Multiscale global atmospheric transport and convective precipitation

Freshwater from precipitation is an indispensable resource for life/nand a driver for many biological and geological processes. In this/ndissertation we present four papers and an investigation in progress/nusing conventional and multiscale global climate models with the/nlong term goal of understanding the physical mechanisms responsible/nfor the observed precipitation trends and distributions. We use the/nmultiscale model because, in contrast to conventional models, it has/nan explicit treatment of cloud and precipitation processes. We test/nthe hypothesis that the multiscale approach should, in principle,/ngenerate more realistic estimates of tracer concentration and/nconvective precipitation fields which are known to be sensitive to/ncloud-scale physical processes./nAtmospheric convection associated or not with precipitation produces/nvertical transport of surface-emitted compounds which can then affect/nconvection itself, and hence precipitation, by altering the atmospheric/nradiative budget or through cloud microphysical processes. In two/npapers we evaluate the multiscale method on simulating the global/ntransport of passive tracers and dust contrasting model outputs and/nin situ observations. We force our climate simulations with/nobservationally derived meteorology. Therefore we can attribute the/ndifferences between the multiscale and the conventional methods to the/ndifferences in vertical transport. The multiscale model simulates/nrealistic atmospheric concentrations of passive tracers and is, to a/nsmall degree, better than the conventional model in the lower and/nupper troposphere. Comparing global maps of passive tracers and dust/nfrom simulations, we find that the multiscale model simulates higher/nconcentrations in the upper troposphere. This characteristic should/nemerge from the overall more efficient vertical transport from/nsynthetic thermals in the explicit cloud treatment./nWe evaluate the multiscale model and a group of the latest versions of/nstandard climate models on the intensity distribution of/nprecipitation. In two papers we focus our analysis on sub-daily/nprecipitation for a highly convective region and contrast model output/nwith observationally derived precipitation rates. The multiscale model/ngreatly improves the simulation of convective precipitation/ndistribution with respect to its conventional counterpart. A comparison/nof the precipitation efficiency between the two models suggests that/nthe improvement should be due to the higher rate of production of cloud/nwater from vertical advection from the multiscale model as opposed to/nthe rate of conversion of cloud water to precipitation from cloud/nmicrophysical processes. Extending the analysis of the precipitation/nintensity distribution to other state of the art climate models, we/nfind that the models which implement mechanisms for inhibiting/nconvective precipitation at low level of instability are more/nrealistic./nWe present work in progress where the multiscale model is the/ntool of choice for understanding the physical processes responsible/nfor the observed precipitation trends in the region of the Indian/nmonsoon and for attributing these trends to anthropogenic aerosol/nemissions. In addition to the realistic representation of convection and/ntransport, the multiscale model can simulate better than other models/nthe intraseasonal precipitation variability thought to be important/nfor the dynamics of the monsoon.

Advisor: Collins, William D.