Application research of wind profile radar in short-term heavy rainfall forecast of typhoon in Fujian Province

Abstract

Based on wind profile radar data, this paper aims at different typhoon processes landed and affected Fujian from 2011 to 2019, according to the nature of typhoon rainstorm, it can be classified into outer precipitation before typhoon landed, main body precipitation and precipitation at the rear of typhoon, the change of the characteristic quantities in approaching time of the occurrence of short-term heavy rainfall was analyzed, and the typhoon case in 2020 was back calculated. The results show that, the characteristics of low-level jet streams (maximum wind speed at low altitude, minimum height of jet streams, and low-level jet stream index), as well as the magnitude of vertical wind shear below 700 hPa, have important indicative significance for the occurrence of short-term heavy rainfall. (1) More than 80 % of short-term heavy rainfall occurred 3 h before the low-level jet stream already existed. The maximum wind speed below 2 km was basically close to a normal distribution, and the occurrence of heavy precipitation showed a bimodal pattern. The percentage of wind speed between 8 and 32 m/s was the highest, exceeding 85 %. The wind direction of the strong wind is mainly NE, SE, and SW. Classification analysis showed that the distribution characteristics of wind speed of the main precipitation were the same as before, but the wind direction SE was higher than NE. The wind speed of pre-landfall precipitation was basically skewed, and the occurrence time of heavy precipitation followed a normal distribution. The percentage of wind speed between 16 and 32 m/s was the highest, and the wind direction was the same as before classification. The maximum wind speed of precipitation in the rear was basically bimodal distribution, with a relatively even distribution, and the wind direction was mainly SE and SW. (2) In the 3 h before the occurrence of short-term heavy precipitation, there was an increase in the maximum wind speed value, a decrease in the minimum extension height, and an increase in the low-level jet stream index I. As short-term heavy rainfall approached, the intensity of the low-level jet stream remained high and its proportion increased. The minimum achievable extension height gradually decreased and remained stable at a low value. In the first 2 h of heavy rainfall, the wind speed reached its maximum, the extension height was the lowest, and the low-level jet stream index I was the highest. Classifying and discussing it, the precipitation in the rear was different, and the lowest height decreased to the lowest at the time of occurrence, at which point the I value reached its maximum. The characteristics of the other two categories were the same as before the classification. (3) The vertical wind shear from the ground to different isobaric surfaces gradually decreased with the increase of height. With the approach of short-term heavy rainfall, the vertical wind shear of each layer basically decreased gradually, after the beginning of heavy rainfall, which decreased to the minimum. The characteristics of main body precipitation were the same as before the classification. Pre-landfall precipitation, in addition to the gradual decrease of vertical wind shear from the ground to 925 hPa, both 850 hPa and 700 hPa increased first and then decreased, vertical wind shear decreased to the minimum after the beginning of heavy rainfall. Precipitation at rear of typhoon, vertical wind shear from ground to 700 hPa increased compared with that before the occurrence of heavy rainfall, while wind shear from ground to 925 hPa and 850 hPa showed the characteristics of decreasing when heavy rainfall occurred. (4)The median values of various physical quantities before the occurrence of typhoon short-term heavy rainfall were selected as the thresholds of short-term heavy rainfall will occur. The intensity of LLJ is about 21 m/s, the lowest height is about 0.65 km, the LLJ index I is about 36 × 10⁻³s⁻¹. Vertical wind shear from the ground to 925 hPa, 850 hPa and 700 hPa are respectively about 15.9 × 10⁻³s⁻¹, 11.2 × 10⁻³s⁻¹ and 5.1 × 10⁻³s⁻¹.