Daren Lu

VHF Doppler Radar Observations of Buoyancy Waves Associated with Thunderstorms

Abstract The Platteville VHF Doppler radar, located on the Colorado piedmont near Platteville, Colorado, continuously measured the vertical wind velocity during a 12-day period in late July and early August 1981. Measurements were made every 2.5 min on the average with range gates centered at 3.3, 5.7, 8.1, 10.5, 12.9, 15.3, 17.7, and 20.1 km above sea level. Periods of active thunderstorms were identified from the PPI maps from the National Weather Service 10 cm weather radar at Limon, Colorado. When no thunderstorm activity was present, the vertical velocity fluctuations were small and erratic. But a few hours after strong thunderstorm activity began, large quasi-sinusoidal wave trains with periods of about 40 min were observed. Power spectra of the vertical velocity time series showed enhancements at all frequencies during thunderstorm activity, but for periods longer than 30 min the enhancements were larger, particularly for the mid-tropospheric range gates from 5.7 to 12.9 km. Some of the implication...

Features of aerosol optical depth with visibility grade over Beijing

The observation of direct solar radiation in an urban district of Beijing was carried out with a PIS spectrometer from March 1993 to March 1995. The aerosol spectral optical depths are deduced from the radiation data. The results show that the seasonal mean aerosol optical depth is larger in spring and summer, and smaller in autumn and winter. It is found that the aerosol spectral optical depth is closely related to surface visibility. For the same visibility level, it has a very weak seasonal variation. This means the seasonal variation of the aerosol spectral optical depth is mainly determined by the variation of surface visibility. The yearly averaged aerosol optical depth spectra under the various visibility levels are given and comparison with calculations from the LOWTRAN 7 aerosol model are made at the same visibility level. The particle-size distributions are retrieved from observed aerosol optical depth spectra.

Small-Scale Rain Nonuniformity and Its Effect on Evaluation of Nonuniform Beam-Filling Error for Spaceborne Radar Rain Measurement

Abstract An observational method has been proposed to sample radar echo with high range resolutions using a ground-based meteorological radar. Utilizing this method, the rainfall echo data with a high range resolution of 125 m was obtained by using an X-band meteorological radar. The analysis of rain nonuniformity strength using these high-resolution radar data shows that the nonuniformity is significant and, even in an instantaneous field of view (IFOV) of 1 km, the reflectivity excursion above 10 dB is common. The simulation of the nonuniform beam filling (NUBF) error of the path-integrated attenuation (PIA) measured by the spaceborne radar has been also implemented using these data. The results show that the PIA encounters mainly underestimation and cannot be neglected; even in 0.5-km IFOV the underestimation can reach up to 50%. The correlation analyses show that the relative PIA error and the standard deviation of rain rate have a power-law relationship with quite good correlation, which might be use...

Numerical simulation on stratospheric gravity waves above mid-latitude deep convection

In this paper, the results of a 3D numerical simulation study of stratospheric gravity wave (GW) excitation above a real storm case which occurred in mid-summer and mid-latitude over China are presented. The model applied is a non-hydrostatic compressible atmospheric model coupled with a bulk cloud-microphysics parameterization. The results demonstrate that the upper tropospheric baroclinic background condition shaped the storm passage and the strong convection developed and was maintained in the upper troposphere. The updrafts penetrated into the tropopause and resulted in significant GWs in the stratosphere. That the tropopause penetration process was closely related to pressure perturbation indicates that the “obstacle effect” proposed by Clark (1986) was responsible for the wave excitation. Three distinct sub-areas—the wave-energizing, the wave-exciting and the wave-bearing sub-area, are found to be responsible for the wave generation. The Area-averaged GW momentum flux component, ρu′w′, was about 0.3 N m−2 at the models altitude near the tropopause. It is also found that the interaction between convection and background flow caused the asymmetric wave distribution. Further investigation reveals that, due to the energy removal function of the GWs, cross tropopause mass transportation can only occur in the early stage of the tropopause penetration process. The mechanism implied in this relationship between the two processes may be, at least in this study, an explanation for the general finding that the tropopause is a transport barrier.

