| In recent years,the extreme cold events occurring in China have a tendency to increase gradually,which has a great impact on people's economy and life.However,different extreme cold events have different weather characteristics and circulation causes.Based on the 0.25°×0.25°temperature grid data in China,the extreme cold events occurring in China during the 54 years from1961 to 2014 are divided into two types,i.e.the extensive and persistent extreme cold events(EPECE)and the cold wave events(CWE).Firstly,the temporal and spatial distribution characteristics of frequency and intensity are studied to obtain the climate characteristics of the two types of events.Secondly,the two types of events were further selected,EPECE and CWE that occurred during the extended winter from November to March of the following year are selected.The characteristics of the atmospheric circulation before and after the formation of the two types of events are compared,and the changes of various weather systems at different altitudes that affect the extreme cold events are analyzed.Then,the configuration of weather systems is studied by isentropic surface analysis.Finally,in order to verify the simulation ability of WRF numerical prediction model for the weather situation of two types of extreme cold events in the early and late stages,as well as the simulation effect for the quantitative index,the WRF model is used to simulate the typical cases of the two types of events.The main results are as follows.(1)According to the study on the spatiotemporal distribution characteristics of the two types of events,there are 55 EPECE and 94 CWE during 1961-2014.EPECE maintain 13 days on average,CWE last four days on average,which shows that EPECE have longer duration.Both types of events are most likely to occur in winter,and the annual frequency shows a downward trend.The mean temperature anomaly distribution of EPECE and CWE during the process period is similar,but the value of EPECE in most areas is not lower than that of CWE because of the longer duration.The maximum cooling amplitudes of EPECE is located in northern China with the lowest temperature of-2.4°C.While the maximum cooling range of CWE is located in North,East and South China with the lowest temperature of-3.6°C.The distribution characteristics of the minimum temperature in the two kinds of events are similar.The minimum value is located in the northeast while the maximum value is located in Hainan.And the 0°C isotherm appears near the Qinling-Huaihe River line.(2)The weather effects of the two types of events are significantly different.On the peak day,the temperature anomaly of EPECE and CWE was negative in most areas of China.The lowest temperature anomaly of CWE is-5°C,and the lowest temperature anomaly of EPECE is below-8°C.Four days after the peak day,EPECE still maintains a strong negative anomaly in the whole country,while the temperature anomaly in some areas of CWE turned positive.It shows that EPECE with longer process,wider range and larger intensity.While during CWE,the temperature drops rapidly,it also rises quickly.The precipitation area is mainly located on the southeast coast in EPECE,and on the southwest and southeast coasts in CWE.(3)In EPECE,the anomalies of tropospheric system appear at-11 day,and the abnormal signals are transmitted to the stratosphere,causing abnormalities in the stratosphere.The center of the polar vortex is deviated to the eastern hemisphere,and along with the anticyclonic Rossby wave breaking(AWB)that extends eastward at-9 days,the energy is transmitted down to the troposphere.As a result,the tropospheric blocking high spread is abnormally eastward,extending to 90°E and staying there.The maximum blocking intensity exceeds 24,the maximum instantaneous blocking frequency exceeds 50% of the climatology,and the maximum large scale blocking frequency exceeds 14% of the climatology.The Siberian High increased in intensity due to the sinking of the upper cold air,reaching a maximum of 1053 hPa.Although the intensity of each circulation system was gradually weakened,it still maintained its previous shape until +7 day.Which makes the cold air outbreak with severe cooling,wide range and long duration.However,the precursor signal appears late in CWE.Abnormal signals appeared in the tropospheric circulation system at-3 days and are transmitted upward.Accompanied by the AWB at 0 day,the energy transmits downward.However,the center of the upper stratospheric polar vortex is always near the pole,and the eastward expansion of AWB is less than that of EPECE.Although the intensity of the blocking high is also very strong which the maximum is more than 20,the blocking is limited to 60°E around the Ural Mountain area in the process of energy transmission.the maximum instantaneous blocking frequency exceeds 45%of the climatology,and the maximum large scale blocking frequency exceeds 12% of the climatology.The intensity of cold air increases in the Siberia area,which increases the Siberian cold high pressure,reaching a maximum of 1050 hPa.This abnormality occurs quickly but cannot be maintained for a long time.After +3 days,the abnormalities of the circulation systems have almost disappeared.It makes that the temperature drop also very drastic,but the duration is short.(4)By using isentropic surface analysis,the polar vortex in EPECE is weaker in intensity,smaller in range and the center location is more east than that of CWE on 555 K and 450 K isentropic surface.The polar vortex deformation of EPECE varies in three stages,while the polar vortex shape of CWE changes little.On the isentropic surface of 450 K and 315 K,the high potential vorticity regions of both kinds of events expand significantly southward,which means the cold air gradually expands southward during the process of sinking.The characteristic of potential vorticity downward propagation is that from the middle troposphere to the middle stratosphere,the high potential vorticity regions of both two types of events have a depression from high to low,and the isentropic surface is obviously inclined downward.The inclination and southward range of EPECE is larger than that of CWE.From the troposphere to the bottom of the stratosphere,the high potential vorticity area is more significantly depressed downward than the upper layer,and the EPECE potential and potential vorticity values are larger than CWE at the same position.(5)According to the simulation results of two typical extreme cold events by WRF,the model can simulate the circulation on 200 hPa and 500 hPa well.But there is a certain deviation between the simulation of the range and intensity of low and high value systems and the actual situation.But there is a certain deviation between the simulation of the range and intensity of the low-value and high-value systems and the actual ones.In EPECE,the simulated polar vortex has larger central value,lower intensity and smaller range than the actual ones on 200 hPa.And the simulated intensity of the low trough near Lake Baikal at 500 hPa is stronger.The pressure ridge strength simulation at500 hPa is lower in CWE.For sea level pressure,WRF can simulate the general situation for both types of events.But for the high-value and low-value center strength,the simulated value is smaller than the actual value.Which means the high pressure is weaker and the low pressure is stronger,and the simulation of the Siberian High area is also smaller than the actual one.For the simulation of circulation index,WRF has a good effect on the simulation of blocking index in both two kinds of event,but the simulated blocking range is too large and the center location is deviated from the actual situation.WRF can also simulate the Siberian High index well which can show changes in the transition period,with the deviation on intensity. |
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