William Blumen - US grants

Atmospheric frontal zones, the transition zone between relatively warm and cold air masses, are highly turbulent. In the this zone, kinetic energy is dissipated and internal waves are generated which may result in severe convective activity or turbulence far from the front. Both of these processes have implications for atmospheric predictability. The principal investigator will employ two approaches in the study of frontal zones. He will analyze observations taken during a recent field program designed to investigate frontal processes known as STORM-FEST, and will use wavelet decomposition as his principal analysis tool. He will also conduct a number of modeling experiments with the goal of isolating the physical processes important for frontal evolution. Many of the important physical processes that govern the behavior of weather systems occur on scales which are not explicitly resolved in numerical prediction models. The ultimate goal of this study is to provide information which will allow the development of representations of such and, thereby, improve numerical weather forecasts.

This proposal on "A Study of Atmospheric Fronts Over Mountainous Terrain" between Dr. William Blumen of the University of Colorado and Professors ZHAO Ming and WU Rongsheng of Nanjing University is jointly sponsored by NSF and the Chinese State Educational Commission (SEDC). The investigators propose to study physical processes that affect atmospheric frontal motions over the earth's orography, specifically the mountainous regions of China. They will emphasize the role of the planetary boundary layer in this process. The investigations will consist of 1) observational and analysis study to document and examine various cases of frontal distortion by orography; 2) a numerical modelling effort to simulate frontal motion over terrain features such as the Tien Shan complex in western China; and 3) a theoretical study to examine the relation between frontal motion and planetary boundary processes. The investigators will carry out the first two phases at Nanjing University, where the data, analysis procedures and numerical model resources are. The Chinese investigators will plan and execute these aspects of the research in consultation with the American investigator. The theoretical phases, including analysis of the numerical experiments, will be carried out jointly through visits. Scientists do not completely understand the physical processes that affect frontal passage over mountainous terrain, complicating numerical weather prediction. The grid resolution and model physics may be insufficient to depict fronts adequately and, therefore, meteorological predictions in the vicinity of large mountain complexes are generally less reliable than elsewhere. The proposed research is of mutual interest to both the Chinese and American investigators. The particular emphasis placed on orographic effects in China will expose Western scientists to meteorological observations that have not been readily accessible, and the sharing of resources contributes to the research program.

Processes that occur in the Earth's boundary layer, the relatively shallow layer in contact with the ground, often have a significant effect on atmospheric fronts and are a source of wave motions and atmospheric oscillations. Data available from a previous field investigation in Kansas in 1995 (MICROFRONTS) consists of high frequency hot-wire anemometer winds and both wind and temperature from sonic anemometers mounted on 10 meter tower arrays. Three frontal passages occurred when the instruments were operative. These data, and other lower frequency observations, will continue to be analyzed to determine the kinetic energy dissipation rate and turbulence characteristics that characterize fronts. These motions occur on subgrid spatial and temporal scales of numerical atmospheric models. The turbulent dissipative contributions are either not prescribed or handled ineffectively by all or most models. The present aim is to provide the necessary data analysis and theoretical understanding that is required to ultimately parameterize turbulent dissipation associated with fronts.

The passage of fronts and the relatively abrupt change in the height of the boundary layer near sunset appear to be sources for the initiation of internal gravity waves and inertial oscillations. Four years of data from the National Oceanic and Atmospheric Administration wind profiler network, and from three profilers maintained in Kansas by the Argonne National Laboratory, will be analyzed by the rotary spectrum technique. The purpose is to extract properties of atmospheric inertial oscillations, and to develop boundary layer models that will establish the relative importance of fronts and boundary layer depth changes in their occurrence. The role of these oscillations on the dynamics of fronts will continue to be investigated by use of frontogenesis models. The source and characteristics of these oscillations has also been recognized as important for long-range modeling of atmospheric dispersion. Ongoing model development for this purpose is of considerable importance in the prediction of urban air quality.

The current data bases will be expanded by participation in a field program to be carried out in southern Kansas in October 1999 (Cooperative Atmosphere-Surface Exchange Study -- 1999). These data will be used with ongoing model developments to both improve and to verify model predictions. The observational arrays are expected to be suitable to acquire data that is required to isolate boundary layer sources for both inertial oscillations and for internal gravity waves.