastal Waves group (David Rogers and Clive Dorman, PI's) at the Scripps Insitution of Oceanography.

Please send comments or questions to me at kate@coast.ucsd.edu