Dynamic and thermodynamic relations of distinctive stratus clouds on the lee side of the tibetan plateau in the cold season

Abstract

Given the large discrepancies that exist in climate models for shortwave cloud forcing over eastern China (EC), the dynamic (vertical motion and horizontal circulation) and thermodynamic (stability) relations of stratus clouds and the associated cloud radiative forcing in the cold season are examined. Unlike the stratus clouds over the southeastern Pacific Ocean (as a representative of marine boundary stratus), where thermodynamic forcing plays a primary role, the stratus clouds over EC are affected by both dynamic and thermodynamic factors. The Tibetan Plateau (TP)-forced low-level large-scale lifting and high stability overEC favor the accumulation of abundant saturated moist air, which contributes to the formation of stratus clouds. The TP slows down thewesterly overflow through a frictional effect, resulting inmidlevel divergence, and forces the low-level surrounding flows, resulting in convergence. Both midlevel divergence and low-level convergence sustain a rising motion and vertical water vapor transport over EC. The surface cold air is advected from the Siberian high by the surrounding northerly flow, causing low-level cooling. The cooling effect is enhanced by the blocking of theYunGui Plateau. The southwesterlywind carryingwarm, moist air fromthe eastBay ofBengal is uplifted by the HengDuan Mountains via topographical forcing; the midtropospheric westerly flow further advects thewarmair downstream of the TP, moistening andwarming themiddle troposphere on the lee side of the TP. The low-level cooling and midlevel warming together increase the stability. The favorable dynamic and thermodynamic large-scale environment allows for the formation of stratus clouds overECduring the cold season. © 2013 American Meteorological Society.