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GEST Highlight: Modeling Diurnal Cycle of Warm-Season Precipitation in GCMs
Myong-In Lee
45
CODE 610.1
Figure: Amplitude and maximum phase of the diurnal cycle of summertime precipitation from (a) the
observations (HPD, 1983–2002) and (b–f) the model simulations with different convection trigger
functions. Arrow length indicates the amplitude (mm d -1 ), and the direction indicates the peak timing of
precipitation in a day in local time. Adapted from Lee et al. (2008).
The diurnal cycle of precipitation is a fundamental component of the warm season climate of the
continental United States; however, it still remains a difficult test for General Circulation Models
(GCMs), where most of them fail to capture the strong signal of nocturnal precipitation over the
Midwest/Great Plains. This indicates our lack of understanding to the governing processes that drive the
continental nocturnal convection. Using a series of GCM sensitivity experiments, it was found that the
modeled diurnal cycle is highly sensitive to the changes in the parameterization of deep convection,
particularly to the convective trigger functions (see Figure above), which are implemented to define
certain conditions to initiate deep convection. Based on a subsequent analysis, it was found that the GCM
result supports the hypothesis on the observed diurnal convection mechanism. Convection appears to be
quite suppressed in the daytime in the contenental inland regions, such as the Great Plains, where the
relatively dry PBL air causes large convective inhibition and an elevation of the LFC. On the other hand,
nocturnal convection can develop, even at the presence of large nocturnal cooling in the boundary layer,
through the free-atmospheric destablization process. Eastward propagating, mesoscale convective systems
from the Rockies might play a significant role in triggering deep convection over the region. This process
is qualitatively consistent with the Atmospheric Radiation Measurement (ARM) sounding observations
over the Great Plains.