| Severe freezing rain disasters not only cause great losses to transportation,hydraulic power and electricity,but also have a great impact on agriculture in China,a large agricultural country.At present,the numerical simulation of freezing has made a big breakthrough,and the spatial and temporal resolution of the simulation has reached 3km×3km and 1h,respectively,but the impact and assessment of freezing disaster on crops are still in the qualitative research stage.The high spatial and temporal resolution of freezing disaster forecast products and crop disaster-causing factors have not been well coupled,resulting in their products not serving agriculture well.Coupled with the serious lag or lack of research on agro-meteorological hazards,it is far from meeting the requirements of government disaster prevention and mitigation.This thesis attempts to simulate and verify the intensity of freeze damage using WRF model coupled with BTC(freezing rain parameterization scheme)based on detailed analysis of meteorological parameter thresholds for low temperature disasters in rape and citrus,reproduce the change process of freeze damage extent and intensity,and couple it into the freeze damage disaster system,and finally establish a high accuracy freeze damage disaster assessment model for rape and citrus,and then quantitatively study the impact of freeze damage on agriculture and disaster assessment,and reduce the losses caused by frost damage disasters to agriculture.The main conclusions are as follows.(1)Through data collection,a set of low-temperature freezing damage indicators was obtained for different fertility stages of oilseed rape and citrus,which encompasses disasters formed by low-temperature hazards and freezing rain gravity leading to plant fracture,etc.For example,the monthly minimum temperature,the number of days of freezing rain duration and the 3-day accumulated freezing rain amount were used to determine the indicator sets of low-temperature freezing damage levels for rape seedling,bud and flowering stages and citrus respectively,which were coupled into the WRF model of the BTC freezing rain parameterisation scheme to develop a high-precision freezing disaster forecasting model for rape and citrus.(2)By comparing simulations and observations of rape and citrus disaster levels in the2008 mega-freeze disaster,the model not only simulates the extent,intensity and thickness of freezing rain occurrence more accurately,but also the high spatial and temporal resolution of the simulated disaster levels far exceeds the interpolated maps of the observed station data.Frost damage in rape seedlings is mainly due to daily minimum temperatures,followed by freezing rain,while freezing rain is the main cause of freezing damage in rape flowering and citrus.The model simulated freeze hazard levels and areas for rape and citrus were generally consistent with observations,indicating that the developed model for forecasting freeze levels for rape and citrus can simulate past freeze hazard levels for rape and citrus very well.(3)Using the 6-hourly 1 ° × 1 ° forecast field information from NCEP/NCAR in January 2018,the model was used to predict the spatial and temporal distribution and development of freezing and low-temperature damage levels in rape and citrus during the reproductive period.The results show that only the first and third of the three freezing events will cause damage to rape and citrus.The third freezing event for rape seedlings is predicted to reach level 4 in Anhui and level 1 in other areas;the freezing event for rape flowering is predicted to be level 2 or higher mainly in northern Guizhou and Hunan.Citrus freeze damage is forecast to be mild,but scattered level 2 to 3 freeze damage will occur in Guizhou,Hunan,Guangxi and Anhui.The predicted areas of freezing hazard levels are consistent with observations,indicating that the developed freezing level forecasting models for oilseed rape and citrus can achieve a seamless integration of catastrophic weather forecasting and agrometeorological hazards. |
