Applications of column models for terminal weather nowcasts
The Planetary Boundary Layer (PBL) is that part of the atmosphere, which is directly influenced by the presence of the earth's surface, and which responds to surface forcing with a time-scale of an hour or less. The Residual Layer (RL) is the portion of the lower atmosphere, which was part of the PBL within the past several hours, and which has become separated from the influence of short-term surface forcing, usually by the formation of a cooler layer at the surface. In the mid-latitudes, the height of the combined PBL and RL is usually 1-2 kilometers. A column model is a one-dimensional prognostic model for the state of a single column of the atmosphere, with special attention to the processes in the lowest few kilometers. It is designed to diagnose and nowcast the vertical structure of the PBL. Important information for ITWS1 nowcast products are the vertical profiles of horizontal wind velocity, temperature, humidity, and turbulent kinetic energy (TKE) in the lowest few kilometers (Sankey, 1994). Traditionally, operational meteorologists have obtained estimates of these quantities by balloon soundings, a measurement process that is not well-suited for continuous updates. We are investigating the possibility of developing an operational column model to obtain this vertical structure information for use in the ITWS. Our approach involves using a combination of sensing technology and analysis techniques that have proven successful in several research programs. Column models are designed to mimic the processes by which the surface forces the processes in the low atmosphere at times when local radiation is a dominant factor. Fluxes are measures of the net rates of these transport processes. The widely used Oregon State University column model (OSUlDPBL) parameterizes the fluxes by gradient transfer techniques (Troen and Mahr!, 1986). This model has provided dependable service in several field experiments, providing information with a vertical resolution of tens of meters. It is not designed to provide a fine-scale description of the stable nocturnal PBL. The French model COB EL has been developed to forecast the occurrence of radiation fog, and therefore concentrates on modeling the stable nocturnal PBL (Bergot and Guedalia, 1994). It uses a prognostic equation to estimate TKE in the stable boundary layer and parameterizes the fluxes in tern1s of the TKE (Duynkerke, 1991). A discussion of the potential uses of the column model in the ITWS is followed by the considerations that motivate the design of an operational column model. The prototype design is described. We conclude with the results of a preliminary evaluation using STORMFEST data (STORM Project Office, 1992) and a discussion of plans for a more comprehensive evaluation.