Publications

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2018
Wang, MC, Zhang GJ.  2018.  Improving the simulation of tropical convective cloud-top heights in CAM5 with CloudSat observations. Journal of Climate. 31:5189-5204.   10.1175/jcli-d-18-0027.1   AbstractWebsite

Using 4 years of CloudSat data, the simulation of tropical convective cloud-top heights (CCTH) above 6 km simulated by the convection scheme in the Community Atmosphere Model, version 5 (CAM5), is evaluated. Compared to CloudSat observations, CAM5 underestimates CCTH by more than 2 km on average. Further analysis of model results suggests that the dilute CAPE calculation, which has been incorporated into the convective parameterization since CAM4, is a main factor restricting CCTH to much lower levels. After removing this restriction, more convective clouds develop into higher altitudes, although convective clouds with tops above 12 km are still underestimated significantly. The environmental conditions under which convection develops in CAM5 are compared with CloudSat observations for convection with similar CCTHs. It is shown that the model atmosphere is much more unstable compared to CloudSat observations, and there is too much entrainment in CAM5. Since CCTHs are closely associated with cloud radiative forcing, the impacts of CCTH on model simulation are further investigated. Results show that the change of CCTH has important impacts on cloud radiative forcing and precipitation. With increased CCTHs, there is more cloud radiative forcing in tropical Africa and the eastern Pacific, but less cloud radiative forcing in the western Pacific. The contribution to total convective precipitation from convection with cloud tops above 9 km is also increased substantially.

Yang, MM, Zhang GJ, Sun DZ.  2018.  Precipitation and moisture in four leading CMIP5 models: Biases across large-scale circulation regimes and their attribution to dynamic and thermodynamic factors. Journal of Climate. 31:5089-5106.   10.1175/jcli-d-17-0718.1   AbstractWebsite

As key variables in general circulation models, precipitation and moisture in four leading models from CMIP5 (phase 5 of the Coupled Model Intercomparison Project) are analyzed, with a focus on four tropical oceanic regions. It is found that precipitation in these models is overestimated in most areas. However, moisture bias has large intermodel differences. The model biases in precipitation and moisture are further examined in conjunction with large-scale circulation by regime-sorting analysis. Results show that all models consistently overestimate the frequency of occurrence of strong upward motion regimes and peak descending regimes of 500-hPa vertical velocity JCLI-D-17-0718.1 regime, models produce too much precipitation compared to observation and reanalysis. But for moisture, their biases differ from model to model and also from level to level. Furthermore, error causes are revealed through decomposing contribution biases into dynamic and thermodynamic components. For precipitation, the contribution errors in strong upward motion regimes are attributed to the overly frequent . In the weak upward motion regime, the biases in the dependence of precipitation on probability density function (PDF) make comparable contributions, but often of opposite signs. On the other hand, the biases in column-integrated water vapor contribution are mainly due to errors in the frequency of occurrence of , while thermodynamic components contribute little. These findings suggest that errors in the frequency of occurrence are a significant cause of biases in the precipitation and moisture simulation.

2005
Collier, JC, Zhang GJ.  2005.  US warm-season rainfall in NCAR CAM3: An event-oriented perspective. Geophysical Research Letters. 32   10.1029/2005gl024217   AbstractWebsite

A modified form of the Community Atmosphere Model, ver. 3 (CAM3) developed at the National Center for Atmospheric Research (NCAR) is validated in its warm-season mean precipitation diurnal cycle for two regions of the United States. For all grid boxes of each region, simulated and observed precipitation records over four four-month periods are separated into discrete precipitation events. These events are binned into mutually-exclusive categories, and the diurnal harmonic for each category is estimated. In this way, the model is validated over the spectrum of precipitation episodes, and biases in the overall seasonal-mean diurnal cycle can be attributed to particular kinds of events. The results of the study indicate that the model's total seasonal precipitation is overwhelmingly weighted in the extremely long events and that these events contain the source of any biases in the seasonal mean.

2003
Zhang, GJ.  2003.  Lagrangian study of cloud properties and their relationships to meteorological parameters over the US southern Great Plains. Journal of Climate. 16:2700-2716.   10.1175/1520-0442(2003)016<2700:lsocpa>2.0.co;2   AbstractWebsite

Hourly satellite cloud data from 18 June to 18 July 1997 over the U. S. southern Great Plains are analyzed to study the scale-dependent cloud properties and their relationships to atmospheric conditions. The observed clouds are classified into high, midlevel, and low clouds according to their top heights. For each cloud type, contribution to the total cloud amount from clouds of different sizes is determined using a Lagrangian cloud classification scheme. It is found that in this continental, convectively active environment, more than half of the total cloud amount is from high clouds, of which 80% comes from clouds with area >4 x 10(4) km(2). For midlevel clouds, more than 50% of the contribution to cloud amount is from small clouds (e. g., cloud area,4 x 10(4) km(2)). Almost all of the low clouds with significant contribution to cloud amount have spatial scales,4 x 10(4) km(2). This suggests that most of the midlevel and low clouds are of subgrid scale to a typical GCM resolution (T42 or T63). It is further found that cloud radiative properties, such as cloud albedo, outgoing longwave radiation, and cloud radiative forcing, have strong scale dependence. Bigger clouds are brighter and have lower outgoing longwave radiation. These results indicate that contributions to the observed cloud radiative forcing are dominated by large cloud systems. The diurnal variation of the cloud properties is also examined. Using concurrent meteorological analysis from NCEP, possible relationships between cloud properties and prevailing meteorological conditions were sought. It is found that clear relationships exist between cloud properties, such as cloud amount and albedo, and the layer-averaged relative humidity, and the relationships vary with cloud scale. In addition, cloud properties for high clouds are well correlated to vertical velocity in the upper troposphere. More large and highly reflective clouds tend to occur in regions of upward motion. Low clouds have a clear correspondence with the lower-tropospheric static stability and temperature. Large and thick clouds prefer to exist where the lower-tropospheric air is cold, statically more stable, and has high relative humidity.