Guang Zhang's Current Projects

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 Zhang-McFarlane scheme. The revised closure couples convection to the large-scale 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 CAPE-based closure limits the CAPE buildup at the beginning of the monsoon season, resulting in the under-simulation 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 quasi-equilibrium assumption proposed by Arakawa and Schubert assumes that convection is controlled by the large-scale forcing in a statistical sense, in such a way that the stabilization of the atmosphere by convection is in quasi-equilibrium with the destabilization by the large-scale 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 quasi-equilibrium 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 large-scale 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 quasi-equilibrium 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 large-scale forcing in the free troposphere is weak. Based on these and other observations, a modification to the quasi-equilibrium assumption is proposed. It assumes that convective and large-scale 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 quasi-equilibrium 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 quasi-equilibrium 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 quasi-equilibrium with the removal of convective instability by convection. Our recent investigation using observational data found that such a quasi-equilibrium 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 Arakawa-Schubert quasi-equilibrium closure. The predicted relationship between the large-scale 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 non-convective 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.