Properties of the convection scheme in NCEP's Eta Model that affect forecast sounding analysis

Model forecast soundings of temperature and humidity can be strongly controlled by feedback from the convective parameterization. Routine examination of forecast soundings from the operational Eta Model has shown that diagnostic parameters important for the prediction of convection, such as cap strength and CAPE, are often affected by the convective scheme, known as the Betts-Miller-Janjic scheme (Betts and Miller, 1986, Janjic 1994). For instance, the shallow convection component of the scheme can mix temperature and moisture from the top of the boundary layer into the free atmosphere, sometimes quickly eroding a capping inversion. Since shallow convection does not produce precipitation, it is not possible to determine where it has occurred based solely upon examination of the QPF field. However, the scheme does produce a distinctive "signature" in the temperature and humidity profile, allowing for identification of the process through examination of the forecast soundings.

At the conference, we will describe aspects of the Betts-Miller-Janjic scheme that impact the vertical structure of temperature and humidity, important in convective forecasting. We will show examples of heating and drying profiles from a 1-D version of the scheme that demonstrate how it can dramatically modify forecast soundings. We will also show example forecast soundings that exhibit typical deep and shallow convective "signatures" to help forecasters recognize situations when the scheme may be modifying the profiles in an unrealistic manner. It is hoped that knowledge of these tendencies will help forecasters adjust forecast parameters to get a better depiction of the convective environment.