The Characteristics And Mechanism Of Diurnal Variations Of Low-level Jet And Heavy Rainfall Over Yangtze-Huai River During June 2011

This study first investigates the synoptic background and horizontal distribution of rainfall during early summer (June and July) by analyzing the ERA-interim reanalysis data and surface automatic weather stations in 2007-2013. It is found that precipitation associated with the southwest vortex accounts for about 52.7% of the total rainfall amount and also 53.1% of the occurrence of heavy rainfall (>50mm/6h). Then analyzing the five heavy rainfall events along the Yangtze River during June 2011, it is found that they all happen under the synoptic background with southwest vortex, meiyu front, southwest low level jet (LLJ) and the western Pacific subtropical high (WPSH), and they all have plenty of water vapor,instability energy and large scale uplift conditions.At the same time, the LLJ and the rainfall have the same evident diurnal variations in the 5 processes, that is being stronger in nocturnal-to-early moring hours and weaker in the afternoon hours. Then analyzing the vertical structure characteristics of the LLJ, it is found that vertical profiles of the horizontal wind speed during the 5 processes have significant shear with a peak around 800-850hPa during the initial stage of the precipitation. With the development of the precipitation and the deepened low trough and the intesified WPSH, the horizontal wind speed of LLJ is significantly intensified in the middle troposphere.The diurnal variations of the LLJ are mainly attributed to the diurnal variations of ageostrophic winds. Then analyzing the surface mesoscale characteristics of the fourth process, it is found that the convectively generated cold outflows play little important roles in initiating and maintaining the rainfall, but it can force the warm moist air to lift along the slope of it, in order to promote development of the convection.Then a convection-permitting simulations (CTL) is carried out using WRF model for the fourth rainfall event which successfully reproduces the observed evolution of precipitation and weather systems and the diurnal variations of rainfall and LLJ. Then three sensitivity simulations (SEN) are also performed:(1) make the solar heating hold off for 12 hours; (2) remove the terrain; (3) turn off the latent heating and cooling.The major conclusion are as follows:(1) the diurnal variations of the LLJ and rainfall is mainly caused by the solar heating; (2) the diurnal variation of the southwestly LLJ is mainly attributed to inertial oscillation at nighttime and vertical mixing in the boundary layer during daytime; (3) The terrain can make the LLJ, low vortex and pricipitation weak by strengthening its friction,and it can make a little contribution to the diurnal variation of rainfall; (4) the latent heating and cooling can enhance the surface precipitation, and it is necessary for the maintain and development and movement of the low vortex.Morever, it can enhance the LLJ in order to promote the movement of the low vortex; (5) horizontal scales of the low-level wind field and relative vorticity are separated using a two-dimensional spectral decomposition based on Fast Fourier Transformation (FFT). The wind field is mainly manifested in the scale of L>200km, but the relative vorticity is mainifested in the both scale of 50km<L<200km and L>200km. The results show that the large scale characteristic of the fourth rainfall is the southwest monsoon, and the mesoscale characteristic is the vortex on the axis of jet stream.