
Last Update  6/25/04 
Guang Zhang's Current Projects
micro
RZM
CAM3
megacity climate new
CAM3
megacity Exp2
CAM3
megacity Exp3
CAM3
megacity Exp4
CAM3
Freezing vs. No Freezing Exp
CAM3
c0 Exp
CAM3
Friction Exp
CAM3 Climatology Diagnostics with Zhang's New Closure (Comparison with Obs)
CAM3 Climatology Diagnostics with Zhang's New Closure (Comparison with Standard CAM3)
CAM3_fv vs. Observations Diagnostics with Zhang's New
Closure
CCSM3 Climatology Diagnostics with Zhang's New Closure (Comparison with Standard CCSM3)

Evaluating the Representation
and Impact of Convective Momentum Transport in CCSM Atmosphere Model (DOE)
 Improving convection parameterization using ARM
observations and NCAR Community Atmosphere Model (DOE)
[current results]
 This study compares the simulation of tropical convection in the
NCAR CCM3 using the original and a revised convective parameterization
closure in the ZhangMcFarlane scheme. The revised closure couples
convection to the largescale forcing in the free troposphere, instead
of to the convective available potential energy in the atmosphere as
employed in the original closure. In addition, a relative humidity threshold
is used for convection trigger. It is shown that the mean precipitation
distribution in the tropical regions for both summer and winter
is in general improved when the new closure is used. During JJA,
the precipitation in the western Pacific monsoon region is
significantly enhanced, alleviating the negative precipitation bias
there in the model. The spurious precipitation in the Arabian Peninsula
desert is completely eliminated. During DJF, the SPCZ is enhanced
considerably. All these changes are desirable in addressing important
model deficiencies. The probability distributions of the precipitation
intensity from the model simulations are compared with that from the
TRMM data. It is shown that over 90% of the CCM3 precipitation is from
light rain with rainfall rate less than 1 mm/hr, whereas the simulation
with the new closure and the TRMM observations show significant
contribution (30~40%) from rainfall rates greater than 2 mm/hr.
Precipitation simulation over the western North
Pacific summer monsoon region and the Arabian Peninsula was examined
in detail to understand the causes of the precipitation biases
in CCM3 over these regions. It is demonstrated that the CAPEbased closure limits the CAPE buildup at the beginning of the monsoon season,
resulting in the undersimulation of the western North Pacific monsoon precipitation. In the Arabian Peninsula, the positive feedback between convection and surface evaporation leads to the spurious heavy precipitation
center there. In addition to the new closure, the use of relative humidity threshold is also found important to the improvement of the simulation.

 Development and evaluation of convection parameterization
using ARM observations (DOE)
[current results]
 The quasiequilibrium assumption proposed by Arakawa and Schubert
assumes that convection is controlled by the largescale forcing in a
statistical sense, in such a way that the stabilization of the atmosphere
by convection is in quasiequilibrium with the destabilization by the
largescale forcing. The assumption was developed largely based on
observations in the tropical maritime environment and has not been
evaluated in midlatitudes. This study examines the quasiequilibrium
assumption in midlatitude continental convection environment using
summertime observations from the Southern Great Plains of the United States.
Two complementary approaches are taken for this purpose. The first
one compares the net time rate of change of convective available potential
energy to that due to the largescale forcing. The second one examines
the contributions to the net change of CAPE from the boundary layer air
and the free tropospheric air above. Results from both the approaches
indicate that the quasiequilibrium assumption is not well suited for
midlatitude continental convection. It is shown that the net change
of CAPE is comparable to and largely comes from that due
to thermodynamic changes of the boundary layer air, while the contribution
from the free troposphere above the boundary layer is negligible.
The analysis also shows that the role of convective inhibition to
suppress convection is the most pronounced when the largescale forcing
in the free troposphere is weak. Based on these and other observations,
a modification to the quasiequilibrium assumption is proposed.
It assumes that convective and largescale processes in the free troposphere
above the boundary layer are in balance, so that contribution from the
free troposphere to changes in CAPE is negligible. This assumption is
then tested using the single column model of the NCAR CCM3 by modifying
the closure in the CCM3 convection scheme. Such a modification
significantly improves the single column model simulation.
The applicability of this new quasiequilibrium assumption to tropical
convection environment is also discussed.
 Convection Parameterization and Climate Simulation
in NCAR Community Climate System Model (NSF)
 Observational Results
[current results]

Convective parameterization in global climate models (GCMs) represents
the collective effect of convection within a GCM grid in terms of the
resolvable scale quantities. It is one of the most difficult problems
facing the GCM community in improving climate simulations and predictions.
Within a convective parameterization, a closure empirically determining
the relationship between convection and the resolved scale fields is
required to close the system of equations. Most of the convective
parameterization schemes nowadays use the quasiequilibrium assumption
proposed by Arakawa and Schubert (1974) for this purpose, which assumes
that statistically the generation of convective instability
by the resolvable scale processes is in quasiequilibrium
with the removal of convective instability by convection.
Our recent investigation using observational data found that
such a quasiequilibrium assumption is not a good approximation
in either the midlatitude continental convection environment
or the tropical oceanic convective environment for averaging
time shorter than a day. This has important implications for
convective parameterization in GCMs. The following figure
is an example showing the prediction of convective stabilization
of the atmosphere based on the ArakawaSchubert quasiequilibrium
closure. The predicted relationship between the largescale
destabilization and convective stabilization is represented
by the slanting line in each plot. The observed relationships
are represented by dots and crosses for convective and nonconvective
periods, respectively.
Madden Julian Oscillation
[current results]


Precipitation and 1000 mb winds at MJO mature phase from CCM3 simulations
This material is based upon work supported by the National Science
Foundation under Grant No. 0204798. Any opinions, findings and
conclusions or recommendations expressed in this material are
those of the author and do not necessarily reflect the views of
the National Science Foundation.
Copyright Guang Zhang.
Last revised: January 20, 2006.


