Atmos. Chem. Phys. Discuss., 13, 13653-13684, 2013
© Author(s) 2013. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Diagnosing the average spatio-temporal impact of convective systems – Part 1: A methodology for evaluating climate models
M. S. Johnston1,4, P. Eriksson1, S. Eliasson2, M. D. Zelinka3, R. M. Forbes5, and K. Wyser4
1Department of Earth and Space Sciences, Chalmers University of Technology, Göteborg, Sweden
2Department of Computer Science, Electrical and Space Engineering, Division of Space Technology, Luleå University of Technology, Kiruna, Sweden
3Program for Climate Model Diagnosis and Intercomparison, Lawrence Livermore National Laboratory, Livermore, California, USA
4Swedish Meteorological and Hydrological Institute, Norrköping, Sweden
5European Centre for Medium-Range Weather Forecasts, Reading, England

Abstract. A~method to determine the mean response of upper tropospheric water to localised deep convective (DC) events is improved and applied to the EC-Earth climate model. Following Zelinka and Hartmann (2009), several fields related to moist processes and radiation are composited with respect to local maxima in rain rate to determine their spatio-temporal evolution with deep convection in the central Pacific Ocean.

Major improvements to the above study are the isolation of DC events in time so as to prevent multiple sampling of the same event, and a revised definition of the mean background state that allows for better characterization of the DC-induced anomalies.

The DC events observed in this study propagate westward at ~ 4 m s−1. Both the upper tropospheric relative humidity and outgoing longwave radiation are substantially perturbed over a broad horizontal extent during peak convection and for long periods of time. Cloud fraction anomaly increases throughout the upper troposphere, especially in the 200–250 hPa layer, reaching peak coverage following deep convection. Cloud ice water content anomaly confined to pressures greater than about 250 hPa and peaks near 450 hPa within a few hours of the DC event but remain enhanced following the DC event. Consistent with the large increase in upper tropospheric cloud ice, albedo increases dramatically and persists for sometime following the DC event.

Applying the method to the model demonstrates that it is able to capture the large-scale responses to DC events, most notably for outgoing longwave radiation, but there are a number of important differences. For example, the DC signature of upper tropospheric humidity consistently covers a broader horizontal area than what is observed. In addition, the DC events move eastward in the model, but westward in the observations, and exhibit an unrealistic 24 h repeat cycle. Moreover, the modeled upper tropospheric cloud fraction anomalies – despite being of comparable magnitude and exhibiting similar longevity – are confined to a thinner layer that is closer to the tropopause and peak earlier than in observations. Finally, the modeled ice water content anomalies at pressures greater than about 350 hPa are about twice as large as in the observations and do not persist as long after peak convection.

Citation: Johnston, M. S., Eriksson, P., Eliasson, S., Zelinka, M. D., Forbes, R. M., and Wyser, K.: Diagnosing the average spatio-temporal impact of convective systems – Part 1: A methodology for evaluating climate models, Atmos. Chem. Phys. Discuss., 13, 13653-13684, doi:10.5194/acpd-13-13653-2013, 2013.
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