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Impact of the subgrid representation of parameterized convection on simulated climatology
  • Published Date:
    2000
Filetype[PDF - 2.82 MB]


Details:
  • Personal Authors:
  • Corporate Authors:
    National Centers for Environmental Prediction (U.S.)
  • Series:
    Office note (National Centers for Environmental Prediction (U.S.)) ; 428
  • Document Type:
  • Description:
    The sensitivity of general circulation models (GCMs) to the subgrid representation of parameterized convection is investigated. Various experiments testing the sensitivity of the simulated climatology to parameter in convective downdrafts were conducted within the framework of a perpetual January run. The mass flux of the parameterized downdraft is varied in magnitude up to the maximum value allowed within the current parameterization framework over the ocean, while over land it is bounded to a specific value. This concept is based on the fact that soil temperature and moisture over land are highly correlated to the amount of parameterized convective precipitation, whereas over the ocean the corresponding subsurface properties in the model are prescribed by the sea surface temperature. The impact of enhanced evaporation of precipitation over the oceans was also discussed. The sensitivity experiment was extended to an ensemble simulation framework for two different summers and winters, in terms of SST over the tropical oceans. The modified scheme with an maximum downdraft over the ocean clearly shows improvements compared to that from the convection scheme with a suppressed downdraft. The modified scheme suppresses the erroneously excessive rainfall north of the equator and increases rainfall over the Southern Hemispheric oceans, which generally agrees better with the observed January precipitation climatology. The improvement is also significant in the low-level wind distribution, particularly easterlies over the equatorial Pacific between 130-160 W, which are too weak in the operational version of the scheme. Enhanced evaporation shows comparable impact to modified downdrafts in that both effects play a role in stabilizing the atmosphere. Ensemble simulations with it, for January of 1997 and 1998, reveal consistent improvement in precipitation anomalies as achieved in the perpetual run. The improvement of simulated precipitation anomalies for July of 1996 and 1997 is also significant in response to SST anomalies over the tropical oceans.

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