Extended Abstract (276K)13B.1
Multi-balanced modes of deep tropical and extratropical convection self organization
Lan Yi, National University of Singapore, Singapore; and H. Lim
Organized convection such as squall lines occurs frequently with strong low-level wind shears. In midlatitudes the stronger the wind shear, the more severe the storm is expected, but this may not be the case in deep tropics. Results from three-dimensional idealized simulations using the US navy's COAMPS™ model show that under three different types of environmental temperature profiles that approximate deep tropical, temperate, and cold atmospheres that have the same amount of convective available potential energy, the balance between low-level wind shear and cold pool, which sustains the convection self organization and development, is realized at three concentrated modes - the warm tropical mode, the temperate mode, and the cold mode. Each mode is characterized by a particular set of temperature lapse rate, low-level wind shear, and humidity content that favors maximum convection development. In the warm tropical mode, wind shear is delicately balanced by weak cold pools and becomes insignificant to convection; the strength of the convection development is negatively correlated with the magnitude of the low-level unidirectional wind shear due to weak atmospheric thermal stratification and low level of free convection induced by abundant boundary layer moisture in the tropical atmosphere. Moreover, the variation of the mid tropospheric (850-400 hPa) moisture in the warm tropical mode exhibits greater sensitivity to convection growth than it does in the temperate and cold modes, with higher midlevel moisture promoting much stronger and longer-lived storms. A diagnosis of cloud thermal forcing in these simulations indicates that the atmospheric thermal stratification (lapse rate) basically decides the vigor and power of the linear convection organization; whereas low-level wind shear and midlevel moisture associated with a certain lapse rate or a certain convective regime act to modify the convection self organization through interaction with cold pool and entrainment of dry surrounding air.
Session 13B, Convection, waves, and precipitation VI
Thursday, 6 May 2004, 10:15 AM-11:45 AM, Napoleon I Room
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