The Diurnal Cycle of Clouds and Precipitation at the ARM SGP Site
Millimeter Wavelength Cloud Radar (MMCR) data from Dec. 1996 to Dec. 2010, collected at the U. S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) program Southern Great Plains (SGP) site are used to examine the diurnal cycle of hydrometeor occurrence. These data are categorized into clouds (-40 dBZe ≤ reflectivity < -10 dBZe), drizzle and light precipitation (-10 dBZe ≤ reflectivity < 10 dBZe), and heavy precipitation (reflectivity ≥ 10dBZe). The same criteria are implemented for the observation-equivalent reflectivity calculated by feeding outputs from a Multiscale Modeling Framework (MMF) climate model into a radar simulator. The MMF model consists of the NCAR Community Atmosphere Model (CAM) with conventional cloud parameterizations replaced by a cloud resolving model (CRM). The annual and seasonal mean composites between observations and the model are compared. Meanwhile, a set of atmospheric states that were created specifically for the ARM Southern Great Plains (SGP) site by Evans et al.  are used to investigate in which atmospheric state or states the MMF does well or poorly. Differences (or errors) in the annual diurnal cycle between observations and the MMF model are decomposed into differences due to the relative frequency of occurrence of states, the daily-mean vertical profile of hydrometeor occurrence and the (normalized) diurnal variation of hydrometers in each state. The results show that a radar simulator combined with the simple reflectivity categories can be an effective approach for evaluating diurnal variations in model hydrometeor occurrence. Based upon the comparison, the MMF only marginally captures observed increases in the occurrence of boundary layer clouds after sunrise in spring and autumn, and does not capture diurnal changes in boundary layer clouds during the summer. Above the boundary layer, the MMF captures reasonably well diurnal variations in the vertical structure of clouds, light and heavy precipitation in the summer but not in the spring. The state-wise analysis shows that the model underestimates boundary layer clouds but overestimates high clouds and precipitation in almost all the states. In terms of the diurnal variation, the MMF does not capture that of low clouds in any state, but does a reasonable job capturing the diurnal variation of high clouds and precipitation in summer states though not in those states which occur in the transition seasons (spring and fall). Overall, the error in individual states and in the annual composite is due primarily to error in the daily mean of hydrometeor occurrence (rather than the diurnal variations). However, errors in the state frequency (that is the distribution of weather states in the model) also play a significant role.
- Atmospheric sciences