A new look at assimilation and boundary-layer modeling effects to improve NOAA ceiling/visibility/convection HRRR forecasts

An accurate short-range cloud and precipitation forecast is a fundamental component of rapidly updating data assimilation/short-range model forecast systems such as the NOAA 3-km High-Resolution Rapid Refresh or the 13-km Rapid Refresh (RAP). To reduce cloud and precipitation spin-up problems, a cloud/hydrometeor non-variational assimilation technique for stratiform clouds was developed within the Gridpoint Statistical Interpolation (GSI) data assimilation system. The goal of this technique was retention into the subsequent model forecast, as appropriate, of observed stratiform cloud and observed clear 3-d volumes.

New observation impact studies show that the ceiling forecasts are particularly improved by use of this cloud/hydrometeor assimilation in the HRRR/RAP model in both summer and winter season.   Daytime 2m temperature and dewpoint forecasts are also improved in the summer period, important for convective storms.

Improved design of the MYNN boundary-layer turbulence scheme is also shown to benefit HRRR/RAP ceiling prediction and is now also being tested in the FV3 3km stand-along regional model and in the NOAA FV3 Global Forecast System.   Improved boundary-layer prediction, also through other parameterization additions for gravity-wave drag and land/lake modeling, is demonstrated and isolated to these modifications.