Pressure forcing: synoptic analyses

In the table below are ETA model 850 mb (about the height of the MABL top) potential height gradients. Gradient of potential height ~ pressure gradient. The synoptic analyses are from Wendell Nuss at NPL.

Point Conception	Point Sur	Cape Mendicino	Cape Blanco
June 19	June 02	June 07 Dynamic height Pressure gradients	July 01
June 23	June 11	June 12 Dynamic height Pressure gradients
June 29	June 17	June 26 Dynamic height Pressure gradients
	June 21

On 6/07, 6/12 and 6/26 the pressure gradient has been estimated. 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.

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 at Scripps Insitution of Oceanography.

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