Improving Flash Flooding Forecast with the Assimilation of PBL Depth Observations from the NexRAD Network: The July 2022 East Kentucky Flood

The structure of the planetary boundary layer (PBL) are important for initiation, development, and organization of convection. High-spatiotemporal-resolution network that directly observe the PBL structure is unavailable now. However, recent studies discovered that clear-sky ZDR observations often show distinct features at the top of the PBL, therefore, they can be used to estimate the depth of the PBL when presented in quasi-vertical profiles (QVPs). The NexRAD network has previously never been systematically used to describe the PBL structure. We have developed an algorithm to detect the PBL depth from ZDR QVPs of the NexRAD Network, and the products are assimilated using the PSU WRF-EnKF system to examine their impact on the forecasts of the flash flooding event happened in east Kentucky in July 2022.

The model configuration in the experiments mimics the operational HRRR, because HRRR notably underestimated the accumulated rainfall in east Kentucky during this event, which is associated with insufficient moisture transport by the southwesterlies in the lower troposphere and the PBL. Ensemble analyses confirm correlations between the total accumulated rainfall and low-level moisture transportation. Additionally, ensemble analyses also show correlations between the PBL depth at various NexRAD locations and the low-level moisture transportation in several key regions. As a result, with an improved match of the PBL depth bewteen the model and the observations after assimilating these observations, low-level moisture transportation is notably changed in the upstream region of the flash flooding, which eventually lead to notably improved rainfall forecasts.