Size distribution and element composition of dust aerosol in Chinese Otindag Sandland

Part physical and chemical characteristics of dust aerosol were determined for samples collected from Otindag Sandland of China in spring, 2001. Number concentration, mass concentration, chemical element concentration and size distribution of aerosol particles with 0.5 μm < Dp < 100.0 μm were examined. The average number and mass concentrations of aerosols were 274.8 cm−3 and 0.54 mg/m for the field period respectively, and 31.4 cm−3 and 0.07 mg/m3 for the non-dusty days. PM10 played a dominant role in the aerosol mass concentrations. The particles with Dp < 8.0 μm accounted for about 93.7% of total aerosol number loading in dusty days. The particle size distributions of aerosols were characterized by bi-modal logarithm normal function in heavy and moderate dusty days, and mono-modal in windblown and non-dusty days. Crustal elements such as Al, Fe, etc. in aerosols almost originated from soils. Pollutant elements of S, Pb, etc. associated with aerosols were affected by remote anthropogenic pollutant sources in upwind regions. Mo, V and Co in aerosols were possibly from other dust sources other than local soils. The aerosols over Otindag Sandland consisted of particles from local soils, upwind pollutant sources and other dust sources.

Feasibility study for joint retrieval of air density and ozone in the stratosphere and mesosphere with the limb-scan technique

Air density is a key sensing object for its high variability especially in the mesosphere, and ozone plays an important role in the physical, chemical, and radiant processes in the atmosphere system. Therefore it is essential to obtain their global vertical distributions jointly with high precision and vertical resolution. There is little work on joint retrieval of air density and ozone distributions using the ultraviolet limb-scan technique, although much work has been done on ozone measurement. Numerical simulations of joint air density and ozone retrieval in the middle atmosphere (20-90 km) are carried out using limb-scattered radiances at four bands (255, 300, 320, and 340 nm). Results show that joint retrieval of dual parameters using the limb-scan technique is feasible with high precision in nearly the whole region concerned, where air density and ozone have a precision of 1%-2% and 3%-5%, respectively, provided that high measurement precision and accurate correction of multiple-scattered radiance at long ultraviolet bands are ensured.

Combined remote sensing of cloud characteristics with surface-based radar, lidar, and all sky imagers over Beijing, China

Clouds have been known as the key components of atmospheric processes in climate, weather, and environment related issues. Owing to its highly complicated processes relating to atmospheric dynamic, macro and microphysical characteristics reveal distinct regional and seasonal features, thus observations at various typical sites are very important to quantitatively understand cloud characteristics with their functions. In this paper, we introduce combined ground-based instruments, i.e., a Mie Lidar, a Ka-band Doppler radar, an IR and a visible all sky imagers, and an automatic weather station, to continuously observe the clouds over Beijing. Synthetic analyses are made to derive the cloud base height, vertical structure, horizontal distribution, radiative effect, etc. About 1 year observation data are used to obtain the statistics of cloud characteristics in this area. A simple introduction of the surface-based remote sensing system and some preliminary results are given.

Intercomparison of rainfall rate retrieval results by SSM/I over west Pacific Ocean

Since late 1980s there have been a series of satellite-borne microwave (MW) radiometers operated for remote sensing of water-related parameters in particular for rainfall observation among them SSMII on DMSP TMI on TRMM and AMSR-E on AQUA are well known instruments. A lot of retrieval schemes have been published for operational and research purposes. Inter-comparison of different retrieval algorithms and their products is an important task for their reliable application with enough accuracy. In this paper we will compare some algorithms for SSM/I products over West Pacific area. The methodology is mainly focusing on the comparison of retrieved hourly rain rate at the spatially and temporally collocated area statistical results and case study will be given.

Small-scale nonuniform rain beam filling errors in spaceborne radar rain measurement

An observational method has been proposed to sample echo data with high range resolutions using a ground-based meteorological radar. Utilizing this method, a quantity of rain echo data with a high range resolution of 125 m was obtained by using an X-band meteorological radar. The computation of rain nonuniformity strength using this high resolution radar data shows that the nonuniformity is significant and even in an instantaneous field of view (IFOV) of 1 km, the reflectivity excursion above 10 dB is common. The simulation of the nonuniform beam filling (NUBF) error of the path-averaged rainrate derived from the path-integrated attenuation measured by the spaceborne radar has been also implemented using this data. The results show that the rainrate encounters mainly underestimation and cannot be neglected, even in 0.5 km IFOV, it can reach over 50%. The correlation analyses show that the rainrate error and the true rainrate have a power relationship with some correlation, which might be used to correct this error partially. The simulation also shows that it is very important to use the high resolution data in studying the NUBF error of the next generation spaceborne radar with a higher across beam resolution (e.g. below 3 km).

Method of microwave remote sensing of rainfall distribution over tropical ocean

Although space-borne passive microwave techniques have been used in global precipitation remote sensing with preliminary success, improvement of retrieval accuracy is still an active subject. As part of Chinese National High-Tech Research and Development Program for Space Technology, an improving method for remote sensing of rainfall distribution over ocean area is investigated. In this paper, the vertical structures of cloud model are established, and the radiative transfer models are briefly introduced, also some comparison results between simulation and observation are discussed.