Abstract
A dual-resolution, hybrid, three-dimensional ensemble-variational (3DEnVAR) data assimilation method combining static and ensemble background error covariances is used to assimilate radar data, and pseudo-water vapor observations to improve short-term severe weather forecasts with the Weather Research and Forecast (WRF) model. The higher-resolution deterministic forecast and the lower-resolution ensemble members have 3 and 9 km horizontal resolution, respectively. The water vapor pseudo-observations are derived from the combined use of total lightning data and cloud top height from the Fengyun-4A(FY-4A) geostationary satellite. First, a set of single-analysis experiments are conducted to provide a preliminary performance evaluation of the effectiveness of the hybrid method for assimilating multisource observations; second, a set of cycling analysis experiments are used to evaluate the forecast performance in convective-scale high-frequency analysis; finally, different hybrid coefficients are tested in both the single and cycling experiments. The single-analysis results show that the combined assimilation of radar data and water vapor pseudo-observations derived from the lightning data is able to generate reasonable vertical velocity, water vapor and hydrometeor adjustments, which help to trigger convection earlier in the forecast/analysis and reduce the spin-up time. The dual-resolution hybrid 3DEnVAR method is able to adjust the wind fields and hydrometeor variables with the assimilation of lightning data, which helps maintain the triggered convection longer and partially suppress spurious cells in the forecast compared with the three-dimensional variational (3DVAR) method. A cycling analysis that introduced a large number of observations with more frequent small adjustments is able to better resolve the observed convective events than a single-analysis approach. Different hybrid coefficients can affect the forecast results, either in the single deterministic or cycling analysis experiments. Overall, we found that a static coefficient of 0.4 and an ensemble coefficient of 0.6 yields the best forecast skill for this event.
Highlights
The accuracy and timeliness of severe weather forecasts are critical to safeguard life and property
Numerical weather prediction (NWP) models still face many challenges in accurately forecasting high-impact weather events such as the existence of biases and errors contained in the initial conditions, which are often derived or downscaled from
A severe convective event associated with the Meiyu front on 30 June 2018, which occurred in the middle and lower reaches of the Yangtze River, was selected to examine the performance of the lightning and radar data assimilation (LRDA)
Summary
The accuracy and timeliness of severe weather forecasts are critical to safeguard life and property. Numerical weather prediction (NWP) models still face many challenges in accurately forecasting high-impact weather events such as the existence of biases and errors contained in the initial conditions, which are often derived or downscaled from 4.0/). Reducing the initial condition biases and errors through data assimilation is crucial to improve forecast skill [3,4,5]. Lightning data from ground-based networks and spaceborne optical instruments are able to identify areas of deep, mixed-phase convection [6,7,8,9,10]. The comparison analysis of lightning data and radar echoes suggests that lightning data can be used to determine the convective activity and its development probability and intensity [7].
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