Daren Lü

Comprehensive Pattern of Deep Convective Systems over the Tibetan Plateau-South Asian Monsoon Region Based on TRMM Data

AbstractDiurnal and seasonal variation, intensity, and structure of deep convective systems (DCSs; with 20-dBZ echo tops exceeding 14 km) over the Tibetan Plateau–South Asian monsoon region from the Tibetan Plateau (TP) to the ocean are investigated using 14 yr of Tropical Rainfall Measuring Mission (TRMM) data. Four unique regions characterized by different orography are selected for comparison, including the TP, the southern Himalayan front (SHF), the South Asian subcontinent (SAS), and the ocean. DCSs and intense DCSs (IDCSs; with 40-dBZ echo tops exceeding 10 km) occur more frequently over the continent than over the ocean. About 23% of total DCSs develop into IDCSs in the SHF, followed by the TP (21%) and the SAS (15%), with the least over the ocean (2%). The average 20-dBZ echo-top height of IDCSs exceeds 16 km and 9% of them even exceed 18 km. DCSs and IDCSs are the most frequent over the SHF, especially in the westernmost SHF, where the intensity—in terms of strong radar echo-top (viz., 40 dBZ) he...

Cloud Determination of All-Sky Images under Low-Visibility Conditions

Abstract The threshold method is commonly used to determine cloud in a sky image. This paper evaluates the method by numerical simulation and shows that the aerosol optical depth (AOD) is a key factor that influences the accuracy. Particularly when the visibility is low, a single threshold method is inappropriate. To improve the accuracy of cloud determination from low-visibility sky images, an integrated cloud-determination algorithm is presented that is based on the fast Fourier transform, symmetrical image features, and threshold methods. The preliminary comparison tests show that the new integrated method improves the ability to determine cloud under lower-visibility conditions.

Characteristics of atmospheric aerosol optical depth variation over China in recent 30 years

This note retrieves the annual and monthly mean 0.75 μm aerosol optical depth (AOD) by using the daily direct solar radiation and sunshine duration data of 47 solar stations from 1961 to 1990. The characteristic of AOD variation over China in recent 30 years was analyzed. The results indicate that AOD increased obviously over China from 1961 to 1990. AOD increased most rapidly over the east part of Southwest China, the middle-and-lower reaches of the Yangtze River and the Tibetan Plateau. The increasing trend of AOD is also relatively distinct in North China, the Shandong Peninsula, east part of Qinghai Province, and coastal areas of Guangdong Province. However, in most parts of Northwest China and Northeast China, the increase of AOD is less significant, while in the west part of the Xinjiang Uygur Autonomous Region and some parts of Yunnan Province, AOD shows decreasing tendency. Generally, AOD reaches its maximum in spring and the minimum appears in summer. As to the linear trend, the maximum occurs in spring but the minimum in winter. Among the 47 stations selected in this note, the largest three stations of AOD are Chengdu, Chongqing and Nanchong, respectively, which all lie in the Sichuan Basin, and the smallest value of AOD occurs in Jinghong located in Yunnan Province.

Retrieval of stratospheric background aerosol scattering coefficient from twilight polarization measurements.

A backward Monte Carlo computation procedure has been used to simulate the intensity and degree of polarization of the twilight sky during 1977. It was found that the single scattering approximation is applicable for detection of stratospheric aerosols by twilight measurement during background periods. Based on the simulation, a scheme is proposed for retrieving the stratospheric aerosol scattering coefficient utilizing the measured degree of polarization at the 0.7-microm wavelength in the zenith direction of the twilight sky. Compared with the in situ measurement, both the retrieved total optical depth and the retrieved profile of the aerosol scattering coefficient below 30 km agree reasonably well with measurements.

Simulation of the stratospheric gravity waves generated by the Typhoon Matsa in 2005

The generation of stratospheric gravity waves (GWs) due to typhoon is simulated by using a meso-scale model (WRF) with a typhoon case, the Matsa in 2005. An 8-day model run that covers the major stages of the Matsa’s development reproduces the key features of the typhoon. For example, good agreements in the typhoon’s track, the intensity, and the spiral clouds, as well as mean state of stratosphere, are seen between the simulation results and the observation. Simulation results clearly show that with typhoon propagates northwestward, pronounced stratospheric GWs are generated continuously in the vicinity of Matsa. The GWs exhibit the typical curve-like wave fronts away from the Typhoon Matsa, and propagate preferentially in the upstream of the background winds. These characteristics reflect that the stratospheric GWs are closely associated with the typhoon, and thus the GWs are referred to as Tropical Cyclone related Gravity Waves (TC-GWs). The results also show that these waves should have a rather large horizontal scale so that the outmost wave fronts can be seen at the distance of ∼1000 km to the typhoon center in the horizontal plane of 20 km. This is consistent with the phenomenon of stratospheric TC-GWs with ∼1000 km horizontal scale disclosed by the previous observational analysis results.

