Classification And Diurnal Variation Characteristics Of Precipitation Echo Observed By Radar In Summer Over Central Tibetan Plateau

This study analyzes the classification and diurnal variation of precipitation echo over the central Tibetan Plateau using observations collected from a vertically pointing C-band frequency-modulated continuous-wave(C-FMCW)radar during the Third Tibetan Plateau Atmospheric Scientific Experiment(TIPEX-III)in July-August 2014,as well as the environmental air conditions using radiosonde observations at Naqu meteorological station.It shows that 51.23% of the total profiles are valid echo profiles,and 35.06% of the valid echo profiles produce precipitation at the ground(precipitation profiles);stratiform precipitation with evident bright-band signature,weak convective precipitation,and strong convective precipitation account for 52.03%,42.98%,and 4.99% of the total precipitation profiles,respectively.Precipitation occurs mainly during afternoon to midnight.The highest occurrence frequency of precipitation appears at 21:00-22:00(local solar time;LST),mainly being stratiform precipitation;the secondary peak appears at 13:00-14:00 LST,mainly being weak convective precipitation;the strong convective precipitation occurs mostly in the afternoon and evening with two peaks at 12:00-13:00 and 17:00-18:00,respectively.During 00:00-11:00LST,precipitation echoes occur less frequently with weaker intensity.Starting from about 11:00 LST,precipitation echoes develop with increased radar reflectivity,enhanced vertical air motion,and elevated echo top.Precipitation echoes occur less frequently at 4-5 km above ground level(AGL)all day and most frequently around 3 km most of the day(except 17:00-20:00 LST).The occurrence frequency of precipitation echoes at 6-9 km increases during 12:00-01:00 LST.Strong echoes and intense upward motion occur mainly at 12:00-14:00,17:00-18:00,and 21:00 LST.During 17:00-18:00 LST,intense upward motion exists at 8-13 km,and the tops of precipitation echo,strong precipitation echo,and intense upward motion reach the highest level.Strong echoes also appear during 01:00-05:00 LST.The evolution of precipitation echoes is closely related to that of the environmental air conditions.In the early morning,almost all the convective available energy(CAPE)is less than 200 J/kg,the neutral buoyancy level(LNB)is low(mostly 3-7 km),and the convective inhibition(CIN)scatters throughout 0-150 J/kg.These indicate disadvantageous environment for convective initiation and development.Around noon,the CAPE increases markedly(median value of 458 J/kg),most of the CINs are less than 20 J/kg,and the environmental lapse rate near the surface(0-400 m)is larger than the dry adiabatic lapse rate.Therefore the occurrence frequency of weak convective precipitation reaches the highest around noon,and sometimes it develops into strong convective precipitation,leading to the occurrence sub-peak of strong convective precipitation.Around the evening hours,the average atmospheric conditions are not as advantageous as those around noon,but larger CAPE values emerge(800-1000 J/kg),the top 10% values of the LNB and the total precipitable water are larger than the corresponding values of the whole sample around noon,suggesting that more favorable thermodynamic and water vapor conditions can appear in the evening.This is consistent with the radar observations that the occurrence frequency of strong convective precipitation reaches its major peak with highest echo top in the evening.