Stratus-impacts on observed long-term cooling-trends of summer max-temperatures in coastal California

Past research in this project has shown that during the past 35 years, summer (June-August, JJA) observed high temperatures in the Central Valley have increased, while those at coastal San Francisco Bay Area (SFBA) sites have cooled, producing a “reverse-reaction” to GHG-induced global warming. The effect is associated with: (a) stronger Central Valley to SFBA temperature gradients, (b) increased marine layer influences (i.e., cold air advection) from the Pacific Ocean; and (c) thicker and longer lasting stratus in SFBA. The purpose of this new effort was to evaluate: (a) how longer lasting stratus decks (i.e., delayed burn-off time) affects the magnitude and timing of daily high temperatures and (b) the relationship between surface temperature and inversion height prior to, and following, stratus burn-off.

Data were received from the Marine Stratus Initiative (MSI) project, in operation from 1995 through the present. These data were made available by MIT Lincoln Lab-oratory via the Monterey office of the NWS. The following are located at San Francisco International Airport (SFO): Pyranometer for surface radiation at 5 min intervals; SODAR for inversion height at 15 min intervals, ASOS for surface temperature at 15 min intervals; and MIT clear-sky surface radiation, calculated from an approximation to Beer's Law at 5-min intervals. The above data were analyzed for 1 June to 30 September, between 12 and 24 UTC (5 AM to 5 PM LST), as this is the SFO stratus season.

Results show a second-order inverse relationship between daily max-temperature and inversion burn off time, i.e., later burn-off results in lower max-temperature. These results will be compared to the observed coastal-cooling rates to deduce what fraction of the cooling could be due to increased stratus duration.