Diagnostic analysis of a rare rainfall/snowfall and freezing process in Henan in early February 2024

From 31 January to 5 February, 2024, Henan Province experienced a rare freezing rain and snow process characterized by a long duration,substantial cumulative precipitation,significant snow depth,low temperatures,and complex precipitation phases.Based on the national meteorological observation data,dual-polarization radar data,and hourly reanalysis data of ERA5,a diagnostic analysis of the event's extremity,phase evolution,and causes of the freezing rain and snow was conducted.The results are as follows.During the process,5 national meteorological stations reached or exceeded the maximum snow depth of the historical records in February.At many stations,the minimum temperature has reached or set a new historical record in early February.Precipitation was observed from at least 68.3% of national meteorological observation stations in a single day,with the longest continuous precipitation at a single station being nearly 29 hours.The stable maintenance of a large-scale circulation system was one of the main reasons for this process.The persistent intrusion of the strong cold air in the lower troposphere provided both dynamic uplift and sustained cooling conditions for the freezing rain and snow process.Exceptionally strong southerly airflow provided ample moisture and momentum,enhancing unstable stratification.For precipitation phase identification,relying solely on a single-layer temperature is insufficient.Instead,the entire atmospheric layer temperature should be considered to predict possible precipitation phases.When the entire atmospheric layer temperature is below 0℃,precipitation is pure snow.If there is a melting layer or the cloud consists of supercooled water droplets,mixed precipitation will occur.Dual-polarization radar's correlation coefficient and differential reflectivity products are useful for distinguishing between sleet and pure snow.A correlation coefficient close to 1 and differential reflectivity between-1 and 0 dB indicates pure snow,while a correlation coefficient less than 1 and differential reflectivity greater than 1 dB indicate sleet.