School of Meteorology (Defense)

Assimilation of radar and surface observations of a developing convective system: Observing system simulation and real-data experiments.

Ryan Sobash
OU School of Meteorology

13 November 2013, 3:00 PM

National Weather Center, Room 4140
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Convective-scale observing system simulation experiments (OSSEs) and real-data experiments were performed to study the impact of radar and surface observations on analyses and forecasts of convective systems using the Weather Research and Forecasting (WRF) model with an ensemble Kalman filter (EnKF). The OSSEs were performed to assess the impact of covariance localization of radar data on the analyses of a developing convective system. Increasing the horizontal localization and decreasing the vertical localization produced analyses with the smallest RMSE for most of the state variables. Prior state correlations between the observations and state variables were used to construct reverse cumulative density functions (RCDFs) to identify the correlation length scales for various observation-state pairs. The OSSE with the smallest RMSE employed localization cutoff values that were similar to the horizontal and vertical length scales of the prior state correlations. Vertical correlations were restricted to state points closer to the observations than in the horizontal, as determined by the RCDFs. Further, the microphysical state variables were correlated with the reflectivity observations on smaller scales than the three-dimensional wind field and radial velocity observations.

As a complement to the OSSEs, the WRF model and the EnKF were again employed to produce analyses and forecasts for the 29 May 2012 convective episode. The frequent assimilation of surface data, especially the use of mesonet data, improved the forecast of CI timing and placement within the domain, especially for convection developing along a surface dry line. Surface data assimilation reduced a surface moisture bias that was present due to model error. Experiments where mesonet data were withheld, or where surface data were assimilated less frequently, produced less accurate forecasts of CI and possessed larger surface moisture errors. The ability of sub-hourly assimilation of mesonet data to improve forecasts of CI has not been previously documented.

Ensemble forecast initialized after two hours of radar and surface data assimilation captured much of the observed convective evolution, including the tracks of several long-lived supercells. Surface data assimilation played a significant role in producing the successful forecasts. Forecasts from an experiment that assimilated only radar data contained several large errors due to a poor representation of the mesoscale environment. Some parts of the forecast were especially sensitive to the assimilation of reflectivity observations and the vertical localization of those observations. Finally, using innovation statistics, several reflectivity biases were identified in the analyses due to errors in the microphysics parameterization, the reflectivity forward operator, and biases in the environmental wind profile. Together, the OSSEs and real-data experiments provide a benchmark for the ability of EnKF systems to produce analyses and forecasts for more complex convective events.

For accommodations based on disability, or more details, please contact the School of Meteorology at 325-6561. All visitors without NOAA or University of Oklahoma identification must register at the registration desk on arrival. Visitor parking is available for all University visitors. However, faculty/staff/students must have a current multi-purpose parking permit. Additional parking is available at the Lloyd Noble Center (LNC) for those individuals who do not have a parking permit. You do not need a permit to park in one of 1,200 spaces reserved for CART bus riders, although you must ride the CART shuttle to park in the reserved area. This area is on the north central side of the Lloyd Noble Center. Elsewhere at the LNC, permits are required.

The University of Oklahoma is a smoke-free / tobacco-free campus.

School of Meteorology (Defense) Seminar Series website