Abstract:Observation data of the boundary-layer tower at the Dali National Climate Observatory from April to May 2008 were used to investigate the characteristics of wind speed, direction, profile, vertical shear, momentum flux, and sensible and latent heat flux in the surface layer before and after the southwestern monsoon burst in 2008. The results show that the main wind direction was southeast in April and northwest in May. The diurnal pattern of wind speed before and after the monsoon burst showed two peaks and one peak, respectively. After the monsoon burst, the wind speed, vertical shear, and momentum flux became lower. The latent heat flux was larger than the sensible heat flux throughout the entire period. The latent heat flux decreased (increased) before (after) the monsoon burst. The peak values of diurnal variation of monthly mean latent flux did not change significantly during the period, although the monthly mean values of sensible heat flux decreased monthly from April to June. The causes of the changed characteristics for wind and fluxes correlated with the weather and land surface conditions.

Abstract:The characteristics and differences of seasonal changes of the tropical cyclone (TC) activities in terms of genesis location, genesis frequency, intensity, and duration are analyzed over the South China Sea (SCS) (5°N-25°N, 110°E-120°E) and the western North Pacific (WNP) (5°N-25°N, 120°E-180°) using the TC datasets for the period 1945-2011 from the Joint Typhoon Warning Center (JTWC). Further examined are the impacts of ENSO events on the interannual variability of TC activities over the two areas from the perspective of TC longitude crossings. The results show that the TC genesis in the SCS is modulated significantly by the SCS monsoon, with a small number of weak TCs forming only south of 10°N during months of January to April when the SCS winter monsoon prevails, which results from favorable dynamical conditions confined to lower latitudes in terms of low-level cyclonic vorticity and weak vertical wind shear. In contrast, the period from June to September is the SCS TC season during which TCs generate most frequently under the impact of the SCS summer monsoon season, with majority of cyclogenesis forming around 17°N. Abrupt northward (southward) shift of TC genesis position accompanied by the apparent increase (decrease) in cyclogenesis frequency takes place in the seasonal transition of May (October), depending on the onset (retreat) of the SCS summer monsoon in relation to drastic changes of the weak vertical wind shear and mid-layer relative humidity higher than 40% in the SCS. With the higher relative humidity concentrating over southern SCS in the mid-layer, the TC genesis positions again return back south of 10°N during November-December. However, the TC genesis in the WNP exhibits a gradually seasonal variation with an evident change only appearing in July. Note that the period July-October is the dominant typhoon season in the WNP. Because weaker vertical easterly wind shear is present over the NWP in almost all months of the year, most TCs generated in this region can reach a stronger intensity and develop into typhoons. But over the SCS, the TC intensity is weaker than that over the NWP, with most TCs evolving into only tropical storms. ENSO events exert a significant impact on changes in regions favorable for TC genesis during different seasons and consequently on geographical variations in TC tracks. During an El Niño event for both developing phase and decaying phase, the departure from climatology of monthly TC longitude crossings is negative anomaly in the SCS and positive anomaly in the WNP. Opposite situation occurs during a La Niña event.

Abstract:The performance of IAP Atmospheric General Circulation Model Version 4 (IAP AGCM4.0) in reproducing the observed features of the East Asian Subtropical Westerly Jet (EASWJ) is examined by analyzing the differences between model simulations and JRA-25 reanalysis data during 1979-2008, the possible reasons for the bias in model performance are analyzed, and the impact of convective parameterization schemes in the model simulations has been further investigated. The results show that the model can reproduce the climatological characteristics of the EASWJ reasonably well, including the spatial structure and the seasonal meridional displacement. However, the model also shows deficiencies in simulating the intensity and location of the EASWJ, with weaker magnitude in both wintertime and summertime, and the model simulated position of the EASWJ is located southward during wintertime and northward during summertime when compared with observations. Analysis of the results demonstrates that IAP AGCM4.0 can capture the seasonal evolution of the EASWJ well, including the northward jump during summer months. Based on Empirical Orthogonal Function (EOF) decomposition of the zonal wind at 200 hPa, both for the observation and model simulation, it is found that IAP AGCM4.0 can reproduce well the spatial structure of the first leading mode from the observation; however, the model fails to capture the interannual variation of the meridional displacement of the EASWJ. Based on the analysis of surface sensible heat fluxes and mid-upper troposphere Meridional Temperature Difference (MTD), it is found that the model's biases in terms of the simulated surface sensible heat fluxes in the southeastern Arabian Peninsula, northwestern Arabian Sea and northern India cause a weaker intensity and northward shift of MTD. The deficiency of IAP AGCM4.0 in reproducing the interannual variation of MTD leads to the biases in the simulation of the EASWJ. Furthermore, different convective parameterization schemes can influence the simulation of MTD, which will in turn directly affect simulation of the EASWJ.

