Forecast Impact of Additional Radiosonde Ascents During the Year of Polar Prediction-Southern Hemisphere (YOPP-SH) Winter Campaign

Antarctic weather forecasts are less skillful than for other parts of our planet, in part due to limited conventional observations. The increasing intensity and scope of activities in this remote region motivate the need for better forecasts. The Year of Polar Prediction in the Southern Hemisphere (YOPP-SH) aims to improve weather prediction for Antarctica and the Southern Ocean. The project is centered around the winter Special Observing Period (SOP) that spanned April 15 - August 31, 2022. During the SOP, seven Targeted Observing Periods (TOPs) featured extra radiosonde launches to capture intense weather events, such as major oceanic cyclones and atmospheric rivers. To investigate the impact of these extra radiosondes in numerical weather prediction, NCAR performed experiments using Polar WRF in the Antarctic Mesoscale Prediction System (AMPS) framework. Two different simulation methodologies were performed. The first consisted of assimilating the standard set of observations into the initial conditions for Polar WRF, and these simulations are termed Routine. The second consisted of assimilating in addition to the standard observations, the extra radiosondes from the TOPs, called Extra. Two data assimilation approaches were used: Multi-Resolution Incremental-Four Dimensional Data Assimilation (MRI-4DVAR) and Three Dimensional Ensemble Data Assimilation (3DEnVAR). The forecasts generated covered all of the seven TOP periods, starting at each 0000 UTC and 12 UTC and had a duration of 5 days.

To quantify the benefits of the additional soundings during the TOPs, statistical analyses on Polar WRF forecast runs that were initialized using both MRI-4DVAR and 3DEnVar were performed. These were generated for both forecasts (Extra and Routine) and were validated against ECMWF 5th generation reanalysis (ERA5) for geopotential height, temperature, wind speed, and specific humidity throughout the troposphere and lower stratosphere. Overall, the extra soundings improve model forecast skill across the Southern Ocean. The positive impacts are much more prominent for latitudes south of 60°S, paralleling the findings from the austral summer YOPP-SH campaign. Ongoing research is verifying the impact of additional soundings for all the seven YOPP-SH TOPs with an additional focus on the main events.