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Liu Huizhi and Feng Jianwu

Abstract

Seasonal and interannual variations of evapotranspiration (ET) and energy exchange were estimated over degraded grassland and cropland land surfaces in a semiarid region of northeastern China using the eddy covariance technique from 2003 to 2008. The peak daily ET, which occurred in August, was 1.5–4.5 mm day−1 for the degraded grassland and 1.5–5.5 mm day−1 for the cropland land surface. Annual cumulative ET was roughly equal to annual precipitation at both sites. However, the annual cumulative ET at the cropland site was slightly larger (about 10–30 mm) than it was at the grassland at the end of each year. More water might come from irrigation at seedtime and from the soil. With the factor analysis technique, the results revealed that the atmospheric water demand was the most important factor in the ET process on a half-hour time scale in this semiarid area. On a seasonal time scale, ET was greatly constrained by surface conductance and precipitation; on an annual time scale, ET was greatly constrained by the total amount of precipitation at both sites. The accuracy of ET estimation using the Penman–Monteith formula in this semiarid area was also discussed.

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Yong Chen, Huizhi Liu, Junling An, Ulrich Görsdorf, and Franz H. Berger

Abstract

Small-scale summer rainfall variability in a semiarid zone was studied by deploying five vertically pointing Micro Rain Radars (MRRs) along a nearly straight line and by using 12 rain gauges in the study area of the Xilin River catchment in China. The spatial scales of 4 and 9 km correspond to the resolution of precipitation radar and rainfall products from satellites. The dataset of the MRRs and rain gauges covers two months in the summer of 2009. Three parameters, that is, spatial correlation, intermittency, and the coefficient of variation (CV), were used to describe the rainfall variability as based on the data from the MRRs and rain gauges. The probability of partial beamfilling in a 4-km (9 km) pixel over a 30-min temporal scale was 17%–20% (28%–37%). More accurate equipment can measure lower rainfall intermittency. For scales of 4 and 9 km, the median CV of the accumulation times that were longer than 3 h with rainfall > 1 mm was 0.17–0.42. The accuracy of areal rainfall measured by different quantities of equipment was also evaluated. One MRR was sufficient for measuring the daily areal rainfall at a 4-km scale, with a fraction of prediction within a factor of 2 of observations of 1.0 and a correlation coefficient of ≥0.58 when daily mean rainfall was >1 mm.

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Kun Yang, Toshio Koike, Hirohiko Ishikawa, Joon Kim, Xin Li, Huizhi Liu, Shaomin Liu, Yaoming Ma, and Jieming Wang

Abstract

Parameterization of turbulent flux from bare-soil and undercanopy surfaces is imperative for modeling land–atmosphere interactions in arid and semiarid regions, where flux from the ground is dominant or comparable to canopy-sourced flux. This paper presents the major characteristics of turbulent flux transfers over seven bare-soil surfaces. These sites are located in arid, semiarid, and semihumid regions in Asia and represent a variety of conditions for aerodynamic roughness length (z 0 m; from <1 to 10 mm) and sensible heat flux (from −50 to 400 W m−2). For each site, parameter kB −1 [=ln(z 0 m/z 0 h), where z 0 h is the thermal roughness length] exhibits clear diurnal variations with higher values during the day and lower values at night. Mean values of z 0 h for the individual sites do not change significantly with z 0 m, resulting in kB −1 increasing with z 0 m, and thus the momentum transfer coefficient increases faster than the heat transfer coefficient with z 0 m. The term kB −1 often becomes negative at night for relatively smooth surfaces (z 0 m ∼ 1 mm), indicating that the widely accepted excess resistance for heat transfer can be negative, which cannot be explained by current theories for aerodynamically rough surfaces. Last, several kB −1 schemes are evaluated using the same datasets. The results indicate that a scheme that can reproduce the diurnal variation of kB −1 generally performs better than schemes that cannot.

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Ping Zhao, Xiangde Xu, Fei Chen, Xueliang Guo, Xiangdong Zheng, Liping Liu, Yang Hong, Yueqing Li, Zuo La, Hao Peng, Linzhi Zhong, Yaoming Ma, Shihao Tang, Yimin Liu, Huizhi Liu, Yaohui Li, Qiang Zhang, Zeyong Hu, Jihua Sun, Shengjun Zhang, Lixin Dong, Hezhen Zhang, Yang Zhao, Xiaolu Yan, An Xiao, Wei Wan, Yu Liu, Junming Chen, Ge Liu, Yangzong Zhaxi, and Xiuji Zhou

Abstract

This paper presents the background, scientific objectives, experimental design, and preliminary achievements of the Third Tibetan Plateau (TP) Atmospheric Scientific Experiment (TIPEX-III) for 8–10 years. It began in 2013 and has expanded plateau-scale observation networks by adding observation stations in data-scarce areas; executed integrated observation missions for the land surface, planetary boundary layer, cloud–precipitation, and troposphere–stratosphere exchange processes by coordinating ground-based, air-based, and satellite facilities; and achieved noticeable progress in data applications. A new estimation gives a smaller bulk transfer coefficient of surface sensible heat over the TP, which results in a reduction of the possibly overestimated heat intensity found in previous studies. Summer cloud–precipitation microphysical characteristics and cloud radiative effects over the TP are distinguished from those over the downstream plains. Warm rain processes play important roles in the development of cloud and precipitation over the TP. The lower-tropospheric ozone maximum over the northeastern TP is attributed to the regional photochemistry and long-range ozone transports, and the heterogeneous chemical processes of depleting ozone near the tropopause might not be a dominant mechanism for the summer upper-tropospheric–lower-stratospheric ozone valley over the southeastern TP. The TP thermodynamic function not only affects the local atmospheric water maintenance and the downstream precipitation and haze events but also modifies extratropical atmospheric teleconnections like the Asia–Pacific Oscillation, subtropical anticyclones over the North Pacific and Atlantic, and temperature and precipitation over Africa, Asia, and North America. These findings provide new insights into understanding land–atmosphere coupled processes over the TP and their effects, improving model parameterization schemes, and enhancing weather and climate forecast skills.

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