Characteristics of the Waterspout in East Dongting Lake on 13 August 2017

Abstract

Based on conventional weather data, automatic weather station data, and the observation of Yueyang Doppler radar, a waterspout occurred in Bianshan waters of East Dongting Lake (Bianshan waterspout for short) on 13 August 2017 is analyzed. Results show that the cold and warm airflows converge in the East Dongting Lake area when the upper East Asian trough forces the cold air southward, and the subtropical high guides the southwestern warm moist flow northward. The quasi stationary front over the north central Hunan Province is northeast to southwest, forming an "S" curve, which is favorable for the convergence of frontal instability energy to the East Dongting Lake area. The special geographical environment is easy to trigger canyon effect, which often leads to increased wind speed and humidity. The strong divergence in front of the upper trough, the deep low-pressure shear from northeast to southwest in the middle and lower layers, strong cyclonic convergence in the boundary layer, and the special topography jointly form a strong convergent upwelling flow field. When three meso-gamma-scale low eddies on the ground move northward to Bianshan waters, influenced by combined effects of the above flow field and the front and back vortices, the second vortex strengthens rapidly and forms a waterspout. The meteorological factors such as wind speed, wind direction, air pressure and visibility recorded by the lighthouse automatic meteorological station in the lake center change significantly when the waterspout passes, while precipitation is only 0.2 mm. Yueyang Doppler radar shows that the centroid of heavy precipitation is low to the north of strong convergence zone, where shear of strong wind is moderate and the radial wind speed over the shear is low. Yueyang Doppler radar wind profiles show that mesocyclone at the height of 0.6 km and the convergent flow fields near the ground at the height of 0.3 km are superimposed when the waterspout formed at 0905 BT. Waterspouts in the southern convergence zone have no storm tracking information, mesocyclones or tornado-type vortices. However, heavy precipitation accompanied by strong subsidence and convergence at the middle and low altitudes often produce both rising and subsidence currents, which are obviously unfavorable for the formation and development of waterspouts that need huge upward pumping. Comparing and analyzing waterspout processes of the Shengjin Lake in Anhui Province and the Dongting Lake in Hunan Province, it is concluded that the funnel-shaped strong lift suction caused by large-scale divergence at high altitude and the deep low-pressure shear from northeast to southwest in the middle and lower layers, and the intense convergence of cyclones and surface cyclones in the boundary layer are the main causes of the waterspout formation.

Keywords

• high-altitude divergence
• mesocyclones in the boundary layer
• strong pumping and suction