Forcing Data for Single Column Models


Here we have analyzed the results from our Single Column Model (SCM) using two sets of forcing data. The forcing data consists of the horizontal advection of temperature, moisture and momentum. For the SCM to produce accurate profiles of temperature and humidity, it is important that the forcing data be as accurate as possible. Unfortunately, several SCM groups participating in ARM have experienced large errors in the temperature and humidity produced by their SCMs.

The first set of forcing data was produced as part of the ARM Operational Data products and uses objective analysis to produce the horizontal advection data from data gathered by radiosondes and wind profilers at the ARM SGP Site. This data is currently stored at CSU. The second set is produced by Prof. Minghua Zhang at SUNY Stonybrook and can be accessed via ftp. This product is derived using a constrained variational scheme. In addition to the objective analysis techniques used by the first data set, heat and moisture fluxes at the surface and top of the atmosphere are used to constrain the data to conserve the column-integrated mass, moisture, static energy and momentum. A manuscript which includes a detailed description of this procedure has been submitted to the Journal of the Atmospheric Sciences for publication (Zhang and Lin, 1996).

Two versions of our SCM were used to test the different forcing data sets. Cloud liquid water (CLW) was not included as a variable in the first version of the SCM, while in the second version it was included as a prognostic variable. We ran each version our SCM for a period of 15 days (Oct 26 - Nov 9, 1994) during the Fall 1994 Intensive Observing Period (IOP) using both sets of forcing data. Graphs of the temperature and specific humidity difference (SCM - Observed) are shown below.
Figures 1-4: Model version without cloud liquid water
Figures 5-8: Model version with cloud liquid water
(Click on the graph for a larger image).

ARM Forcing without CLW
Temperature Difference		 Zhang Forcing without CLW
Temperature Difference
Figure 1 Figure 2


ARM Forcing without CLW
Humidity Difference		 Zhang Forcing without CLW
Humidity Difference
Figure 3 Figure 4


ARM Forcing with CLW
Temperature Difference		 Zhang Forcing with CLW
Temperature Difference
Figure 5 Figure 6


ARM Forcing with CLW
Humidity Difference		 Zhang Forcing with CLW
Humidity Difference
Figure 7 Figure 8

Model temperature errors are significantly reduced when the constrained variational forcing data (Zhang) is used compared to the standard objectively analyzed products (ARM). There appears to be very little, if any, improvement in the model humidity fields when the variational forcing data is used. It is important to remember that the model is not perfect, and even with exact forcing data, there will be some difference between model results and reality.