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Juan Huo and Daren Lu

Abstract

Naked-eye observation of cloud cover has widely resisted automation. Replacement of human observation by instruments is an inexorable trend for the development of ground-based macroscopic cloud observation. In this paper, cloud covers from an all-sky imager (ASI) are compared with those from a meteorological observer (MO) through field experiments performed at three sites in China. The correlation coefficient between ASI and MO is 0.77 for all cases. The ASI cloud fractions have great agreement with MO for clear sky, overcast sky, and sky loaded with low- and middle-level clouds. About 78% of the ASI cases had deviations between ±1 tenth compared to MO cloud cover. High-level cloud (or aerosol) is the main reason causing this difference. It is partially due to MO, who takes aerosol as high, thin cloud. Another reason might be that ASI made a wrong estimation for high-level cloud (or aerosol) because of its detector and the cloud-determination algorithm. Distinguishing high, thin cloud from aerosol is a challenge, and is the main problem that needs to be resolved for future developments of ASI. A new, improved method is discussed at the end of this paper.

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Juan Huo and Daren Lu

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Unlike 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 over both land and ocean. Comparisons reveal that the mean occurrence frequency, mean cloud thickness, range of cloud-base and cloud-top height, IER, and ice water content of cirrus in summer were generally greater in winter, and greater over the O area than over the L area. However, the relationship between IER and temperature over land is close to that over ocean.

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Juan Huo and Daren Lu

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.

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Daren Lu, T. E. VanZandt, and W. L. Clark

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 implications of these observations on the relations between thunderstorms and buoyancy waves in the free atmosphere are discussed.

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Ling Zhang, Daren Lu, Shu Duan, and Jinli Liu

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 used to partially correct this error. 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).

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Xiushu Qie, Xueke Wu, Tie Yuan, Jianchun Bian, and Daren Lu

Abstract

Diurnal 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) height, ice-particle content, and lightning flash rate—is the strongest. DCSs over the TP are relatively weak in convective intensity and small in size but occur frequently. Oceanic DCSs possess the tallest cloud top (which mainly reflects small ice particles) and the largest size, but their convective intensity is markedly weaker. DCSs and IDCSs show a similar diurnal variation, mainly occurring in the afternoon with a peak at 1600 local time over land. Although most of both DCSs and IDCSs occur between April and October, DCSs have a peak in August, whereas IDCSs have a peak in May.

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Jinhuan Qiu, Xiangao Xia, Jianghui Bai, Pucai Wang, Xuemei Zong, and Daren Lu

Abstract

A 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 WRR ratio. The method is applied to calibrate radiation measurements at Xianghe, China, during 2005. It is estimated from the derived transfer coefficients on 36 clear and clean days that uncertainties of DSI, GSI, and SSI measurements are within −4.0% to 2.9%, −5.9% to 2.4%, and −6.1% to 4.9%, respectively. The calibration is further validated based on comparisons of AOT at 750 nm retrieved from HDSI/GSI to Aerosol Robotic Network (AERONET) AOT products. The maximum deviation between two AOT products is 0.026. The unique advantage of this method lies in its potential applications in correcting historic radiation measurements and monitoring radiometer performance.

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C. M. Platt, S. A. Young, A. I. Carswell, S. R. Pal, M. P. McCormick, D. M. Winker, M. DelGuasta, L. Stefanutti, W. L. Eberhard, M. Hardesty, P. H. Flamant, R. Valentin, B. Forgan, G. G. Gimmestad, H. Jäger, S. S. Khmelevtsov, I. Kolev, B. Kaprieolev, Da-ren Lu, K. Sassen, V. S. Shamanaev, O. Uchino, Y. Mizuno, U. Wandinger, C. Weitkamp, A. Ansmann, and C. Wooldridge

The Experimental Cloud Lidar Pilot Study (ECLIPS) was initiated to obtain statistics on cloud-base height, extinction, optical depth, cloud brokenness, and surface fluxes. Two observational phases have taken place, in October–December 1989 and April–July 1991, with intensive 30-day periods being selected within the two time intervals. Data are being archived at NASA Langley Research Center and, once there, are readily available to the international scientific community.

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