Diurnal Variability of
the Hydrologic Cycle and Radiative Fluxes:
Comparisons Between Observations
and a GCM
Journal of Climate, in press
Xin Lin, David
A. Randall, and Laura
D. Fowler
Department
of Atmospheric Science
Colorado State University
Fort Collins, CO 80523
Abstract
The simulated diurnal cycle is in many ways an ideal
testbed for new physical parameterizations. The purpose of this paper is
to compare observations from
Tropical
Rainfall Measurement Mission,
the
Earth Radiation Budget Experiment,
the
International Satellite Cloud Climatology Project,
the
Clouds and the Earth's Radiant Energy System Experiment, and
the
Anglo-BRazilian Amazonian Climate Observation Study with the diurnal
variability of the Amazonian hydrologic cycle and radiative energy budget
as simulated by
the Colorado
State University general circulation model (CSU GCM), and
to
evaluate improvements and deficiencies of the model physics.
The model uses a prognostic cumulus kinetic energy to
relax the quasi-equilibrium closure of the Arakawa-Schubert cumulus parameterization.
A parameter, alpha, is used to relate the CKE to the cumulus mass flux.
This parameter is expected to vary with cloud depth,
mean shear, and the level of convective activity, but up to now
we have used a single constant value for all cloud types. The results of
the present study show clearly that this approach cannot yield realistic
simulations of both the diurnal cycle and the monthly-mean climate state.
Improved results are obtained using a version of the model in which alpha
is permitted to vary with cloud depth.
Figure 1: Observed and simulated global maps of the first diurnal harmonic
of DJF total precipitation rates: (a) TRMM TMI; (b) TRMM PR; (c) Alpha8;
(d) Alpha9. The units are mm/day and the contour intervals are 0.4 mm/day.
Arrows pointing upward indicate maxima at local midnight, those pointing
to the right indicate maxima at 06 LST, etc. Note that TRMM TMI and PR
data only have values between 40S and 40N. Alpha8 and Alpha9 are two GCM
simulations with different convective adjustment times. Alpha8 tends to
be more realistic comparing with Alpha9.
Figure 2: Diurnal time series of observed and simulated DJF total, convective,
and stratiform precipitation rates over the Amazon (mm/day).
Figure 3: A descriptive model showing the diurnal variation of convection
over tropical summer continents.
For further information and comments, contact
lin@atmos.colostate.edu or visit
Xin Lin's homepage.