Abstract:The authors used the NCEP reanalysis system to make integrated analysis by region and have found environmental conditions associated with tornadoes taken from Year-book of Meteorological Disasters in China (Xu Xiaofeng, 2005-2012) for the period of 2004-2009. Then an index has been formulated from analyzing kinds of parameters derived from proximity soundings. Results show that favorite significant tornado environments were concentrated in the east of China. The coordinating between low and high situation was important as well as instability energy accumulation and vapor transport. It was also proven that tornadoes in China occur in conditions accompanied by moderate convective available potential energy (CAPE), high wind shear (0-6 km layer shear and 0-1 km layer shear) and low cloud base. The results of this study were used to create a universal tornadic index based on NCEP reanalysis data and designed to forecast activity in tornadic environment. The quality of this index was tested with tornado activities in China for the period from 2010 to 2011.

Abstract:The relationships of the general circulation and East Asian winter monsoon (EAWM) index anomalies [e.g., the East Asian trough location index (CW), Siberian High intensity index (SH)] with the winter snowfall (WS) over the Tibetan Plateau (TP), as well as the interdecadal characteristics, during 1961-2010, are analyzed. The results show that: 1) The abnormalities of winter planetary waves in the Northern Hemisphere (NH) impact on the WS over the TP on interdecadal scales; 2) The variations of the planetary wave pattern in the NH, “Three Troughs/Ridges in winter”, are the background circulation for the WS change over the TP on interdecadal scales; 3) The WS over the TP and the EAWM indices indicate significant correlation on interdecadal scales; 4) The WS increases over the TP with a decreasing trend of the EAWM during 1961-1986; 5) The WS over the TP, the CW and SH, all show an abrupt change around 1986. After this abrupt change, the intensity of the winter “three troughs” pattern demonstrates a stronger variation, while the amount of WS over the TP declines.

Abstract:The tropical cyclones (TCs) that occurred during 2002-2011 in the northwestern Pacific Ocean and the connection between the moisture and their ability to subsequently undergo rapid intensification (RI) are investigated with the NCEP FNL (Final Analysis) data and the best-track datasets of the JTWC (Joint Typhoon Warning Center), which is organized into 24-h intervals of either RI and slow intensification/constant intensity (non-RI) periods. The environmental and internal distributions of moisture and its transport at each interval are statistically analyzed to reveal their significant influence on the intensity change in the subsequent 24 hours. The results show that relative humidity area-averaged from the radius of 3° to 10° latitude at 900 hPa (RH_3-10) can clearly distinguish RI from non-RI, which means the sensitive vertical level of moisture in the northwestern Pacific Ocean is lower than that in the Atlantic Ocean. In addition, larger RH_3-10 values and smaller horizontal moisture flux (F_all) mark the beginning of RI. With the consumption of the convection development in RI, the environmental humidity decreases and the moisture flux is enhanced. The obvious differences of the moisture distribution and transfer between the TC inner-core and outer-rainband region are also studied, showing that the net convergence region of moisture flux in RI is larger than that in non-RI. The correlative analysis also indicates that intensity forecasts and RI predictions in particular may be aided by the use of the moisture and its flux, both in the environmental and internal region.

