Deep Convective Transition Characteristics in the NCAR CCSM and Changes Under Global Warming.

Sandeep Sahany, J. David Neelin, Katrina Hales, Richard B. Neales

J. Clim., 27, 9214-9232. Paper (1.4 MB).

© Copyright 2014 by the American Meteorological Society.

Abstract Tropical deep convective transition characteristics, including precipitation pickup, occurrence probability and distribution tails related to extreme events are analyzed using uncoupled and coupled versions of the Community Climate System Model (CCSM) under present-day and global warming conditions. Atmospheric Model Intercomparison Project-type simulations using a 0.5 degree version of the uncoupled model yield good matches to satellite retrievals for convective transition properties analyzed as a function of bulk measures of water vapor and tropospheric temperature. Present-day simulations with the 1.0 degree coupled model show transition behavior not very different from that seen in the higher resolution uncoupled version. Frequency of occurrence of column water vapor (CWV) for precipitating points shows reasonable agreement with the retrievals, including the longer-than-Gaussian tails of the distributions. The probability density functions of precipitating grid points collapse toward similar form when normalized by the critical CWV for convective onset in both historical and global warming cases. Under global warming conditions, the following statements can be made regarding the precipitation statistics in the simulation: (i) as the rainfall pickup shifts to higher CWV with warmer temperatures, the critical CWV for the current climate is a good predictor for the same quantity under global warming with the shift given by straightforward conditional instability considerations; (ii) to a first approximation the probability distributions shift accordingly, except that (iii) frequency of occurrence in the longer-than-Gaussian tail increases considerably, with implications for occurrences of extreme events, and thus (iv) precipitation conditional averages on CWV and tropospheric temperature tend to extend to higher values.

Citation Sahany, S., J. D. Neelin, K. Hales and R. B. Neale, 2014: Deep Convective Transition Characteristics in the NCAR CCSM and Changes Under Global Warming. J. Clim., 27, 9214–9232. doi:10.1175/JCLI-D-13-00747.1

Acknowledgments. This work was supported in part by National Science Foundation Grant AGS-1102838, National Oceanic and Atmospheric Administration Grants NA11OAR4310099 and NA14OAR4310274, and Department of Energy grant DESC0006739. We thank J. E. Meyerson for graphical assistance and S. Krueger for discussions and clarifications.

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