The influence of convective parameterization on model forecasts of an East Coast cyclone

The sensitivity of numerical forecasts of coastal cyclogenesis and frontogenesis to the choice of model cumulus parameterization (CP) scheme is examined for the 17 February 2004 East Coast cyclone event. Cold-air damming, a developing coastal surface cyclone, and an upper-level trough combined to present a challenging winter weather forecast scenario, a situation that was further complicated by inconsistent numerical model predictions. The most pronounced area of disagreement between model runs was the treatment of a coastal cyclone and an associated coastal front, features that would affect the location and timing of precipitation and influence precipitation type. Due to the differences in the model forecasts, it was hypothesized that the Eta model's Betts-Miller-Janjić (BMJ) cumulus parameterization (CP) scheme was dictating the location and intensity of the initial coastal cyclone center in model forecasts. Thus, forecasts for this case were re-run with the Workstation Eta model using the Kain-Fritsch (KF) CP scheme to test this hypothesis.

The experimental model forecasts reveal that the Eta model CP scheme did play a major role in the forecast for this case, affecting the quantitative precipitation forecast (QPF) as well as the strength, location, and structure of coastal cyclogenesis and coastal frontogenesis. Model forecasts employing the KF CP scheme revealed a relatively uniform distribution of convective precipitation oriented along the axis of an inverted trough and coastal front. In contrast, forecasts produced using the BMJ scheme resulted in a weaker coastal front, as well as the development of multiple distinct closed cyclonic circulations in association with more localized convective precipitation centers. In order to further investigate the role that the CP schemes played in the model forecasts, an additional BMJ model forecast was run with the shallow mixing component of the scheme disabled. This model forecast bore a closer semblance to the KF forecast than to the original BMJ run.