Mesoscale Mechanisms, Radiative Impact, and the Influence of Topography on the Cloud-Clearing Phenomenon Over the Subtropical Southeast Atlantic

Horning, Zachary Joseph

Publication

Mesoscale Mechanisms, Radiative Impact, and the Influence of Topography on the Cloud-Clearing Phenomenon Over the Subtropical Southeast Atlantic

Horning, Zachary Joseph

Abstract

The abrupt clearing of large portions of marine stratocumulus clouds have been documented along the Namibian coastline over the subtropical southeast Atlantic (SEA). During these events, stratocumulus clouds are rapidly eroded thousands of kilometers offshore. Since stratocumulus clouds play an important role in the climate system, any event that acts to disrupt these clouds may be climatologically significant. Previous studies documented the influence of gravity waves, enhanced shear, and strong offshore flow as possible mechanisms associated with rapid dissipation of low clouds, but the mechanisms underlying the rapid cloud-clearing events over the Southeast Atlantic are not clear. Weeklong high-resolution simulations using the Advanced Regional Weather Research and Forecasting (ARW/WRF) model were conducted across periods with observed cloud-clearing events (active periods) and periods without observed cloud-clearing events (null periods). Simulations explore the feasibility of previously identified cloud-clearing mechanisms, radiative impact of the events, and the influence of topography on the regional circulation and cloud fields. Relative to the null-period simulations, simulations of the active periods exhibit stronger offshore flow, which appears to westwardly advect a sharp gradient in marine boundary-layer (MBL) height; the MBL east of this sharp gradient becomes too shallow to support cloud. The movement of this gradient westward leads to rapid cloud erosion in the simulations. The climatological impact of clearing events was found to be statistically significant at the 95% confidence interval, with shortwave cloud radiative forcing values in the null periods averaging roughly -63 W m−2 and roughly -42 W m−2 during the active periods. Furthermore, the steep topography was found to hinder clearing events; decreasing the topography was found to increase the extent of clearing and increasing the topography was found to decrease the extent of clearing. This is due to the coastal topography in the SEA acting to shelter the MBL from strong offshore flow. Topographic sensitivity simulations further demonstrated that this stronger than normal offshore flow is the primary mechanism responsible for cloud- clearing in the southeast Atlantic. This is due to a strong correlation in MBL height and u-wind speed. As the topography is decreased, this allows for less sheltering of the MBL from the strong offshore flow; u-wind increases significantly and therefore so does the extent of clearing. As the topography is increased, there is more sheltering of the MBL from the offshore flow; u-wind decreases and therefore so does the extent of clearing.