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Climate Convection feedbacks: some further issues.

The editorial essay by Shaw et al. (1998) discusses the possible role of deep convective transports of sulfate particles in regulating climate. They suggest that a 30% increase in the biogenic sulfate aerosol, cycled through enhanced deep convection in a warmer climate, upon sinking back into the convective boundary layer (BL), could increase the albedo of BL clouds by 1.6% by providing additional cloud condensation nuclei and increasing the droplet number concentration (Charlson et al., 1987). Given a 30% BL cloud fraction, this would increase the global albedo by 0.5%, and provide a significant negative feedback on climate change. There are many links in the convective components of this argument, where we need a better understanding of the processes and the climatic equilibrium involved (Betts, 1990). I will discuss some of them. The first is whether there will be more transport of air (carrying biogenic sulfur) out of the BL in a warmer climate. This is not proven, and indeed the reverse could be true. While it is true that deep convection is more intense when fueled by a higher BL equivalent potential temperature (as evidenced by lightning: Williams, 1994), this depends equally on the cooler overlying vertical atmospheric structure, which provides the convective available potential energy (CAPE). In a warmer climate, where the entire tropical tropospheric temperature will be warmer (with an equilibrium structure which is only slightly unstable to a warmer moist adiabat), there is no certainty that the mean CAPE will increase much. Certainly any increase in mean CAPE is a distinct issue from the spatial variation in mean CAPE, which we see in our present climate, associated with spatial variations in sea surface temperature and in the land surface boundary. Furthermore, the global vertical transport of BL air with biogenic sulfur depends on a global vertical mass transport out of the BL by deep convection, not on the intensity of individual convective systems. The upward mass transport depends on the frequency of deep convection as well as the intensity of typical system. For example fewer more intense convective systems or a greater number of less intense ones could carry the same mass transport. Do we know whether the net vertical mass transport by convection would increase in a warmer climate? One study of the climatic equilibrium of the tropics (Betts and Ridgway, 1989) in fact says the net mass circulation in the tropics will decrease in a climate with a warmer sea surface temperature (SST).

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Betts, A. K., 1998: Climate-Convection feedbacks: some further issues. Climatic Change. 39, 35-38. DOI 10.1023/A:1005323805826.