Xiangde Xu

A New Integrated Observational System Over the Tibetan Plateau

| 1492 SCIENTIFIC BACKGroUNd. Because it is the largest and highest plateau in the world, the Tibetan Plateau plays an important role in the global atmospheric energy and water cycles. The large water resource stored in this region and its downstream transports support nearly 40% of the world population, including China, India, and many Southeast and East Asian countries. The Tibetan Plateau accounts for nearly one-quarter of the total land area of China, but meteorological observational and operational stations are scarce over this vast land. This has contributed to the uncertainties in the prediction of high-impact weather over the plateau and regions downstream, and in the monitoring and diagnostics of global climate. In 1979, China carried out a field experiment to investigate the role of the Tibetan Plateau in the global atmospheric circulation. It was this experiment that provided, for the first time, a qualitative understanding of general aspects of Tibetan Plateau meteorology and its role in the Asian Monsoon. In 1998, Chinese and Japanese scientists jointly conducted the second atmospheric science experiment over the plateau, under the sponsorship of GAME/ Tibet, TiPEX, and CAMP/Tibet (these various components of the experiment were summarized in detail by Ma et al. in the companion to this article in this issue of BAMS). Significant progress was made during these experiments. Improved descriptions of the Tibetan Plateau’s land–atmospheric interaction and various hydrometeorological processes have been studied quantitatively using more sophisticated datasets and modeling tools. These research activities and field experiments are now being extended into a new phase: the Tibetan Observation and Research Platform (TORP), also discussed in the companion paper by Ma et al. With the understanding of the Tibetan Plateau’s importance in the regional and global atmospheric circulations gained from these field experiments, it is highly desirable to establish a set of more permanent and operational observing networks. Consequently, a New Integrated Observational System over the Tibetan Plateau (NIOST), supported by the Chinese–Japanese joint AFFILIATIoNS: Xu, r. zhang, shi, s. zhang, bian, Cheng, and ding—State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China; KoiKe— University of Tokyo, Tokyo, Japan; lu—NOAA Earth System Research Laboratory, Boulder, Colorado; li—Tianjin Institute of Meteorological Science, Tianjin, China CorrESPoNdING AUThor: Chungu Lu, NOAA Earth System Research Laboratory, David Skaggs Research Center, GSD7, Boulder, CO 80305 E-mail: chungu.lu@noaa.gov

Measurements of fine-scale structure at the top of marine stratocumulus

During the Dynamics and Chemistry of the MarineStratocumulus (DYCOMS) experiment in July–August 1985, the NCAR Electra aircraft flew a series of flight legs just at the top of the marinestratocumulus cloud decks that cap the mixed layer off the coast of southernCalifornia. Because of the corrugated structure of the cloud-top, the aircraft, which was flown at a nearly constant level and adjusted only to maintain its altitude at the average cloud-top height, was alternately within and above the clouds – roughly half the time in each domain. These legs were used to examine the structure of the cloud-top by compositing the segments on either side of the cloud/clear-air interface, which was identified by the transitions of liquid water measured by the Forward Scattering Spectrometer Probe (either increasing or decreasing) through a threshold of 0.04 × 10-3 kg m-3.An equivalent vertical distance (EVD) from the cloud-top was obtained from the horizontal flight legs by estimating the average slope of the cloud-top from the cloud-top radiation temperature. The results show that a near discontinuity occurs in variables across cloud top over an EVD of 0.3 m, but that above this, the air has already been modified by boundary-layer air. Thus, cloud-top is not the limit of mixing of boundary-layer air. This mixing may extend to tens of metres or more. The bulk Richardson number in the vicinity of cloud-top increases from near zero within the cloud to about 1.2 at an EVD of 3–6 m above cloud. Fluctuations of the three velocity components within cloud are nearly equal; above cloud the vertical component structure function is about half the horizontal components. The scalar structure functions are about an order of magnitude higher above cloud than in cloud. The structure parameters of temperature and humidity measured just below cloud-top agree reasonably well with predicted values based on a previously-developed model for the clear convective boundary layer. Above cloud, the scalar structure parameters are much larger, but their interpretation is questionable, since this region does notcontain isotropic turbulence.

