Analysis of Weather Element Characteristics and Air Pollution Status during Continuous Fog Days in Liaoning

[Objective] The aim was to discuss the formation, duration and disappearance of fog. [Method] Based on the weather data and physics of Liaoning Province, and considering the continuous 4 foggy days in 11 places in Liaoning from November 29 to December 2, 2009, detailed analysis of this consecutive foggy days was given proceeding from the weather situation, element characteristics, especially the occurrence, duration and disappearance of dense fog. [Result] This dense fog was because that Liaoning was in the saddle pressure field. The weak cold air divergence set the ground in many weak mild scale convergence areas. The high temperature in the early time resulted in melting snow and long duration of dense fog days, along with radiation low temperature and southwest convergence transmission. In the meantime, because of the existence of small wind speed and existence of upper inversion temperature layer, it was inconvenient for the divergence of wet air; in addition, there was dense fog in north China. The upper southwest airstream transmitted wet air in the north China to the air of Liaoning, increasing the intensity of dense fog in Liaoning. [Conclusion] The study offered reference for the further forecast of air pollution.

THE DESIGN AND CORRECTION OF A QUANTITATIVE METHOD OF SNOW ESTIMATE BY RADAR

An optimization method is based to design a snowfall estimate method by radar for operational snow warning, and error estimation is analyzed through a case of heavy snow on March 4, 2007. Three modified schemes are developed for errors caused by temperature changes, snowflake terminal velocity, the distance from the radar and calculation methods. Due to the improvements, the correlation coefficient between the estimated snowfall and the observation is 0.66(exceeding the 99% confidence level), the average relative error is reduced to 48.74%, and the method is able to estimate weak snowfall of 0.3 mm/h and heavy snowfall above 5 mm/h. The correlation coefficient is0.82 between the estimated snowfall from the stations 50 to 100 km from the radar and the observation. The improved effect is weak when the influence of the snowflake terminal velocity is considered in those three improvement programs, which may be related to the uniform echo. The radar estimate of snow, which is classified by the distance between the sample and the radar, has the most obvious effect: it can not only increase the degree of similarity, but also reduce the overestimate and the undervaluation of the error caused by the distance between the sample and the radar.The improved algorithm further improves the accuracy of the estimate. The average relative errors are 31% and 27% for the heavy snowfall of 1.6 to 2.5 mm/h and above 2.6 mm/h, respectively, but the radar overestimates the snowfall under1.5 mm/h and underestimates the snowfall above 2.6 mm/h. Radar echo may not be sensitive to the intensity of snowfall, and the consistency shown by the error can be exploited to revise and improve the estimation accuracy of snow forecast in the operational work.

Effect of Mesoscale Land Use Change on Characteristics of Convective Boundary Layer:Semi-Idealized Large Eddy Simulations over Northwest China

Although large-scale topography and land use have been properly considered in weather and climate models, the effect of mesoscale and microscale heterogeneous land use on convective boundary layer(CBL) has not been fully understood yet. In this study, the influence of semi-idealized strip-like patches of oases and deserts, which resemble irrigated land use in Northwest China, on the CBL characteristics, is investigated based on the Weather Research and Forecasting(WRF)-large eddy simulation(LES) driven by observed land surface data. The influences of soil water content in oases on aloft CBL flow structure, stability, turbulent kinetic energy(TKE), and vertical fluxes are carefully examined through a group of sensitivity experiments. The results show that secondary circulation(SC)/turbulent organized structures(TOS) is the strongest/weakest when soil water content in oases is close to saturation(e.g.,when the oases are irrigated). With the decrease of soil water content in oases(i.e., after irrigation), SC(TOS) becomes weak(strong) in the lower and middle CBL, the flux induced by SC and TOS becomes small(large), which has a dramatic impact on point measurement of eddy covariance(EC) fluxes. The flux induced by SC and TOS has little influence on EC sensible heat flux, but great influence on EC latent heat flux. Under this circumstance, the area averaged heat flux cannot be represented by point measurement of flux by the EC method, especially just after irrigation in oases. Comparison of imbalance ratio(i.e., contribution of SC and TOS to the total flux) reveals that increased soil moisture in oases leads to a larger imbalance ratio as well as enhanced surface heterogeneity. Moreover,we found that the soil layer configuration at different depths has a negligible impact on the CBL flux properties.