Iowa State University: Forecasting nocturnal mesoscale convection over the Great Plains

Final Report

One of the most challenging forecast problems over the Great Plains is the prediction of the development and intensification of nocturnal convection. It has long been recognized that thunderstorms are more common at night than during the day over this region. This contradicts our usual view of thunderstorms being promoted by daytime heating.

One reason for the anomalously high frequency of nocturnal thunderstorms over the Great Plains is the relationship between convection and the nocturnal low-level jet (LLJ) that commonly occurs over the plains. Related work, which was conducted under an earlier Outreach Partners Project, included the analysis of case studies of these interactions between the LLJ and mesoscale convection. During the project, a data set containing all of the hourly profiler wind observations for 1991 and 1992 was collected, and an hourly climatology of the LLJ was developed.

Because mesoscale convection over the Great Plains is a regional problem that requires a regional perspective, three NWS forecast offices participated in this Outreach Cooperative Project (Des Moines, Minneapolis, and Chicago). Research activities accomplished as part of the project includes the following:

• A climatology of low-level jet (LLJ) occurrences was compiled for the summers of 1991 and 1992, and the relationship between the LLJ and the synoptic pattern for the warm season of 1992 was analyzed. The results indicated that the LLJ is promoted by the warm sector of an extratropical cyclone, and is suppressed by a subtropical ridge or polar high. The LLJ analysis was also extended to include an analysis of strong southerly wind events that may affect the development of deep convection, but which do not necessarily meet the definition of a low-level jet. The researchers found that these southerly wind events occur under similar synoptic patterns as the LLJ, but show much less diurnal variability than the LLJs.
• A climatology of mesoscale convective complex (MCC) occurrences during 1993 was also developed. The results indicate that the number of MCCs for 1993 was about average or even slightly below compared to previous years. This result confirms previous findings that flood (or drought) years do not necessarily have more (or fewer) MCCs than other years.
• An examination of archived profiler data has found that the normalized frequency of the LLJ for the Midwestern flood summer of 1993 was about twice that of the summer of 1992. In particular, the peak of the 1993 floods is evidenced by a dramatic increase in the frequency and persistence of the strongest LLJs. The researchers also found that the LLJ was associated with a mid- and upper-level (500-200 mb) height perturbation extending upstream to the eastern Pacific.
• Recent concerns about contamination of the wind profiler data due to echoes from migratory birds were also addressed. Preliminary indications are that the contamination is most severe during the spring and autumn months, during the night, and during the strongest LLJ events.