Abstract:The impact of real vegetation cover changes in typical years on the regional climate of the Zoigê Plateau in 2010 was studied by using the Weather Research and Forecasting (WRF) model and the surface classification data of satellite remote sensing in the years 1981, 1990, 2000, and 2010. The main conclusions are: 1) The simulated latent heat flux has clearly changed in association with different surface classification data; the dominant climate response is the variation of surface temperature; and the local change is more significant than the regional average. 2) The change is driven by changes in both transpiration efficiency and albedo, with changes in transpiration efficiency dominating in Zoigê in summer. 3) After comparing the simulated profiles of temperature and humidity among different surface data, we concluded that simulated changes in vegetation cover affect temperature and humidity at low levels, but impose only small disturbance on temperature at high levels.

Abstract:Based on observed and reanalysis data, this study analyzes the spatiotemporal variations of monthly surface temperature in winter in China, their interrelationship, and the related anomalous circulation on the interannual timescale. The atmospheric internal mechanisms for the interannual variation of surface temperature is also discussed. The dominant interannual mode is uniform across China, except in the Tibetan Plateau, for all winter months, while the second mode is a north-south dipole mode with opposite sign. The dominant interannual modes of surface temperature in January and February are closely correlated to each other in phase, but are to some extent negatively correlated with that in the prior December. The dominant interannual modes of monthly surface temperature in winter are also linked to the opposite-sign dipole anomalous patterns of surface temperature in the Eurasian continent. The anomalous temperature signals associated with the nationwide winter surface temperature anomalies may extend up to high levels in the troposphere, with the most intense signals at the surface. The surface temperature anomalies may induce the air temperature anomalies from the near surface to the upper troposphere via the upward longwave radiation flux anomalies emitted from the underlying surface. In all winter months, the intensified westerly over the northern Eurasian continent hinders the southward intrusion of cold air from high latitudes, and then results in a deep warm anomaly over winter in China, and an even larger part of the Eurasian continent.

Abstract:Based on the dataset of hourly rain gauge records in central East China for the period 1961-2012, the spatiotemporal characteristics of summer rainfall events in terms of their durations are analyzed. The results show that the rainfall duration exhibits a short-long-short meridional pattern in central East China, i.e., short-duration rainfall events (1-6 h) prevail in southern and northern China, whereas long-duration rainfall events (>6 h) contribute more than 60% to the total rainfall in the region between about 28°N and 37°N. As the duration increases, the occurrence of light rainfall events with intensity weaker than 1 mm/h decreases, while the occurrence of moderate rainfall events with intensify between 1.1 mm/h and 10.0 mm/h increases. Rainfall events with intensity stronger than 10.0 mm/h tend to be characterized by relatively short duration in the region south of about 35°N. The total summer rainfall amount increases in southern China due to the increasing trends of both intensity and occurrence, and decreases in northern China with decreasing occurrence but increasing intensity. The rainfall duration generally shows an increasing trend in central East China, especially in the region between the Yangtze River and Yellow River. The significant increase of total rainfall amount in this region from the late 1970s to early 1990s is mainly attributed to the increase of long-duration rainfall amount, while no obvious interdecadal abrupt change is detected in the short-duration rainfall amount. The significant increase of total rainfall amount in southern China after the early 1990s is related to both the increase of long-duration and short-duration rainfall amount, but the impact of the former is more wide-ranging and northward.

Abstract:Frequent heavy rainfall (HR) can cause flood disasters in summer, which seriously affect the development of society and the economy in China. Based on a large number of studies on summer precipitation and synoptic characteristics of summer HR in China, the main advancements in summer HR research, from the climatological standpoint, are summarized. In particular, progress in studying certain aspects such as HR-related interannual and interdecadal variations and changing trends, the characteristics of associated large-scale circulations and their causes, and the evolutionary tendency under global warming, are reviewed. In addition, this paper also highlights some scientific problems relating to the climatology of summer HR over China that require further research.