Retrieval of vertical distribution of tropospheric refractivity through ground-based GPS observation

In the present reported study, the vertical distributions of local atmospheric refractivity were retrieved from groundbased GPS observations at low elevation angles. An improved optimization method was implemented at altitudes of 0–10 km to search for a best-fit refractivity profile that resulted in atmospheric delays most similar to the delays calculated from the observations. A ray-tracing model was used to simulate neutral atmospheric delays corresponding to a given refractivity profile. We initially performed a “theoretical retrieval”, in which no observation data were involved, to verify the optimization method. A statistical relative error of this “theoretical retrieval” (−2% to 2%) indicated that such a retrieval is effective. In a practical retrieval, observations were obtained using a dual-frequency GPS receiver, and its initial value was provided by CIRA86aQ_UoG data. The statistical relative errors of the practical retrieval range from −3% to 5% were compared with co-located radiosonde measurements. Results clearly revealed diurnal variations in local refractivity profiles. The results also suggest that the general vertical distribution of refractivity can be derived with a high temporal resolution. However, further study is needed to describe the vertical refractivity gradient clearly.

Simulation of the stratosphere-troposphere exchange process in a typical cold vortex over Northeast China

A mesoscale weather research and forecasting (WRF) model was used to simulate a cold vortex that developed over Northeast China during June 19–23, 2010. The simulation used high vertical resolution to reproduce the key features of the cold vortex development. Characteristics of the associated stratosphere-troposphere exchange (STE), specifically the spatiotemporal distribution of the cross-tropopause mass flux (CTF), were investigated using the Wei formula. The simulation results showed that the net mass exchange induced by the cold vortex was controlled by stratosphere-to-troposphere transport (STT) processes. In the pre-formation stage of the cold vortex (i.e., the development of the trough and ridge), active exchange was evident. Over the lifecycle of the cold vortex, STT processes prevailed at the rear of the trough and moving vortex, whereas troposphere-to-stratosphere transport (TST) processes prevailed at the front end. This spatial pattern was caused by temporal fluctuations of the tropopause. However, because of the cancellation of the upward flux by the downward flux, the contribution of the tropopause fluctuation term to the net mass exchange was only minor. In this case, horizontal motion dominated the net mass exchange. The time evolution of the CTF exhibited three characteristics: (1) the predominance of the STT during the pre-formation stage; (2) the formation and development of the cold vortex, in which the CTF varied in a fluctuating pattern from TST to STT to TST; and (3) the prevalence of the STT during the decay stage.

Physical Properties of High-Level Cloud over Land and Ocean from CloudSat–CALIPSO Data

AbstractUnlike other cloud types, high-level clouds play an important role, often imposing a warming effect, in the earth–atmosphere radiative energy budget. In this paper, macro- and microphysical characteristics of cirrus clouds, such as their occurrence frequency, geometric scale, water content, and particle size, over northern China (land area, herein called the L area) and the Pacific Ocean (ocean area, herein the O area) are analyzed and compared based on CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) products from 1 January 2007 to 31 December 2010. Over both areas, statistical analysis shows that cirrus occurrence approached 33% in summer whereas it was only ~10% in winter, >50% of cirrus cloud thicknesses were in the range of ~(0.25–1.5) km, there were >98% ice particles in high-level clouds, and temperature had a closer linear relationship with ice effective radius (IER) than height. Also, the seasonal difference of this linear relationship is minor ove...

A New Method to Calibrate Shortwave Solar Radiation Measurements

AbstractA method is proposed to simultaneously calibrate shortwave (0.3–4 μm) global, direct, and scattering solar irradiance (GSI, DSI, and SSI, respectively) measurements. The method uses the World Radiation Reference (WRR) as a calibration standard and on-site radiation measurements as inputs. Two simple but effective techniques are used in the calibration. The first is to scale SSI and GSI detection sensitivities under overcast skies, which is based on the assumption that SSI should be equal to GSI if DSI is completely scattered and absorbed. The second is a new method to retrieve aerosol optical thickness (AOT), using the ratio of horizontal DSI (HDSI) to GSI measurements under clear and clean conditions. Thereafter, retrieved AOTs are used to drive a radiative transfer model to calculate atmospheric transmittance and then a ratio of GSI to the transmittance. Deviation of this ratio to the WRR is regarded as an indicator of GSI uncertainty, and the calibration transfer coefficient is derived as the W...

The Impact of Surface Properties on Downward Surface Shortwave Radiation over the Tibetan Plateau

The complexity of inhomogeneous surface-atmosphere radiation transfer is one of the foremost problems in the field of atmospheric physics and atmospheric radiation. To date, the influence of surface properties on shortwave radiation has not been well studied. The daily downward surface shortwave radiation of the latest FLASHFlux/CERES (Fast Longwave And Shortwave Fluxes_Time Interpolated and Spatially Averaged/Clouds and the Earth’s Radiant Energy System) satellite data was evaluated against in situ data. The comparison indicated that the differences between the two data sets are unstable and large over rugged terrain compared with relatively flat terrain, and the mean absolute error of the satellite products reaches 31.4 W m−2 (12.3%) over rugged terrain. Based on the SSF (single satellite footprint)/CERES product, the influence of surface properties on the distribution of downward surface shortwave radiation (DSSR) was analyzed. The influence of surface properties on DSSR over the Tibetan Plateau is about twice as large as that in two other regions located at the same latitude (eastern China-western Pacific and subtropical North Pacific). A simulation was carried out with the help of the I3RC (International Intercomparision of Three-Dimensional Radiation Code) Monte Carlo 3D radiative transfer community model. The results showed that DSSR increases as surface albedo increases. Moreover, the impact of surface albedo on DSSR is larger if the spatial distribution of clouds is more non-uniform. It is hoped that these results will contribute to the development of 3D radiative transfer models and the improvement of satellite inversion algorithms.