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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 28, issue 10
Ann. Geophys., 28, 1827–1846, 2010
https://doi.org/10.5194/angeo-28-1827-2010
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 28, 1827–1846, 2010
https://doi.org/10.5194/angeo-28-1827-2010
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.

  06 Oct 2010

06 Oct 2010

Sensitivity of the simulated precipitation to changes in convective relaxation time scale

S. K. Mishra1,2 and J. Srinivasan3,4 S. K. Mishra and J. Srinivasan
  • 1National Center for Atmospheric Research, Boulder, CO, USA
  • 2Department of Computer Science, University of Colorado, Boulder, CO, USA
  • 3Divecha Centre for Climate Change, Bangalore, India
  • 4Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India

Abstract. The paper describes the sensitivity of the simulated precipitation to changes in convective relaxation time scale (TAU) of Zhang and McFarlane (ZM) cumulus parameterization, in NCAR-Community Atmosphere Model version 3 (CAM3). In the default configuration of the model, the prescribed value of TAU, a characteristic time scale with which convective available potential energy (CAPE) is removed at an exponential rate by convection, is assumed to be 1 h. However, some recent observational findings suggest that, it is larger by around one order of magnitude. In order to explore the sensitivity of the model simulation to TAU, two model frameworks have been used, namely, aqua-planet and actual-planet configurations. Numerical integrations have been carried out by using different values of TAU, and its effect on simulated precipitation has been analyzed.

The aqua-planet simulations reveal that when TAU increases, rate of deep convective precipitation (DCP) decreases and this leads to an accumulation of convective instability in the atmosphere. Consequently, the moisture content in the lower- and mid- troposphere increases. On the other hand, the shallow convective precipitation (SCP) and large-scale precipitation (LSP) intensify, predominantly the SCP, and thus capping the accumulation of convective instability in the atmosphere. The total precipitation (TP) remains approximately constant, but the proportion of the three components changes significantly, which in turn alters the vertical distribution of total precipitation production. The vertical structure of moist heating changes from a vertically extended profile to a bottom heavy profile, with the increase of TAU. Altitude of the maximum vertical velocity shifts from upper troposphere to lower troposphere. Similar response was seen in the actual-planet simulations. With an increase in TAU from 1 h to 8 h, there was a significant improvement in the simulation of the seasonal mean precipitation. The fraction of deep convective precipitation was in much better agreement with satellite observations.

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