rpt28

Report 28: Interannual Tropical Rainfall Variability in General Circulation Model Simulations Associated with the Atmospheric Model Intercomparison Project

Sperber,

The interannual variability and Reproducibility of rainfall over the Indian subcontinent, the Sahel and the Nordeste region of Brazil have been evaluated from the suite of AMIP simulations. Rainfall variations over the Nordeste region are the most readily captured owing to the intimate link with Pacific and Atlantic SSTs. The precipitation variations over the Indian subcontinent and the Sahel are relatively less well captured by the models respectively. Additionally, an Indian monsoon wind shear index was calculated for each model. The models are generally more adept at simulating the variability of the wind shear index than that associated with the rainfall over this region, indicating that the models exhibit greater fidelity at capturing the large-scale dynamical fluctuations. For each region improved skill scores and enhanced Reproducibility result for those models that qualitatively simulate the observed rainfall/SST correlation pattern which is dominated by an ENSO teleconnection in the Pacific Ocean. Accordingly for this subset of the models, the enhancement in skill and Reproducibility occurred mainly during years of strong El Niño and La Niña conditions.

A suite of six ECMWF AMIP runs (differing only in their initial conditions) have also been examined. The Indian monsoon rainfall exhibits a consistent response during 1987 and 1988, while during other years differences are simply not very predictable, possibly because of internal chaotic dynamics that are associated with intraseasonal monsoon fluctuations and/or land surface process interactions. In this case the Reproducibility is poor indicating that the average intramodel spread is greater than the temporal variability of the ensemble mean. For the Sahel and the Nordeste the Reproducibility increases to 2.4 and 9.0 respectively indicating a robust response to the boundary conditions for this model.

Five SUNY/NCAR Genesis low resolution initial condition realizations were also examined. In this model the Nordeste rainfall also exhibited the largest Reproducibility, however for all regions the Reproducibility was smaller than that for the ECMWF model. The relationship of the all-India and Sahel rainfall/SST teleconnections with horizontal resolution, convective closure and numerics has been evaluated. Models with resolution >=T42 performed more poorly than lower resolution models. The higher resolution models were predominantly spectral. At low resolution, spectral vs. gridpoint numerics performed with nearly equal verisimilitude. At low resolution, moisture convergence closure was slightly preferable to other closure techniques. At high resolution, the models that utilized a moisture convergence criteria performed very poorly suggesting that moisture convergence may be inappropriate for models with horizontal resolution >=T42. (pdf file)

UCRL-MI-123395