Vol.21 No.1 JOURNAL OF TROPICAL METEOROLOGY March 2015 Article ID: 1006-8775(2015) 01-0092-09 THE DESIGN AND CORRECTION OF A QUANTITATIVE METHOD OF SNOW ESTIMATE BY RADAR JIANG Da-kai (蒋大凯)1, 2, MIN Jin-zhong (闵锦忠)1, CAI Kui-zhi (才奎志)2 (1. Nanjing University of Information Science & Technology, Nanjing 210044 China; 2 Shenyang Central Meteorological Observatory, Shenyang 110016 China) Abstract: 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 is 0.82 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 under 1.5 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. Key words: weather forecast; radar-based snowfall estimate; optimization technique; correlation CLC number: P412.25 Document code: A mm/h meets the standard of meteorological disasters. 1 INTRODUCTION Currently, however, the automatic meteorological ob- Heavy snow has great impacts on people's lives serving station is unable to provide 1-h snowfall in win- and social economy and is a disastrous weather condi- ter, and the manual observation can provide 3-h or 6-h tion of high social concern. Heavy snow has happened cumulative snowfall only in some stations, which cannot frequently in the southern part of northeast China in re- fulfill the need of operational snow early warning. With cent years, especially for the heavy snow rarely paral- construction of the meteorological station networks in leled in history in 2007, 2009 and 2010. "Meteorologi- China, the Doppler radar has covered most provinces in cal Disaster Warning Signal and Prevention Guide" of East China. Products of the Doppler radar can provide a "Procedures on the Release and Dissemination of Early wide range of precipitation information in a short time. Warning Signals for Meteorological Disasters" (Decree Therefore, products of the new-generation Doppler radar No.16 of China Meteorological Administration) speci- can be applied with combination of conventional obser- fies that the heavy snow warning signal should be re- vation data to investigate the radar-based snowfall quan- leased when the snowfall intensity within 12 h is above titative estimate technique in order to fulfill the need of 4 mm. That is to say, the snowfall intensity above 0.3 operational snow early warning. Radar-based snowfall quantitative estimate tech- Received 2013-10-15; Revised 2014-12-01; Accepted 2015-01-15 niques have been explored by many foreign scholars. [1] Foundation item: Program for Key Fundamental Research of Boudala et al. developed ice water content and precipi- China (2013CB430102); Specialized Project for Forecasters from tation rate retrieval algorithms as a function of tempera- China Meteorological Administration (CMAYBY2012-012); ture and radar reflectivity factor using ice particle spec- Specialized Project for Public Welfare Sectors of Industry from tra measured in stratiform ice clouds in mid-latitude and CMA (GYHY201006001); Project for Research on Agricultural Arctic regions. Wandishin et al. [2] produced short-range Science and Technology, Bureau of Agriculture, Liaoning ensemble forecasts of different precipitation type, which Province (2011210002) is very skillful in forecasting rain and snow but it is on- Biography: JIANG Da-kai, Ph. D. candidate, forecaster, primari- ly moderately skillful for freezing rain and unskillful for ly undertaking research on snow forecast. ice pellets, with an advantage of being able to discrimi- Corresponding author: MIN Jin-zhong, e-mail: [email protected] nate between different precipitation types. Gray et al. [3] du.cn No.1 JIANG Da-kai (蒋大凯), MIN Jin-zhong (闵锦忠), et al. 93 summarized the snow Z-I relationship for different tem- to determine a Z-I relationship suitable for every snow perature and different form of snow. For Chinese re- process. So how should one design a snowfall estimate searchers, most attention has been paid to the rainfall scheme by radar for operational use? According to the rather than the snowfall in the aspect of radar-based current status of the operational snow early warning, the quantitative estimate technique (Ji et al.[4]; Zhang et al.[5]). observed snowfall is the hourly manual observation, Kuang et al. [6] estimated the snowfall of a snowstorm while the radar completes a volume scan in every 6 weather process that occurred in Shijiazhuang city in minutes. Therefore, relationship can be constructed be- 2009 with radar data and the Kalman filter method, in- tween the 1-h snowfall of every station and the radar e- dicating that this method is suitable for large-range con- cho, which is used for snowfall estimate in the next tinuous snowfall estimate, but not suitable for short-term hour. For the precondition, it is assumed that influenc- local snowfall estimate. Jiang et al. [7] analyzed the fea- ing factors of snow estimate, such as crystal structure tures of Doppler radar products of heavy snow process- and degree of freezing and aggregation, remain the es in the southern Northeast China, uncovering that 1-h same in an hour. The specific method is using an opti- snowfall correlates positively with snow reflectivity and mization method. Firstly, a Z-I relationship is assumed, that the echo intensity of heavy snow is usually less which is changed every hour in order to get the best [8] than 30 dBZ. Zhang et al. showed that the value of b consistency of hourly snowfall Hi estimated by radar e- is 1.6 and the value of a is 1 000 to 1 500 in the Z-I cho to the observed hourly snowfall Gi. Besides, the de- relationship of radar echo. In this paper, an optimization gree of consistency depends on the discrimination func- method is based to design a snowfall estimate method tion (Zhang et al. [8]). In this paper, the discrimination by the radar for operational snow early warning, and an function is experiment of snow estimate is performed for a case of heavy snow on March 4, 2007, followed by error esti- mation to correct the scheme for radar-based snowfall estimation. Finally, an appropriate Z-I relationship is 2 where Σ(Hi-Gi) is the square of deviance, indicating the built to support the timely and accurate release of oper- non-linear effects of the Z-I relationship. This discrimi- ational early warning. nation function is used to modify the values of a and b 2 DATA in the Z-I relationship until the CTF2 reaches its mini- mum value, when the Z-I relationship determined by a The data used are based on the reflectivity of the and b is optimal (Zhang and Dai[12]; Zheng et al.[13]). Yingkou CINRAD-SC Doppler radar products, intensive observation data of 1-h snowfall, conventional observa- 4 CASE APPLICATION tion data for the heavy snow rarely paralleled in history This paper uses the echo intensity data detected during 12:00-19:00(Beijing Time, the same hereafter) in above the observation station at a 0.5° elevation angle Liaoning province on March 4, 2007 as well as size by Yingkou CINRAD-SC Doppler radar for the ex- spectrum of precipitation during 2007-2009 of the laser tremely heavy snow process during 12:00-19:00 on spectrometer Parsivel produced by the OTT company March 4, 2007. Considering that the resolution of opera- from Germany. tional PUP products has four bins and conditions of ob- 3 SCHEME DESIGN FOR SNOW ESTIMATE servation station, echo intensity of every station used is BY RADAR the average of data from 5×5 bins (about 3 to 22 km2) above the observation station, which is combined with [1] The study of Boudala et al. indicated that the val- intensive observation data of hourly snowfall to calcu- ue of a is 160-3 300 and the value b is 1.5-2.2 in the late the Z-I relationship by means of the optimization Z-I relationship of snow estimate by radar. These method. changes depend on various parameters, such as crystal Taking into account that the radar detection at a structure, degree of freezing and aggregation, degree of 0.5° elevation may be blocked, comparison of riming, snowflake humidity, density, terminal velocity, radar-based snowfall estimate is performed using the as well as changes in the size distribution of snowflakes observation station located at certain distances from the [9] [10] (Rasmussen ).