Diurnal cycle of tropical precipitation in a general circulation model
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abstract
Hourly averaged precipitation rates from an ensemble of the Community Climate Model version 3 (CCM3) simulations for the 44-month period from January 1998 through August 2001 are compared to observations from the Tropical Rainfall Measuring Mission (TRMM) satellite. In order to have adequate sampling by the satellite, comparisons are made for 15 longitude 10 latitude boxes and for larger geographical areas within the tropics. The temporally and spatially averaged hourly precipitation rates from CCM3 and from TRMM are fit to the diurnal harmonic by the method of linear least squares regression, and the phases and the amplitudes of the diurnal cycles are compared. The model's diurnal cycle is too strong over major land masses, particularly over South America (200% too large), and is too weak over many oceans, particularly the northwestern tropical Pacific (57% too small). The model-satellite phase differences tend to be more homogeneous. The peak in the model's diurnal harmonic consistently precedes that of the observations nearly everywhere. Phase differences are large over Australia, Papua New Guinea, and Saharan Africa, where CCM3 leads TRMM by 4 hours, 5 to 6 hours, and 9 to 11 hours, respectively. The model's phase and amplitude biases likely have effects on its hydrologic cycle and its surface and atmospheric energy budgets. Thus the causes for the model's biases need to be investigated. Copyright 2004 by the American Geophysical Union.
