Parameterized updraft mass flux as a predictor of convective intensity

Numerical weather prediction models provide valuable guidance for forecasters. However, meteorological features with characteristic wavelengths less than about 4 times the distance between model grid points are not resolved by the model grid. With the current configuration of the NCEP's Eta model, this means that phenomena with characteristic scales less than about 130 km are not resolvable. Smaller-scale features must be represented in parameterized form.

When forecasters use today's models to help with predictions of severe convection, they examine a model's depiction of processes that are well-resolved by the model (i.e., mesoscale and larger-scale processes). In addition, they examine single-grid-point soundings and common convective indices derived from these soundings. However, in terms of a model's prediction of convection itself, the only guidance that traditional NWP products provide is a parameterized-convective rainfall amount. This is unfortunate because convective rainfall totals appear to be poorly correlated with convective intensity. For example, while severe convection certainly can produce heavy rainfall, factors such as rapid cell movement, high wind shear, or dry layers in the atmosphere can diminish rainfall totals at a given location even when violent convective overturning occurs overhead. Limiting numerical guidance to convective rainfall amount is also unsatisfying because convective parameterization schemes routinely diagnose important characteristics of convective environments that could be much better indicators of convective intensity than the parameterized rainfall.

In semi-operational experimental runs of the Eta model at the National Severe Storms Laboratory, we have investigated the potential value of non-traditional forecast parameters provided by the Kain-Fritsch convective parameterization scheme. In this study, we discuss the utility of the parameterized updraft mass flux predicted by this scheme in forecasting severe convection. In particular, we define the updraft mass flux in the context of the KF scheme and discuss the relationship between this parameter and the thermodynamic characteristics of an input sounding. In addition, we evaluate the correlation between model-predicted updraft mass flux and convective intensity derived from radar data, standard surface observations, and local storm reports.