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Xinghua Bao
and
Fuqing Zhang

Abstract

More than 6000 independent radiosonde observations from three major Tibetan Plateau experiments during the warm seasons (May–August) of 1998, 2008, and 2015–16 are used to assess the quality of four leading modern atmospheric reanalysis products (CFSR/CFSv2, ERA-Interim, JRA-55, and MERRA-2), and the potential impact of satellite data changes on the quality of these reanalyses in the troposphere over this data-sparse region. Although these reanalyses can reproduce reasonably well the overall mean temperature, specific humidity, and horizontal wind profiles against the benchmark independent sounding observations, they have nonnegligible biases that can be potentially bigger than the analysis-simulated mean regional climate trends over this region. The mean biases and mean root-mean-square errors of winds, temperature, and specific humidity from almost all reanalyses are reduced from 1998 to the two later experiment periods. There are also considerable differences in almost all variables across different reanalysis products, though these differences also become smaller during the 2008 and 2015–16 experiments, in particular for the temperature fields. The enormous increase in the volume and quality of satellite observations assimilated into reanalysis systems is likely the primary reason for the improved quality of the reanalyses during the later field experiment periods. Besides differences in the forecast models and data assimilation methodology, the differences in performance between different reanalyses during different field experiment periods may also be contributed by differences in assimilated information (e.g., observation input sources, selected channels for a given satellite sensor, quality-control methods).

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Xinghua Bao
and
Fuqing Zhang

Abstract

The NCEP–NCAR reanalysis, NCEP Climate Forecast System Reanalysis (CFSR), 40-yr ECMWF Re-Analysis (ERA-40), and interim ECMWF Re-Analysis (ERA-Interim) products are evaluated with sounding observations from an enhanced radiosonde network available every 6 h during the Tibetan Plateau Experiment (TIPEX) conducted from 10 May to 9 August 1998. This study uses more than 3000 high-quality, independent rawinsondes at 11 stations (which were not assimilated in any of the reanalyses), which represents the first time that such a comprehensive evaluation is performed to assess the quality of these four most widely used reanalysis products over this region, which is highest in the world and crucial to the global climate and weather.

Averaging over the entire three-month period, it is found that each reanalysis dataset produces mean values of temperature and horizontal winds consistent with the verifying soundings (indicating relatively small mean bias); however, there are considerable differences (biases) in the mean relative humidity. On average, except for temperature at higher levels, both newer-generation reanalyses (CFSR and ERA-Interim) have smaller root-mean-square (RMS) error and bias than their predecessors (NCEP–NCAR and ERA-40). With some exceptions, the RMS errors of all variables for both CFSR and ERA-Interim (verifying with soundings) are similar in magnitude to the RMS difference between these two reanalyses, all of which are approximately twice as large as the corresponding observation errors. It is also found that there are strong diurnal variations in both RMS error and mean bias that differ greatly among different reanalyses and at different pressure levels.

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Xinghua Bao
,
Fuqing Zhang
, and
Jianhua Sun

Abstract

This study explores the diurnal variations of the warm-season precipitation to the east of the Tibetan Plateau over China using the high-resolution NOAA/Climate Prediction Center morphing technique (CMORPH) precipitation data and the Global Forecast System (GFS) gridded analyses during mid-May to mid-August of 2003–09. Complementary to the past studies using satellite or surface observations, it is found that there are strong diurnal variations in the summertime precipitation over the focus domain to the east of the Tibetan Plateau. These diurnal precipitation cycles are strongly associated with several thermally driven regional mountain–plains solenoids due to the differential heating between the Tibetan Plateau, the highlands, the plains, and the ocean. The diurnal cycles differ substantially from region to region and during the three different month-long periods: the pre-mei-yu period (15 May–15 June), the mei-yu period (15 June–15 July), and the post-mei-yu period (15 July–15 August).

In particular, there is a substantial difference in the propagation speed and eastward extent of the peak phase of the dominant diurnal precipitation cycle that is originated from the Tibetan Plateau. This diurnal peak has a faster (slower) eastward propagation speed, the more (less) coherent propagation duration, and thus covers the longest (shortest) distance to the east during the pre-mei-yu (post-mei-yu) period than that during the mei-yu period. The differences in the mean midlatitude westerly flow and in the positioning and strength of the western Pacific subtropical high during different periods are the key factors in explaining the difference in the propagation speed and the eastward extent of this dominant diurnal precipitation cycle.

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Tingting Qian
,
Ping Zhao
,
Fuqing Zhang
, and
Xinghua Bao

Abstract

The spatial variability and diurnal propagation of mean precipitation in the summer rainy seasons (from 2003 to 2010) over the Sichuan basin (SCB) and adjacent mountainous regions are examined using high spatiotemporal resolution satellite-derived precipitation estimates. The SCB is located just east of the Tibetan Plateau (TP) and is prone to heavy precipitation that often peaks over nighttime and early morning. The large-scale environment over the SCB during the rainy season is characterized by weak low- to midtropospheric convergence in the lee of the TP and by the upper-tropospheric jet stream to the north. Under this flow configuration, the study links the unique diurnal variations in the precipitation pattern and propagation to the unique topography in this region. It is found that during the rainy season, the local diurnal precipitation maximum moves primarily downslope and southeastward, from over the TP in the daytime to SCB at night. A secondary maximum moves northeastward downslope of the Yunnan–Guizhou Plateau toward the SCB from late evening to the early morning. The movement of precipitation over the SCB and the adjacent regions is closely tied to multiple regional-scale mountain–plain solenoids because of the large contrast in terrain heights between the basin and surrounding mountain ranges.

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Yali Luo
,
Renhe Zhang
,
Qilin Wan
,
Bin Wang
,
Wai Kin Wong
,
Zhiqun Hu
,
Ben Jong-Dao Jou
,
Yanluan Lin
,
Richard H. Johnson
,
Chih-Pei Chang
,
Yuejian Zhu
,
Xubin Zhang
,
Hui Wang
,
Rudi Xia
,
Juhui Ma
,
Da-Lin Zhang
,
Mei Gao
,
Yijun Zhang
,
Xi Liu
,
Yangruixue Chen
,
Huijun Huang
,
Xinghua Bao
,
Zheng Ruan
,
Zhehu Cui
,
Zhiyong Meng
,
Jiaxiang Sun
,
Mengwen Wu
,
Hongyan Wang
,
Xindong Peng
,
Weimiao Qian
,
Kun Zhao
, and
Yanjiao Xiao

Abstract

During the presummer rainy season (April–June), southern China often experiences frequent occurrences of extreme rainfall, leading to severe flooding and inundations. To expedite the efforts in improving the quantitative precipitation forecast (QPF) of the presummer rainy season rainfall, the China Meteorological Administration (CMA) initiated a nationally coordinated research project, namely, the Southern China Monsoon Rainfall Experiment (SCMREX) that was endorsed by the World Meteorological Organization (WMO) as a research and development project (RDP) of the World Weather Research Programme (WWRP). The SCMREX RDP (2013–18) consists of four major components: field campaign, database management, studies on physical mechanisms of heavy rainfall events, and convection-permitting numerical experiments including impact of data assimilation, evaluation/improvement of model physics, and ensemble prediction. The pilot field campaigns were carried out from early May to mid-June of 2013–15. This paper: i) describes the scientific objectives, pilot field campaigns, and data sharing of SCMREX; ii) provides an overview of heavy rainfall events during the SCMREX-2014 intensive observing period; and iii) presents examples of preliminary research results and explains future research opportunities.

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