GSI-based Hybrid Ensemble-Variational Data Assimilation for Global, Hurricane and Storm-scale Numerical Weather Prediction

Xuguang Wang
University of Oklahoma
8 August, 1 pm in 2155

GSI-based hybrid ensemble-variational data assimilation (DA) system including both 3DEnVar and 4DEnVar improved US NWS global forecast significantly. This seminar will discuss our recent research and development of the system for a variety of
scales including global, hurricane and storm scales.

A method which effectively increases the size of the ensemble incurring minimum cost is implemented in GSI-based 4DEnVar. Experiments with GFS configured similarly to the 4DEnVar pre-implementation test were conducted. It was found that the GFS forecast was further improved.

Research and development have also been made to extend the GSI-based hybrid DA system for regional modeling systems. The hybrid EnKF-Var DA system is extended with the operational HWRF modeling system to improve high-resolution, convection-allowing tropical cyclone prediction. An end-to-end, continuously cycled hybrid DA system was developed using a newly developed directed moving nest strategy. Experiments were conducted for hurricane Edouard (2014) in which all operational observations including conventional in-situ data, satellite wind, tcvital, satellite radiances, and tail Doppler radar observations were assimilated. It was found that: a) the dual resolution hybrid DA improved upon the coarser, single resolution hybrid; b) vortex initialization and relocation in the control and relocation of the ensemble background improved the forecasts; c) using 4DEnVar DA in the TDR-involved cycles improved the intensity forecasts for early lead times compared to 3DEnVar DA; and d) the hybrid system improved intensity forecasts relative to operational HWRF during the intensification period due to the alleviation of the “spin-down” issue because the hybrid better analyzed the structures of an intensifying storm. Issues associated with the vortex initialization will also be discussed.

The GSI-based hybrid DA system is also extended with the convection resolving WRF ARW and NMMB models for multi-scale assimilation of both conventional in-situ observations and radar observations to improve convective scale hazardous weather forecasts over the Continental US (CONUS). Systematic comparison of GSI based 3DVar and EnKF was first conducted in the context of multiscale data assimilation where scales ranging from convective scales to synoptic scales were resolved by both the model and the observations. The cases include many examples of both discrete cellular convection and organized Mesoscale Convective Systems (MCSs). It was found that convective scale precipitation forecasts initialized by GSI-based EnKF were much more skillful than GSI-based 3DVar. The positive impact of assimilating radar data lasted much longer in EnKF than 3DVar. Design, implementation and performance of the GSIbased convection-allowing ensemble based data assimilation and forecast system in support of the 2015 PECAN field experiment will also be discussed.

In addition, a new method to directly assimilate reflectivity observations was proposed and implemented in the GSI-based EnVar system. This new method avoids the use of the tangent linear and adjoint of the nonlinear reflectivity operator and therefore overcomes the issues of using hydrometeors or logarithm of hydrometeors in GSI variational minimization. The newly proposed method is examined on the analysis and prediction of the 8 May 2003, Oklahoma City, tornadic supercell storm at 2km model grid. Both the probabilistic forecast of a strong low-level vorticity and the maintenance of the strong updraft and vorticity in the new method are more consistent with the reality than in option-logarithm and option-mixing-ratio. Detailed diagnostics suggest that a more realistic cold pool due to the better analyzed hydrometeor mixing ratios in the new method than in other methods leads to the constructive interaction between the surface gust front and the updraft aloft associated with the mid-level mesocyclone.

Challenges, implications and future research under the NGGPS paradigm will be discussed at the end of the seminar.