Advanced Research WRF (ARW) modeled low-level jet climatology compared to observed climatologies

Nocturnal low-level jets (LLJs) are common features observed in the Great Plains region of the United States. LLJs play a key factor in initiating and sustaining mesoscale convective systems and other severe convective storm modes in the Great Plains. The LLJ in the Great Plains can also be a key source of moisture transport into the region which is crucial for severe weather development and shown important for widespread flooding. Knowing the climatology of such events is important so an understanding of the importance of the LLJ in severe weather can be furthered.

Several observational studies have been conducted to determine the climatology of LLJs over the Great Plains. However these studies are limited due to the spatial restraints. Using point measurements makes it nearly impossible to determine the spatial structure of LLJs. Using a NWP model lessens this restraint, though grid spacing and frequency of the model output is still problematic using operational forecasts.

This study investigates how well the Advanced Research WRF (ARW), represents the LLJ climatology of the region centered on the ARM site along the Kansas/Oklahoma border using 6 months of data (June-Sept.) with output every 3 hours. NCAR's operational ARW is a 36/12 km two-way nest (12km grid analyzed) initialized at 0000 UTC from 40 km Eta grib data. Preliminary results indicate that the ARW has similar climatology characteristics (i.e. frequency, max time occurrence, dominant direction) to the previous observational studies. To forecast LLJs, accurate representation of the PBL is crucial, which is also important for being able to forecast many high impact events. If the WRF can be shown to produce a similar climatology to that what has been observed, we gain more confidence in WRF and its PBL parameterizations.

Since the ARW shows promise of representing the LLJ climatology of the ARM site closely to what has been found in the observational studies, the ARW could be used to get a better understanding on the frequency of the LLJ over other sites within the Great Plains. This information could be further used to understand the importance of the LLJ in moisture transportation. It is also possible for the ARW to be used to further investigate the forcing mechanisms of LLJs which is yet not fully understood. A better understanding of LLJ development could lead to greater improvement in forecasts and severe weather initiation.