Improving land surface soil moisture and energy flux simulations over the Tibetan plateau by the assimilation of the microwave remote sensing data and the GCM output into a land surface model

The land surface soil moisture is a crucial variable in weather and climate models. This study presents a land data assimilation system (LDAS) that aims to improve the simulation of the land surface soil moisture and energy fluxes by merging the microwave remote sensing data and the general circulation model (GCM) output into a land surface model (LSM). This system was applied over the Tibetan Plateau, using the National Centers for Environmental Prediction (NCEP) reanalysis data as forcing data and the Advanced Microwave Scanning Radiometers for EOS (AMSR-E) brightness temperatures as an observation. The performance of our four data sources, which were NCEP, AMSR-E, LDAS and simulations of Simple Biosphere Model 2 (SiB2), was assessed against 5 months of in situ measurements that were performed at two stations: Gaize and Naqu. For the surface soil moisture, the LDAS simulations were superior to both NCEP and SiB2, and there was more than a one-third reduction in the root mean squared errors (RMSE) for both of the stations. Compared with the AMSR-E soil moisture retrievals, the LDAS simulations were comparable at the Gaize station, and they were superior at the Naqu station. For the whole domain intercomparison, the results showed that the LDAS simulation of the soil moisture field was more realistic than the NCEP and SiB2 simulations and that the LDAS could estimate land surface states properly even in the regions where AMSR-E failed to cover and/or during the periods that the satellite did not overpass. For the surface energy fluxes, the LDAS estimated the latent heat flux with an acceptable accuracy (RMSE less than 35 W/m 2 ), with a one-third reduction in the RMSE from the SiB2. For the 5-month whole plateau simulation, the LDAS produced a much more reasonable Bowen Ratio than the NCEP, and it also generated a clear contrast of the land surface status over the plateau, which was wet in the southeast and dry in the northwest, during the monsoon and post-monsoon seasons. Because the LDAS only uses globally available data sets, this study reveals the potential of the LDAS to improving the land surface energy and water flux simulations in ungauged and/or poorly gauged regions.

On the use of GPS measurements for Moderate Resolution Imaging Spectrometer precipitable water vapor evaluation over southern Tibet

Received 26 April 2011; revised 7 October 2011; accepted 7 October 2011; published 15 December 2011. [1] Precipitable water vapor (PWV) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) near‐infrared measurements from Terra and Aqua satellites have not been evaluated over southern Tibet, which has an elevated and complex topography. This study uses ground measurements from a network of 22 GPS receivers to evaluate the accuracy of MODIS PWV over this region. The evaluation shows that both the Terra and Aqua MODIS PWV have a high correlation with GPS measurements, but they generally tend to overestimate water vapor under clear‐sky conditions, with scale factors from 1.06 to 1.19 and root mean square errors from 2.1 to 3.19 mm. The overestimation is mainly attributed to the systematic bias of MODIS PWV at sites below 3000 m, which increases with increasing water vapor, especially during the monsoon season, when the summer monsoon brings a large amount of water vapor from the tropical ocean to southern Tibet. In contrast, MODIS PWV at sites above 3000 m matches well with the GPS PWV. A linear fit model that was proposed and employed in previous studies is used to correct the errors of MODIS PWV at low sites. The performance of MODIS PWV is improved at each low site after the correction. The corrected MODIS PWV can represent the monthly variation of daily average GPS PWV within a 2% error. This evaluation study should facilitate the use of MODIS‐derived water vapor in the models that calculate radiative forcing and simulate climate change over Tibet and its surrounding areas.

Localized 3D-structural features of dynamic-chemical processes of urban air pollution in Beijing winter

The Beijing City Air Pollution Observation Field Experiment (BECAPEX) is described with emphases on the “point-surface” research approach and composite analysis. The analysis results of measurements from four observation sites across the Beijing urban area from January to March indicate that the overall impact of urban emission sources in the heating season is significant, and the staggered impact of urban emission sources has different features at observation sites over different parts of Beijing in both heating and non-heating seasons. The pollutants NOx, SO2 and CO in the urban boundary layer have the in-phase variation features over a large area. O3 concentrations at different sites have the same variation trend but its change is reversed phases with above pollutants. The pollutants over the urban area in heating and non-heating seasons also have the synchronous variation trend. The comprehensive sounding of BECAPEX indicates that pollutants and aerosol vertical profiles are closely correlated to the vertical structure of the large-scale inversion layer in the urban boundary layer over the urban area. The localized 3D-structural features of local urban polluting processes associated with the peripheral areas are discussed with a “point-surface” comprehensive sounding technique.

