Model Parameter Choices and Their Impact on Precipitation Characteristics
Simulating precipitation is very diffcult for global atmospheric models because precipitation requires accurate handling of many different processes to achieve success. Problems occur with errors in rain location, rates, and timing. A team of U.S. Department of Energy scientists from Pacific Northwest National Laboratory used a regional weather model to explore how global models will behave when used with higher resolutions more typical of regional atmospheric models. An important parameter in simulating clouds is the “convective timescale,” which represents how quickly convective clouds act on the surrounding atmosphere and whose resolution dependency is uncertain. Combining physics packages from the global Community Atmosphere Model version 5 (CAM5) with the regional Weather Research and Forecasting model for realistic weather conditions, they found that a shorter timescale results in a more accurate precipitation amount over the central United States during the simulated period. However, this short timescale worsens the precipitation diurnal cycle, with the convection too tightly linked to the daytime surface heating, and thus occurring too close to noon. Longer timescales greatly improve the diurnal cycle but result in less precipitation and thus produce a low bias. To investigate the simulated precipitation occurrence, strength, and diurnal cycle, the team compared the model results with observations using a grid with approximately ¼° grid spacing for a period in late April and May 2011 during the Midlatitude Continental Convective Clouds Experiment (MC3E). The analysis of rain rates shows that with longer timescales, the frequency distribution of rain can be improved, particularly for the extreme rain rates. Ultimately, without changing other aspects of the physics, a decision between accurate diurnal timing and rain amount must be made when choosing an appropriate convective timescale due to structural deficiencies in the cloud portion of the model. This information is important for designing climate models to operate at ¼° resolution during the next few years.
Gustafson, W. I., P.-L. Ma, and B. Singh. 2014. “Precipitation Characteristics of CAM5 Physics at Mesoscale Resolution During MC3E and the Impact of Convective Timescale Choice,” Journal of Advances in Modeling Earth Systems 6(4): 1271-87. DOI: 10.1002/2014MS000334.