| Poyang Lake?PL?is the largest freshwater lake in China,which is about 3800 km2 in summertime.The lake-effect precipitation is significant over the PL and its surrounding areas.It can induce or enhance the heavy rain and cause serious disasters.Meanwhile,the lake-effect precipitation is also a difficult point in operational weather forecasts,which troubles the weather forecasters.Many achievements have been made on the lake-effect precipitation over the Great Lakes and the Great Salt Lake in North America.However,PL is about 1000 km south of them,and the environmental and synoptic backgrounds of the PL are also significantly different from the lakes in North America.Moreover,so far in China there are little researches on the lake-effect precipitation over PL.Therefore,in this paper the climate characteristics and favorable environmental conditions of lake-effect precipitation over PL are analyzed,based on the radar data,the sensible and latent heat flux data detected by an eddy correlation system on a tower near PL,the sounding data,the regular observation data and the atmospheric circulation data.Then,through the studies of typical cases over PL with the observation and simulation analysis the mechanisms of lake-effect precipitation are diagnosed.The main conclusions are as follows.After reviewing 653,687 pictures of radar reflectivity during 2010–2018,it is found that there are 184 lake-effect precipitation cases over PL,including 105 initiation cases?the precipitation or storm is mainly initiated from PL?and 79 enhancement cases?the existed rainstorm is enhanced when it crosses PL?.Most of the cases could last for 2–3 hours.The initiation type of cases mainly occur from June to August,mostly in July and August,while the enhancement type of cases primarily happen from April to June,mostly in June.The two types of lake-effect cases seldom appear in late autumn and winter.Moreover,the initiation type of cases mainly occur in13:00–01:00?UTC,the same below?.The diurnal variation of the enhancement type of event is less significant than that of the initiation type,with a relatively high peak in 16:00–00:00.The initiation type of lake-effect precipitation cases mostly occur under three synoptic patterns:the low trough pattern?44.7%?,the edge of the West Pacific subtropical high?WPSH??34.3%?and the westerly wind pattern?15.2%?,while the enhancement type mainly occurs under two synoptic patterns:the low trough pattern?50%?and the westerly wind pattern?40%?.In the low trough pattern,PL is located in the front or behind of the 500 hPa low trough,with the southerly wind and the weak temperature advection at 950 hPa.In the pattern of WPSH edge,PL is located in the north,south or west edge of the main body of WPSH,with the southerly or easterly wind and the weak temperature advection at 950 hPa.In the westerly wind pattern,PL is located in the approximately zonal parallel contour lines at 500 hPa,with the easterly or southerly wind and the weak temperature advection at 950 hPa.The air temperature difference between 2m and 850 hPa??35?T850?are mostly?>62%?within 6–10?for both the two types of lake-effect precipitation,but the mean?35?T850 is about 8.3?in the initiation type,which is about 1?higher than that of the enhancement type.For the convective instability,the requirement of the initiation type is also larger than that of the enhancement type.97.8%?79.5%?of the initiation?enhancement?type cases occur in the presence of convective instability layer between the surface and 850 hPa,and 37.8%?28.2%?of initiation?enhancement?cases occur in the condition that the whole layer below 850 hPa is a convective instable layer.A case of an initiation type of lake-effect precipitation over PL was analyzed in detail,which lasted from 18:00 on August 12 to 02:00 on August 13,2015.When the storm occurred,PL was situated within the relatively straight westerly wind in front of the northeast-southwest oriented trough at 500 hPa,with the weak easterly wind and the weak temperature advection at 925 hPa.Results shows that there was a large convective available potential energy?CAPE?,an obvious convective instable layer below 700 hPa and a low lifting condensation level?LCL?,which was favorable for the development of convections.Furthermore,the temperature difference between PL and its surrounding areas was more than 4?at night,generating the lake-land breeze circulation,triggering the convections and finally forming the initiation type of lake-effect precipitation event.The numerical simulation has successfully reproduced this lake-effect event,and has verified the effect of the lake-land breeze circulation.In contrast,in the sensitive simulation,when the water body of PL was replaced by arable land,the lake-effect event did not occur because of the absence of the obvious lake-land circulation.Moreover,a case of an enhancement type of lake-effect precipitation over PL was analyzed,which lasted from 12:00 to 16:00 on May 13,2015.When the case occurred,PL was located within a small shallow trough and in front of the southern trough at 500 hPa,with the southwesterly wind jet and the weak temperature advection at 925 hPa.There was a small CAPE and a large?183 J?convective inhibition energy?CIN?.A convective stable layer existed between925 hPa and 850 hPa,which was not favorable for the development of convections.After the sunset,the front of an existed convective storm swept over PL from west to east.The combination of evaporation,cold pool and radiation cooling jointly caused the land around PL to cool dramatically,and formed a temperature difference of about 6?between the PL and its surrounding land,generating the lake-land breeze circulation.Moreover,the heating and moistening of PL increased the air temperature by about 1.2?and the humidity by about 0.48 g kg-1?equivalent to 0.5?increase in dewpoint temperature?in the boundary layer.This increased the pseudo equivalent potential temperature at near-surface and 925 hPa,destroying the convective stable layer between 925 hPa and 850 hPa and forming a deep convective instable layer from the surface of PL to 700 hPa.Finally,the unfavorable convective conditions changed to favorable convective conditions.The control simulation and the sensitive simulation in which the water body of PL is replaced by arable land have further confirmed the above-mentioned mechanisms for the enhancement type of event.The influence of different parameterization schemes on the simulation of lake-effect precipitation over PL was also studied in this paper.Using WRF model,the simulations with 5land surface schemes and 4 planetary boundary layer schemes were carried out to simulate a initiation type of lake-effect precipitation event over PL.Results show that several schemes combinations have successfully reproduced the lake-effect events,including the YSU boundary and Noah land surface,the MYNN2 boundary and Noah land surface,the MYJ boundary and Noah RUC land surface,the QNSE boundary and RUC land surface,the YSU boundary and Noah-MP land surface,and the MYNN2 and Noah-MP land surface. |
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