Influence of diabatic potential vorticity anomalies upon warm conveyor belt flow. Part II: 3-5 January 2005

The use of conveyor belt theory has been shown to have considerable application in understanding the distribution of clouds and precipitation in extratropical cyclones. Conveyor belts represent large-scale airstreams that occur as a result of the configuration of the structure of synoptic systems. However, mesoscale features can modify the strength and path of conveyor belts. One example of this phenomenon is the production of latent heat release and potential vorticity (PV) due to convection along the warm conveyor belt.

The case of 3-5 January 2005 will be presented. In this case, heavy snowfall was forecast to occur across a large part of Nebraska and Iowa. During the event, convection developed over the southern Plains within a region of strong warm air advection to the north of a synoptic warm front. Although heavy snowfall did result from the winter storm in the area forecast to occur by operational numerical models. This talk will focus on a 12 hour period from 0900 to 2100 UTC on 5 January during a break in the snowfall. This time period is 6 hours after convective development over the southern Plains. During this time frame, significant snowfall was forecast but failed to materialize to the north of the where the strongest convection was occurring. The study utilizes data from the North American Regional Reanalysis from the National Climate Data Center and a simulation of the case using WRF-ARW to examine the effects convection may have had on the failed development of the heavy snowfall during the 12 hour period. This was accomplished using piecewise PV inversions to isolate the flow resulting from the diabatically produced PV from the convection. The magnitude and orientation of the flow difference between numerical forecasts and observed data appears to indicate a redistribution of moisture due to the convection. The effects from the southern Plains convection appear to have decreased after 1800 UTC 5 January at which time heavy snow redeveloped across portions of Nebraska and Iowa near 0000 UTC 6 January. Increased knowledge and awareness of these scale interactions within the context of conveyor belt theory has the potential to improve forecasts in winter events that feature convection.