How variable is deep convective depth?

Monday, 15 December 2014: 5:45 PM
Steven C Sherwood1, Nidhi Nishant2, Daniel Hernandez-Deckers2 and Anand Chitresh2, (1)University of New South Wales, Climate Change Research Centre, Sydney, NSW, Australia, (2)University of New South Wales, Climate Change Research Centre, Sydney, Australia
Sherwood et al. (2014) recently reported a correlation between an index of lower-tropospheric mixing and equilibrium climate sensitivity in 43 climate models, and presented a physical argument as to why the presence of this type of mixing would cause low cloud amount to have a negative temperature-dependence. Here we explore this mixing, and the vertical spread of convective heights generally, from a variety of angles. First, we present evidence from temperature trends that deep convection in the tropics reaches a broader spread of heights with more dispersion than in climate models, and evidence from satellites that cloud populations in the narrow Pacific and Atlantic ITCZ regions are significantly less deep than those over the warmer Indo-Pacific warm pool oceans. On the other hand lower-tropospheric mixing within midlatitude cyclones may be reasonably well simulated, based on examination of a single GCM. We further present simulations using the WRF regional model of overturning in the Pacific region, showing a dipolar depth structure with both deep and shallow overturning. These simulations show that low-cloud amount is strongly reduced by the invigoration of convective motions on small scales. Finally we present an argument based on dynamical studies of individual convective thermals that may account for why convective dispersion is typically underestimated in convective parameterisations.