A Study On The Method Of Helicity Calculating Of Doppler Velocity

Heavy rains occur frequently in china, it is difficult to predict, because of their developing rapidly, mightiness intensive , causing suddenly, character being not significant. Observation and simulation show, wind whose vector turns clockwise with height is critical factor associated with storms. Helicity is the physical parameter that describes the rotation and motion of airflow in the direction of the movement in the wind field, and it in the atmosphere can be expressed as the temperature advection under some conditions, so the amplitudes and changes of helicity are a good indicator heavy rain and severe convective weather.The development of storms depend on air vertical movement, and the motion of widespread atmosphere usually combine macroscale convergence and divergence overlap of warm and cold advection under various wind speeds. The article retrieve average wind field by VAD(Velocity-Azimuth Display) method ranging of 30km, then calculate helicity of storm to describe warm and cold advection quantitatively, and extract convergence and divergence of low level by EVAD technology quantitatively. To combine the both is used to supervise widespread atmosphere. Because of spatial and time high resolution of Doppler radar, to develop software of calculating and showing helicity real time will improve nowcasting level.This paper analyzed storm helicity and divergence of heavy rain during plum rain on June 14,2004. By contrast, another general thunderstorm case occurring in Nanjing city on July 4, 2007 was compared and their different were discussed to find some forecast index of heavy rain and severe storm. The following conclusions can be drawn:(1) There is apparently a positive correlative relationship between storm helicity under 3km above the ground and reflective power, and exists negative correlative relationship between storm helicity and divergence (2) At onset of storm, there exists many layers of positive and negative helicity respectively and value are very little. With developing of storm, the value of storm helicity increase and layers decrease rapidly.(3) The layers of positive and negative helicity (warm and cold advection) increase and decrease alternately, this may be related to the process of unstable energy reconstruction during the development of storm. Analysis demonstrated , when storm helicity integrate with divergence, it is possible to describe warm and cold advection quantitatively and supervise the propagation and evolution of storm availability. Helicity is an important physical variable for nowcasting.