Extended Abstract (316K)8.6
Impact of temporal SST variations on a stratus-capped marine boundary layer
Tracy Haack, NRL, Monterey, CA; and S. D. Burk
Global-scale coupled air-sea interaction processes that affect the climate have been well documented, but many subtle mesoscale coupled air-sea processes remain to be explored. In this study, prior to addressing the fully coupled air-sea problem, we investigate the impact of short-term SST variations on a modeled stratus-capped marine boundary layer (MABL). The SST time history is taken directly from measurements at a buoy (M1) in Monterey Bay, CA during a two-week period beginning 16 May 1999. The initial vertical profiles of wind, temperature, specific humidity, and cloud water content are taken from an intercomparison study of a stratus-topped MABL in which large eddy simulation (LES), cloud resolving, and 1D models participated (Moeng et al 1966; Bechtold et al 1966). We use the 1D version of the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPSTM) in this study. Prior to examining the impact of the temporal SST variation, we test COAMPSTM against this intercomparison study case and demonstrate that the model forecasts the mean and turbulent profiles quite well as compared with the LES results; thus, we establish a benchmark (Case B) having fixed SST and weak upward surface buoyancy flux.
Sensitivity tests subsequently are performed using the observed temporally varying M1 SSTs that increase from 9°C to 12.4°C during the two-week simulation. The bulk of the increase (~3°C) occurs over a 72-hour period near the middle of the period. In the first sensitivity simulation (Case S), the initial SST is sufficiently cooler than the air temperature that the surface layer remains stable, or near neutral, throughout the forecast. By adding a constant increment to the M1 SST time series data, we produce cases having differing surface stability, and are able to investigate the impact of both diurnal radiative forcing and time varying SST upon the marine cloud properties and variability. Thus, increasing the M1 values in the mean by a 3°C increment produces a stable to unstable transition (Case T) during the forecast; an additional 2°C increase produces a case with unstable surface forcing throughout (Case U). Each case shows varying levels of diurnal behavior due to cloud/radiation interactions; these diurnal changes in the MABL tend to be most prominent prior to the large ramp up in SST. After the SST jump, elevated surface fluxes and larger turbulent mixing cause the state variables to display more unsteady behavior, as well as dampen the diurnal signal in the integrated cloud liquid water (ICLW) amount. The stable case (S) produces the most pronounced diurnal signal in ICLW and the greatest cloud layer impact upon crossing the SST jump: peak mixing ratio of cloud liquid water decreases by 25%. In the unstable case (U), the cloud structure shows minimal change across the SST jump with peak cloud water mixing ratio diminishing by only 11%. Although these preliminary results are illuminating, much work remains.
Session 8, Clouds and the Marine Boundary Layer
Friday, 8 August 2003, 9:30 AM-12:10 PM
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