Comparison of China Southwest Vortex Structures Based on AHI Observations as well as ERA5 and NCEP GFS All-sky Simulations

This study first modifies a conventional method for locating the center of the Southwest Vortex (SWV) and applies it to center identification in the fifth generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) and the Global Forecast System (GFS) analysis from the U.S. National Centers for Environmental Prediction for an SWV event on June 26-27, 2020. Significant discrepancies are found between the SWV trajectories in ERA5 and GFS. Secondly, according to observations from the Advanced Himawari Imager (AHI), a row of isolated clouds over the eastern Tibetan Plateau exhibits characteristics of mountain gravity waves and generates SWVs as they propagate eastward; the SWV related cloud/rain structure in cloud-top pressure and cloud type varies consistently with the observed lower brightness temperatures. Thirdly, using ERA5 and GFS as inputs to radiative transfer model, we generate all-sky simulation of AHI channel-13 brightness temperatures. On the synoptic scale, the ERA5 simulations more closely match the observed brightness temperatures, whereas on the small-scale cloud/rainband structure of the SWV, the GFS-simulated brightness temperatures more closely resemble observations. The ERA5 liquid water clouds extend to altitudes as high as the 140-hPa level, with temperatures as low as -32°C. The AHI liquid water cloud tops are located in the middle and upper troposphere, while the GFS liquid water cloud tops are confined to the lower troposphere.