A moderately coupled land data assimilation method (MCLDA) implemented in the NOAA Operational Weather Prediction Models - RAP and HRRR

Initialization methods are needed for all components of earth-system models from medium-range to decadal predictions, and also for only few-hour forecasts in support of safety (e.g., severe weather) and economic (e.g., energy) applications. Strongly coupled land-atmosphere data assimilation (SCDA) producing balanced initial conditions across the land-atmosphere system has not yet been introduced to operational numerical weather prediction (NWP) models, and most NWP systems around the world have evolved separate data assimilation procedures for the atmosphere vs. land/snow system components. This method has been classified as weakly coupled data assimilation system (WCDA). In the NOAA operational weather models, a moderately coupled land/snow-atmosphere assimilation method (MCLDA) has been implemented, a step forward from the WCDA towards the SCDA. In this method soil state variables (soil temperature/moisture, snow temperature, snow water equivalent) are not included in the analyzed variables in the Gridpoint Statistical Interpolation (GSI) data assimilation system, as should be done in the SCDA. However, the atmosphere and land or snow conditions are both updated within the same data assimilation step using the same set of observations. Using this assimilation method, surface state variables have cycled continuously for 6 years since 2015 for the 3-km NOAA High-Resolution Rapid Refresh (HRRR) model and for 10 years since 2011 for the 13-km NOAA Rapid Refresh (RAP) model. The method has been modified and refined over these years to achieve more accurate initial state of soil and snow variables for better predictions in the first couple hours into the model simulation. To quantify the impact of MCLDA a set of month-long experiments were conducted with and without this MCLDA for both winter and summer seasons using the 13-km RAP model with atmosphere (50 levels), soil (9 levels) and snow (up to 2 layers if present) on the same horizontal grid. Improvements were evident for 2-m temperature for all times of day out to 6-12h for both seasons but stronger in winter. Better temperature forecasts were also shown in the 1000-800 hPa layer corresponding roughly to the boundary layer. The results from this study will be presented at the meeting.