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Iowa State University: "Forecasting nocturnal convection based on coincidence of the low level jet and a thermodynamic precursor"

Final Report

Nocturnal convection constitutes a significant forecast challenge over the Great Plains during the warm season, and the relationship between this convection and the nocturnal low-level jet (LLJ) has been the subject of several studies. It is widely acknowledged that the nocturnal LLJ is a major factor in the development and intensification of these storms. The objective of this project was to examine the usefulness of microwave wind profiler observations in forecasting nocturnal convection over the Great Plains, by providing better knowledge of the LLJ and its effects on atmospheric dynamics and thermodynamics.

The research centered around two main concepts: (1) the use of normalized returned power as an indication of atmospheric thermodynamic properties, and (2) the advantages of the hourly time resolution available from the profilers in identifying the nocturnal low-level jet.

(1) Normalized returned power: The profilers are clear-air radars which rely on the dependence of the radio refractive index on atmospheric temperature and moisture. In general, the larger the gradient of the atmospheric temperature or moisture, the stronger will be the discontinuity of the refracted index, causing increased reflection of electromagnetic energy back to the radar. It follows that the vertical and horizontal distribution of returned power may provide an indication of the atmospheric moisture distribution. Using two case studies of mesoscale convection in the Kansas-Nebraska region, the researchers found that the LLJ coincided with gradients of normalized returned power, presumably due to enhanced low-level moisture in the jet. Although there is not a precise, quantitative link between the power and water vapor, there appears to be a qualitative relationship that may be of operational value.

(2) Hourly climatology of the low-level jet: Hourly profiler observations for the warm seasons (April through September) of 1991 and 1992 were used to develop a high-time resolution climatology of the LLJ. This climatology was compared to a previous LLJ climatology by Bonner (1968), who used six- and twelve-hourly radiosonde observations. In general, the findings of this project were consistent with the earlier one, although there were some notable differences. The University of Kansas study also found that "jet events" tended to be of relatively short duration, typically about 4 hours. This implies that many of the jet occurrences may be missed by the conventional radiosonde network, and that the microwave profilers represent a uniquely useful tool for observing the LLJ.