Diurnal Variation Of Precipitation Based On KAZR Observation At The SACOL Site

As one of the most common and important weather events,precipitation has an important impact on the global water cycle,energy balance of the ground-atmosphere system,and climate change.The complex topography of northwest China is sensitive to the influence of climate change,and there is a trend of gradual warming and humidifying under global warming,and the climate and ecosystem changes in this region caused by the spatial and temporal distribution of precipitation have received extensive attention from scholars.Precipitation is an essential component of the water cycle and plays a crucial role in global climate change and energy balance.Diurnal cycle is the most basic periodic signal of precipitation,resulting from the integrated influence of atmospheric thermal and dynamical processes on water cycle,which involves the interaction between large-scale systems and the microphysical processes of atmospheric aerosol-cloud-precipitation formation interactions Based on the magnitude of vertical velocity and the growth rate of precipitation particles,precipitation can be classified into convective and stratiform precipitation,and their radar echo characteristics also have significant differences.Millimeter-wave radar can obtain the three-dimensional structural characteristics of precipitation systems by accepting precipitation particle echo signals,and combined with radar echo feature identification,precipitation areas can be classified into convective and stratiform precipitation,which is a powerful tool for detecting precipitation.Due to the difference in microphysical characteristics and thermodynamic development process,there is a significant difference between convective and stratiform precipitation diurnal variation.Based on radar detection and retrieval,the diurnal variation of macro and microphysical features of different types of precipitation is analyzed,which is important for understanding the development mechanism of different types of precipitation and improving the model precipitation parameterization scheme in semi-arid regions.In this paper,we focus on the diurnal variation of different types of precipitation in semi-arid regions based on Ka-band Zenith Radar(KAZR)at Semi-Arid Climate and Environment Observatory of Lanzhou University(SACOL).The main research contents and conclusions of this paper are as follows:(1)The precipitation retrieval algorithm based on the Ka-band radar of the Atmospheric Radiation Measurement(ARM)site is improved by removing background noise and clutter,correcting the precipitation radar data,determining locally applicable precipitation identification thresholds in combination with rain gauges,identifying the height and time of the melting layer using the characteristics of the melting layer in the radar observation images,categorize the precipitation into convective and stratiform precipitation.A systematic precipitation identification and retrieval algorithm is proposed using the KAZR observation at the SACOL site.The retrieval precipitation rate is compared with the rain gauge to verify the robustness of the algorithm.(2)Based on the precipitation retrieval algorithm,eight years of continuous retrieval products from 2014 to 2021 are obtained using the KAZR at the SACOL,including rain rate,precipitation duration,maximum 15 d BZ echo height,precipitation type,and other characteristics.The diurnal variation,monthly variation,and interannual variation of precipitation in semi-arid regions are analyzed.Overall,the average annual precipitation at the SACOL is 297 mm with a duration of 106 hours.The seasonal variation of precipitation corresponds well with the regional climate.The hot and rainy periods,both concentrated in July and August,are mainly influenced by the warm and humid water vapor brought by the northward shift of the western Pacific subtropical high pressure in summer.The diurnal variation of precipitation is mainly concentrated in the afternoon and at night,and there is a good correspondence between the diurnal variation of precipitation rate,duration,and maximum 15 d BZ height(Maxht15).The peak diurnal variation of precipitation from year to year is also concentrated in the afternoon and at night.(3)The diurnal and monthly variation of convective and stratiform precipitation were further analyzed according to the classification of precipitation types.Overall,stratiform precipitation dominates,accounting for about 70% of the total rainfall and60% of the total precipitation duration.The monthly variation of convective precipitation and stratiform precipitation share common features.They are both longer,stronger,and have higher Maxht15 in summer,mainly due to the abundant warm and humid moisture brought by the northward shift of the western Pacific subtropical high pressure and the thermodynamic condition caused by the enhanced solar radiation in summer.However,there are also some differences in the monthly variation.Convective rainfall has greater monthly differences compared with stratiform rainfall,and a significantly higher precipitation rate in July and August than in other months.In addition,we find that there is almost no stratiform precipitation in March and November,and mainly local weak convection is dominant.(4)Due to the differences in the thermodynamic processes and microphysical characteristics of convective and stratiform precipitation,there are significant differences in their diurnal variation.Convective precipitation is easily driven directly by surface sensible heat flux.The diurnal variation of precipitation rate and duration is more concentrated in the afternoon.But Maxht15 is fluctuating and corresponds poorly with precipitation rate and duration.Except for 2016,the diurnal variation of stratiform precipitation rate,duration,and Maxht15 is similar to the diurnal variation of overall precipitation,which is concentrated at night and in the late afternoon.There is a“survivorship bias” in the diurnal variation of the melting layer height,which is affected by multiple factors such as lolar radiation,latent heat release,inversion of temperature,etc.(5)For the diurnal variation of precipitation in 2016,further statistics of its summer convective and stratiform precipitation differences compared with the rest of the year,it is found that precipitation is mainly convective type precipitation and the phenomenon of severe reduction of stratiform precipitation.Using NCEP reanalysis data,we analyze the meteorological background in 2016 and find that summer northwest airflow,anomalous continental warm high pressure,downward vertical velocity field and water vapor dispersion,and other weather conditions cooperated will result in less water vapor and insufficient lifting conditions in 2016,which were not conducive to the formation of widespread and long-lasting stratiform precipitation,resulting in precipitation mainly triggered by local thermal convection in the late afternoon,with small amounts and short duration.