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JCSDA Partner Activities Toward the Assimilation of GPM Observations: GPM Data Assimilation Status and Plans: Update from NASA/GMAO
From the Fall 2014 issue of the JCSDA Quarterly
Scientists at the Global Modeling and Assimilation Office (GMAO) at NASA Goddard Space Flight Center are investigating the assimilation of Global Precipitation Mission (GPM) Microwave Imager (GMI) radiance measurements. Current efforts are expanding the all-sky (clear, cloudy, and precipitating) data assimilation methodology under development in the GSI system while also determining how the system is reacting to the assimilation of only clear-sky observations. The ultimate goal is to routinely assimilate these data to improve weather forecasts including severe storms and hurricanes as well as to improve global cloud and precipitation analyses.
The all-sky methodology has been implemented and extended to use GMI radiances. Since the observations have sensitivity to both clouds and precipitation, the existing infrastructure in the GSI has been extended to be more directly compatible with the GEOS-5 system. First, the GEOS-5 moist physics have been linearized and implemented as part of the GSI solution. Second, the analysis has been extended to include cloud ice and liquid as control variables. These directly relate to the prognostic cloud variables of the GEOS-5 model as well as the linearized physics.
Initial results have focused on a test case using Hurricane Arthur, which reached hurricane status on July 3, 2014, and was the first Atlantic hurricane to be observed by the GPM Core Observatory satellite. The normalized 37 GHz polarization difference, which anti-correlates to observed cloud liquid water path, is shown in Figure 1a. By comparing the observations to the modeled cloud water in Figure 1b, it is seen that in multiple regions of the Gulf, clouds are either displaced or missing. Also, it is seen that the model underestimates the liquid cloud amount in Hurricane Arthur, located off the coast of South Carolina. By incorporating the GMI radiances under all-sky conditions, it is seen in Figure 1c that the analysis is indeed moving toward the observations via the generation of clouds in the analysis increment of cloud water. While this is only a single case, further investigation is ongoing to ensure that these increments are done in a manner that is physically consistent with the mass and wind fields.
Figure. The (a) normalized polarization difference of the GPM/GMI 37 GHz bands, (b) background cloud liquid water path, and (c) corresponding analysis increment of cloud water projected into cloud liquid water path space for the GPM orbit observing Hurricane Arthur at analysis time 1200 UTC on 3 July 2014.
Additionally, the reaction of the system to the clear-sky measurements from GMI and the TRMM Microwave Imager (TMI) is being considered. This effort has found that these observations have a net drying effect, which is resulting from a skewed distribution of the difference between the observations and the background fields. This has led to a reevaluation of quality-control procedures that have been extended from heritage microwave imagers.
With the convergence of these efforts and further advancements in data assimilation methodology (e.g., 3D/4D Variational/Ensemble Hybrid systems, advances in observation error modeling), the GMAO is on the verge of being able to routinely assimilate these observations in real time. Deliverables including read routines and BUFRization tools have been coordinated with and delivered to JCSDA partners.
Will McCarty, Min-Jeong Kim, and Jianjun Jin, NASA/GMAO