Numerical Simulations And Predictions On The High And Low Temperature Events On The Shanghai-Nanjing Expressway

With the rapid development of society and economy in China, the highway construction has been accelerating and the mileage has been increasing. Meanwhile, the rate of the traffic accidents caused by the unfavorable meteorological conditions on the highway has been rising rapidly and it has brought great loss to the lives and property of people. In order to reduce the loss and ensure the traffic safety on the highway, it seems very important to ascertain the formation mechanism of the high and low temperature stress along highways and develop their accurate meteorological forecasting techniques.Based on the real-time data monitored by the automatic meteorological monitoring system (AMMS) on the Shanghai-Nanjing Expressway, In this paper, some high and low temperature weather events were filtered out and simulated by using WRF 3.2 (Weather Research and Forecast for Version 3.2),a new generation of mesoscale numerical forecasting model. A set of researches was showed that the simulation results were satisfactory after the verifications from the observed values. After the analysis on the simulations and observations, the physical mechanisms of the engendering of these high and low temperature weather events were explained and the influence of the underlying factors under the special environment of the highway on the formation of high and low temperature stress were discussed. Consequently, the forewarning parameters of the high and low temperature weather on the Shanghai-Nanjing Expressway were extracted and the forecast models at some stations were established.The results of numerical simulation and statistical analysis on the filtered 30 typical high temperature weather events showed as followed:(1) The selective simulations on the high temperature weather along the Shanghai-Nanjing Expressway by using WRF model were feasible. (2)After the statistical analysis on the output physical variables from the WRF model,the numerical forewarning indices of the high temperature weathers on the Shanghai-Nanjing Expressway were extracted as follows:the surface temperature at 14:00 (BJT) on the previous day of the predicted day is above 40℃, the latent heat flux on the ground at 14:00 (BJT) on the previous day of the predicted day is above 350 W·m-2, the air relative humidity near the ground at 14:00 (BJT) on the previous day of the predicted day is below 60%, the sensible heat flux at the surface at 08:00 (BJT) on the predicted day is negative and its absolute value is above 70 W·m-2, and the horizontal wind speed on ground at 08:00 (BJT) on the predicted day is below 3m·s-1. (3)Based on the above numerical forewarning indices, the forecasting models of the high temperature weathers at Meicun Station and Heyang Station were established. The air temperature values at 14:00(BJT) per day from July to August in 2010 were predicted by the established regression equations and it was indicated that the forecast accuracy of the forecasting models was satisfactory.The diagnostic analysis on the simulation results of the high temperature weather process from Jul.4th to Jul.6th in 2008 indicated that:(1) The effect of the temperature advection on the warming at the ground was very small in this high temperature weather event. (2) When the air was stable over Jiangsu province and there was strong sinking movement in the vertical direction, it was favorable to the forming of high temperature weather. (3) In this high temperature weather process, the low relative humidity in the high and low level of the atmosphere, the small horizontal wind speed at the ground and the strong downward long-wave radiation were key factors that resulted into a rapid rising of the air temperature in the studied area. (4) The influence of topography on the high temperature weather along the Shanghai-Nanjing Expressway was mainly reflected from some foehn effects and the impact of different land-use types and the change of air temperature at the surface was displayed by the different values of the obtaining sensible heat and the releasing latent heat during the high temperature weather process occurred.Considering the impact of topographic characteristic of underlying surface, the sensitivity tests of coupling different land surface schemes (SLAB, NOAH and RUC) to the WRF model were conducted. The results were described as followed:(1) The integrated simulations coupling the land surface parameterization schemes to WRF model were closer to the reality and the sensitivity of the high temperature weather simulations to the land surface physical processes was significant. The simulated temperature values of Meicun Station based on the three different land surface schemes were close to the observed values and when the NOAH scheme was coupled into the WRF model, the prediction accuracy of the simulation on the air temperature was highest. (2) From the simulations of coupling the three different land surface schemes, it was showed that the sensible heat flux was high and the latent heat flux was low along the Shanghai-Nanjing Expressway. The integrated simulation of coupling NOAH scheme was more satisfactory. (3) The vertical movement simulated by the WRF model was sensitive to the selection of different land surface schemes and NOAH scheme gave more reasonable results than the others. (4) In the simulations on the relative humidity, there were some differences between each other for the three schemes and the SLAB scheme had more ideal effect than the NOAH scheme or the RUC scheme. (5) The horizontal wind speed at ground surface simulated by the three land surface schemes was small, which was conducive to the emergence of high temperature stress at the surface. The simulation to wind speed by WRF model was sensitive to the selection of different land surface processes. Results simulated by the NOAH scheme had a better effect than the others.The numerical simulations and statistical analysis on 30 typical low temperature weather events showed that:(1) The selective simulations on the low temperature weather along the Shanghai-Nanjing Expressway by using WRF model were feasible. (2)The numerical forewarning indices of the low temperature weathers on the Shanghai-Nanjing Expressway were extracted as follows:the surface temperature at 05:00 (BJT) on the day before the predicted day along the Shanghai-Nanjing Expressway is below -3℃, the temperature advection value on 850hPa at 05:00 (BJT) is below -1×10-3K/s, the long wave flux downward at ground surface at 05:00 (BJT) is below 230W·m-2, the relative humidity near the surface at 23:00 (BJT) on the day before the predicted day is below 45%, the upward heat flux at ground surface is above -50 W·m-2 and below 0 W·m-2 at 23:00 (BJT). (3)Under the assistance of SAS (the Statistical Analysis System), an statistical analysis software, the forecast models of the low temperature weathers at Yuqi Station and Zhenjiang Station were established with a multiple linear regression method. The air temperature values at 05:00 (BJT) per day in January of 2010 were predicted by the established regression equations and it was indicated that the forecast accuracy of the forecasting models was satisfactory.The observed data from AMWS and the numerical simulations of a low temperature weather process from Jan.23th to Jan.25th in 2008 indicated that:(1) Jiangsu Province was in the wind field of northwest airflow in the upper and lower air and the northern cold air was continuously transmitted to Jiangsu. As a result, the air temperature dropped significantly. (2) There was strong cold advection in the southern part of Jiangsu and the regions along the Shanghai-Nanjing Expressway were controlled by the cold advection.The relatively stable thermal background was provided for the formation, development and maintenance of the low temperature weather by the cold advection. (3) There was hardly vertical movement in the lower troposphere, so the adiabatic cooling was not the main reason why caused the engendering of this low temperature weather event. (4) The relative humidity was low in the high and low level of the atmosphere in the simulated area. The wind speed at ground surface was small, so the heat exchange between the upper and lower layers of the atmosphere was not significant, which was favorable to radiation cooling at night.By the reasonable selecting of the parameterization and land physical process schemes, this study was innovative to simulate the high and low temperature weather events under the complicated weather backgrounds on Shanghai-Nanjing Expressway by using WRF model combined the real-time data from AMMS with NCEP reanalyzed meteorological data. Considering the weather backgrounds of the different scales, it was objective and comprehensive to diagnose the simulated fields of different physical variables, explain the physical mechanisms of the high and low temperature weathers engendering in the special periphery environment of the Expressway and discuss the effect of underlying factors such as topography, water systems, vegetation, land use types in these processes using DEM model in ArcGIS software. Under the assistance of WRF model, a set of the sensitivity tests for terrain, land surface physical processes and land use types in the high and low temperature weather events were used to verify their effect quantitatively. This study is accurate, systematic in the method and explicit in the physical mechanism and it will provide an important scientific reference for the development of traffic meteorological forewarning and forecasting on highways in China.