一次晚春降水相态变化特征及成因

Characteristic of Phase Conversion and Its Cause on a Rainfall Process in Late Spring

应用常规观测资料、自动站资料、雷达资料及1°×1°NCEP再分析资料,对2013年4月19日河北省一次晚春回流降水相态变化特征及成因进行了分析。结果表明:河北中南部地区相继受东西2股冷空气影响,前期东路冷空气从850 hPa以下入侵,之后西路冷空气随700 hPa高空槽东移加剧了中南部地区的降温,使大范围降水出现相态变化;地面温度对本次降水过程的雨雪相变指示性不大,但地面气压的变化早于地面温度,对预报具有一定的指示意义;700 hPa低空急流对中空暖层的形成起决定作用,当暖层消失,降水相态完全转为降雪,925 hPa上-2℃温度特征线与降雪区对应较好,通过温度的垂直结构来辨别相态更为可靠;雷达回波的0℃层亮带出现高度在4.7 km左右,其高度的快速降低与降水相态转变时间一致。

There was an obvious rairrfall process with the typical return flow on April 19, 2013 in the middle and south of Hebei Prov- ince, which had a complicated conversion in precipitation palterns. The spatial distribution of precipitation palterns was rain in the north and snow in the south of Hebei Province, which was rare in the history. Thus, based on the conventional observation data, auto- matic meteorological obsewation data, Doppler rarlar data and reanalysis data from NCEP at 1° × 1° spatial resolution, the variation characteristic of the precipitation phase in late spring and the cause of phase transformation were analyzed. The results showed that two cold air from the east and west directions affected the central and south of Hebei Province. The cold air from the east path invaded mairily below 850 hPa in earlier stage, while the cold air from the west path intensified temperature going down in the central and south region with lower trough onT00 hPa moving to the east,which resulted in the precipitation types conversion in larger regions. The indi- cation of the temperature above the ground 2 meter for the precipitation types conversions was limited because of the wide ranges of tem- perature from -4 ℃ to 7 ℃, but the change of ground pressure was earlier than that of the temperature, thus the change of pressure was more meaningful to forecast the temperature going down. The low- level jet on 700 hPa played a decisive role in the formation of the middle - level warm layer, where the solid particles of rairrfall could be mehed to liquid. On the contrary, the rain converted into the pure snow when the warm layer disappeared. The - 2 ℃ isotherm of 925 hPa corresponded better with the snow area, so it was re- liable to distinguish the phase of rairrfall by the vertical profile of the temperature. Furthermore, the bright band of the 0 ℃ layer the rain and snow mixture appeared over about 4.7 kin, and the time of quickly declining of the 0 ℃ layer height was corresponding with the time of phase conversion of rairrfall. Therefore, the change of the 0 ℃ layer height considered as an indicator of the phase conver- sion of rainfall.