HR: 0830h
AN: A21E-1016    [PDF]
TI: Orographic Influences on the Annual Cycle of Namibian Stratocumulus Clouds
AU: * Richter, I
EM: richter@atmos.ucla.edu
AF: University of California, Los Angeles, Department of Atmospheric Sciences, 7127 Math Sciences Bldg., Los Angeles, CA 90095-1565 United States
AU: Mechoso, C R
EM: mechoso@atmos.ucla.edu
AF: University of California, Los Angeles, Department of Atmospheric Sciences, 7127 Math Sciences Bldg., Los Angeles, CA 90095-1565 United States
AU: Farrara, J D
EM: farrara@atmos.ucla.edu
AF: University of California, Los Angeles, Department of Atmospheric Sciences, 7127 Math Sciences Bldg., Los Angeles, CA 90095-1565 United States
AB: The impact of African orography on the annual cycle of stratocumulus incidence off the coast of Namibia is examined. To this end, we perform two experiments with the UCLA atmospheric general circulation model (AGCM). Since the UCLA AGCM produces a very realistic annual cycle and geographical distribution of marine stratocumulus clouds, it is well suited for this task. In one experiment, No-Orography, orographic surface heights are set to sea level over the African continent, while the other experiment, Control, features realistic orography everywhere. After the initial adjustment, Control and No-Orography are run for 20 and 3 years, respectively, and their climatological monthly means are examined. Compared to No-Orography, Control shows a significant increase in stratocumulus cloud incidence over the Namibian stratus region (defined here as the area 0$^{\circ}$E to 10$^{\circ}$E, 20$^{\circ}$S to 10$^{\circ}$S). Differences elsewhere are found to be small. Computing the area average of stratocumulus incidence over the Namibian stratus region, we find that the difference between Control and No-Orography is significant from July through November, with the maximum occurring in August, where the incidence in Control is about 40% higher than in No-Orography. Analysis of the bulk static stability (defined here as the potential temperature at 700 hPa minus the potential temperature at 1000 hPa) for the same region reveals that an increased static stability in Control occurs during the same months as the increase in stratocumulus incidence. This is consistent with the notion that static stability is favorable to the maintenance of stratocumulus clouds. The difference in static stability is mainly due to an increase in the potential temperature at 700 hPa in Control (since sea surface temperatures are prescribed in the model, the potential temperature at 1000 hPa can only vary to a small extent between the two experiments). An analysis of the terms in the thermodynamic energy equation for the region shows that horizontal advection is the dominant factor contributing to the greater heating at 700 hPa in Control. Comparing the 700 hPa wind and temperature fields between the two experiments, it is evident that orography acts to break up zonal symmetry over the southern part of the African continent and to the west of it. In accordance with linear barotropic theory, anticyclonic circulation is found over southwest Africa. This anticyclonic circulation advects warm air poleward toward the Namibian stratocumulus region. The same result is obtained from an analysis of the stationary eddies. It is concluded that African orography interacts with the mean flow to generate an increased bulk static stability off the Namibian coast, which is conducive to the maintenance of stratocumulus clouds.
DE: 3307 Boundary layer processes
DE: 3309 Climatology (1620)
DE: 3319 General circulation
DE: 3337 Numerical modeling and data assimilation
DE: 3384 Waves and tides
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting