Modeled onshore and offshore California coastal-cooling: General Circulation and mesoscale effects

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Thursday, 27 January 2011: 11:45 AM
Modeled onshore and offshore California coastal-cooling: General Circulation and mesoscale effects
612 (Washington State Convention Center)
B. Lebassi, College of New York, New York, NY; and J. E. Gonzalez and R. Bornstein

Investigations with the RAMS mesoscale model with a horizontal grid resolution of 4 km on an innermost grid over the LA Basin were undertaken to investigate the joint effect of long-term changes of GHG-concentrations and land-use land-cover changes on coastal temperatures and flows. Comparisons of present (2001-5) and past (1996-1970) summer (JJA) climate conditions showed significant increases in sea breeze flows and coastal cooling, which supports the hypothesis that the observed “coastal cooling” is a possible direct “reverse reaction” result of the GHG warming.

General circulation and synoptic scale forcing are the main external drivers of the RAMS model, through its initialization and BC processes. During summer, the semi-permanent Pacific High pressure system is located between Alaska and Hawaii, while a thermal low is located over southeastern California. The resulting horizontal pressure gradient creates an onshore sea breeze flow from the high to the low. The LA Basin sea breeze peaks in summer, when SSTs are low along the northern California coast in the southward flowing coastal California ocean current, in which the temperature of upwelled water reaching the surface varies from about 287 to 291 K.

Results of the changes over the past 35 years showed that the large scale Pacific High was generally unchanged in position and orientation, but it did show a diminished central pressure. Simulated SSTs generally warmed (but at rate less than at inland sites), at a rate that increased to the south, while JJA average 1700 LT NCEP temperatures at 1000 hPa showed a cooling off the central California coast that extended upwards to about 925 hPa. The increased sea breeze created a separate on-shore “coastal cooling,” which was equal in magnitude and location with that found in a separate observational study. Thus the observed “coastal cooling” reverse-reaction was separate from, and not due to, the imposed NCEP large-scale input to the RAMS simulations, which was located north of the mesoscale cooling region.