863 The physics behind Q1 and Q2

Thursday, 27 January 2011
Washington State Convention Center
William M. Gray, Colorado State Univ., Fort Collins, CO

Yanai et al.'s (1973) famous paper discusses the fundamental need for very large upward or sub-grid scale eddie flux of heat and moisture in precipitating tropical disturbances and wave troughs. This observation requires that there be large up-and-down mass recycling at cumulus cloud base (~ 950 mb) which is an order of magnitude or more larger than the mean broad-scale disturbance upward vertical motion at this level. Yanai, et al.'s observations have been well verified by the author (Gray, 1973) and by others. The physical understanding of this phenomenon requires that one envisage strong and frequent and organized evaporation driven downdraft action within the tropical disturbance and wave trough.

Cumulus cloud models well verify the need for large mechanical cloud-base mass forcing for deep cumulus development (Lopez, 1973). The magnitude of this required large upward mass forcing (2-5 m/s) under individual developing deep cumulus is an order of magnitude or greater than the low level broader scale frictional (or CISK) convergence. This large cumulus scale upward mass forcing can only be explained by the existence of many mass producing downdrafts into the boundary layer. The deep level of mass convergence (up to 300 mb) into the typical tropical disturbance or cloud cluster well supports the existence of such evaporation driven downdraft action.

The wider the cloud-base and the stronger the cloud base upward mass forcing the less the inhibiting updraft entrainment and the deeper and more intense the convection unit can penetrate into the upper troposphere. The spread out of the downdrafts cooler and drier air within the disturbance's boundary layer induces enhanced evaporating ocean energy flux which is fundamental to the tropical disturbance's later ability to transform itself into a tropical cyclone.

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