Construction Of A Theoretical Model Of Winter Precipitation Type And The Influence Of Terrain On The Distribution Of Precipitation Types

Winter precipitation seriously threats the operation of power,transportation,communication and social economy,and the extent of their impacts depends on the different precipitation types.The existing diagnosing and forecasting model does not comprehensively consider the physical mechanisms of precipitation formation,which cannot accurately distinguish the winter precipitation types,especially in complex terrain.Therefore,developing a theoretical model of winter precipitation types,studying the influence of terrain on the distribution of precipitation types,and accurately simulating the winter precipitation type in the mountainous rain-snow transition zone is undoubtedly challenging research with important scientific research significance.This paper statistically analyzes the physical mechanisms and conditions of freezing rain,ice pellets,and sleet.Next,a theoretical model of Precipitation Phases(TPP model)containing the complete thermodynamic process precipitation phase is established.By validating the TPP model and the sensitivity experiments of topography,the influence of topography on the distribution of winter precipitation types and their fall zone is revealed,and two conceptual models of the distribution of winter precipitation types in mountainous environments are developed.The main results are as follows:1)Freezing rain,ice pellets,and sleet can all be formed by “melting”,“warm rain”and “edge” mechanisms.The differences in cloud top height and the air temperature/relative humidity profile are the main causes for different precipitation types.The freezing rain in China is dominated by the “supercooled warm rain”,while sleet is dominated by “melting” and ice pellets are dominated by “melting” and “edge” mechanism.The higher the station altitude,the lower the “melting” percentage and the higher the “warm rain mechanism”.In the “melting mechanism”,the increase of melting layer thickness,air temperature(),and relative humidity(RH)is favorable to freezing rain,and the increase of subfreezing layer thickness or temperature decrease is favorable to ice pellet.In the “warm rain mechanism”,the higher cloud-top height and sub-cloud temperature <-5? are favorable to ice pellets or sleet.In addition,there are 838 events that occurred at greater than 0?(hereafter warm freezing rain and denoted by WFR),which accounts for 9% of the total freezing rain observations.Nearly 3%(266 observations)of freezing rain events occurred when the near-surface wet-bulb temperature is greater than 0?,and neither the air temperature()nor the wet-bulb temperature(can fully explain the occurrence of the WFR.2)By considering the non-equilibrium heat exchange between falling raindrops and the atmosphere,a theoretical model of raindrop temperature is developed to compensate for the shortcoming of other precipitation-type models.The temperature lag between raindrops and the atmosphere is the main cause of WFR.The obtained from transient equilibrium theory differs significantly from the real raindrop temperature.The magnitudes of the difference between raindrops and the atmosphere(and)are determined by the raindrop diameter(D),temperature lapse rate(?),and relative humidity(RH).Meanwhile,the ?also determines whether the temperature lag is positive or negative when raindrops reach the surface.An increase in D,increase in ?,and decrease in RH enhanced the and and thus prompted the occurrence of the WFR.The evaluation results of the model show that the calculated is more accurate than and in distinguishing between rain and freezing rain.3)A new theoretical model for the precipitation phase(TPP)is established by considering the melting process,refreezing process of the mixture precipitation,the non-equilibrium raindrop temperature variations,and the spontaneous refreezing process of supercooled raindrops.Compared with the traditional precipitation type diagnosis model,the TPP model can more accurately predict the types and amounts of winter precipitation within the rain-snow transition zone,such as freezing rain,ice pellets and sleet.On this basis,the TPP model was used to quantitatively investigate the influence of meteorological conditions on different precipitation types.The results show that as the and RH increased,the precipitation types on warm layer based gradually changed from wet snow to slush and rain.As the in the subfreezing layer decreases or the RH increases,the precipitation types gradually changes from slush to ice pellets.In the frontal zone,with the increase of ? or RH in the warm layer,the width of the rain-snow transition zone and freezing rain zone are widened and the snow zone is reduced.With the increase of ? in the subfreezing layer,the mixed precipitation(slush and wet snow)zone disappears;the solid precipitation(snow,wet snow and ice pellets)zone also moves toward the front,but the change of RH in the subfreezing layer has little effect on the spatial distribution of ground precipitation types.4)By coupling the TPP with WRF model,the distribution influence mechanism of winter precipitation types in quasi-stationary front weather and three-dimensional idealized terrain are investigated.When the terrain is much lower than the cold air,the terrain has less influence on the wind field,vertical temperature profile and precipitation types distribution.As the terrain rises,the blocking effect gradually increases,and the cold air accumulation on the windward slope lowers the near-surface air temperature,and increases the thickness and intensity of the subfreezing layer,while the leeward slope is the opposite.The freezing rain on the windward slope gradually transformed into ice pellets and refreezing wet snow,and the freezing rain on the leeward slope is gradually replaced by rain.Meanwhile,the freezing rain area is reduced and leads to the movement of the atmospheric icing area from mountain top to both sides of the mountain.The ice events on the windward and leeward slopes almost disappear completely.In addition,the temperature profile and the distribution of precipitation types on both sides of the mountain are less affected by the terrain raised.5)The conceptual model of the “melting process” and “warm rain process” of winter precipitation types in mountainous environments are established.Compared with the conceptual model of winter precipitation types in plains,the new model expands its application scenarios in complex terrain and refines the categories of precipitation types.In the melting model,freezing rain occurs on the whole mountain when the terrain is under the cold air,and the amount of freezing rain is slightly higher on the windward slope and mountain top than on the leeward slope and both sides of the mountains.Conversely,with the increase of the blocking effect,freezing rain occurs on the part of the mountain below the cold air.The precipitation types transform into wet snow or slush where the warm air dominates the mountain part.The windward slope transforms from freezing rain to ice pellet and dry snow,and the leeward slope changes from freezing rain to rain,slush,and wet snow.In the warm rain model,when the mountain is below the cold air,the amount of freezing rain is increased on the windward slope and decreased on the leeward slope.The precipitation types in the area behind the front are expressed as freezing rain with ice pellets or freezing rain with snow.Conversely,freezing rain occurs only on the windward slope and on both sides of the mountain,and the precipitation types on the rest of the hill are rain.The precipitation types in the upstream area of the mountain are rain and snow(if the ground temperature is >0?)or freezing rain and snow.