The Dynamic and Thermodynamic Structures Associated with a Series of Heavy Precipitation Events over China during January 2008

Abstract In the winter of 2008, China experienced once-in-50-yr (or once in 100 yr for some regions) snow and ice storms. These storms brought huge socio economical impacts upon the Chinese people and government. Although the storms had been predicted, their severity and persistence were largely underestimated. In this study, these cases were revisited and comprehensive analyses of the storms’ dynamic and thermodynamic structures were conducted. These snowstorms were also compared with U.S. east coast snowstorms. The results from this study will provide insights on how to improve forecasts for these kinds of snowstorms. The analyses demonstrated that the storms exhibited classic patterns of large-scale circulation common to these types of snowstorms. However, several physical processes were found to be unique and thought to have played crucial roles in intensifying and prolonging China’s great snowstorms of 2008. These include a subtropical high over the western Pacific, an upper-level jet stream, and tem...

Variability and Trends of High Temperature, High Humidity, and Sultry Weather in the Warm Season in China during the Period 1961–2004

Abstract Using the daily maximum air temperature and mean humidity observations at 394 surface weather stations across China, the changes in the annual number of days of high temperature weather (HTW), high humidity weather (HHW), and sultry weather (STW) in China over the period 1961–2004 are studied. The results indicate that there were considerable spatial differences and temporal variability of HTW, HHW, and STW across China. Under a climatic mean condition, a notable feature is that southeastern China is the region of collocation of high values of the annual number of days of HTW, HHW, and STW, as well as the region of the most significant variabilities of these parameters. About 55% of the stations in China have increasing trends of the annual number of days of HTW. Most stations in China show decreasing trends of the annual number of days of HHW and are mainly located either in the area south of 30°N or in northern and northeastern China. The stations with increasing trends of the annual number of ...

Extreme precipitation events in East China and associated moisture transport pathways

Interannual variation of summer precipitation in East China, and frequency of rainstorms during the monsoon season from 1961 to 2010, are analyzed in this study. It is found that the two variables show opposite trends on a decadal time scale: frequency of rainstorms increases significantly after the 1990s, while summer precipitation in East China decreases during the same period. Analysis of the spatial distribution of summer rainstorm frequency from 1961 to 2010 indicates that it decreases from the southeast to the northwest at the east edge of the large-scale topography associated with the plateaus. Spatial distribution of rainstorms with daily rainfall greater than 50 mm is characterized by a “high in the southeast and low in the northwest” pattern, similar to the staircase distribution of the topography. However, the spatial distribution of variation in both summer precipitation and frequency of extreme rainstorms under global warming differs significantly from the three-step staircase topography. It is shown that moisture characteristics of summer precipitation and extreme rainstorms during the monsoon season in East China, including moisture transport pathways, moist flow pattern, and spatial structure of the merging area of moist flows, differ significantly. Areas of frequent rainstorms include the Yangtze River Valley and South China. Column- integrated moisture transport and its spatial structure could be summarized as a “merging” of three branches of intense moist flows from low and middle latitude oceans, and “convergence” of column-integrated moisture fluxes. The merging area for moist flow associated with rainstorms in the high frequency region is located slightly to the south of the monsoonal precipitation or non-rainstorm precipitation, with significantly strong moisture convergence. In addition, the summer moist flow pattern in East China has a great influence on the frequency of extreme rainstorms. Moisture flux vectors in the region of frequent rainstorms correspond to vortical flow pattern. A comparison of moisture flux vectors associated with non-rainstorms and rainstorms indicates that the moist vortex associated with rainstorms is smaller in size and located to the south of the precipitation maximum, while the moist vortex associated with non-rainstorms is larger and located to the north. It is shown that column- integrated moist transport vortices and the structure of moist flux convergence have significant impacts on the north-south oscillation of frequent rainstorm areas in East China, which is synchronized with the maximum vorticity of moisture transport and the minimum of convergence on the decadal time scale. Synthesis of moisture transport pathways and related circulation impacts leads to a conceptual model of moisture flow associated with rainstorms.

Study on variational aerosol fields over Beijing and its adjoining areas derived from Terra-MODIS and ground sunphotometer observation

This paper presents a comprehensive observation technique on derived aerosols data from mobile sunphotometer graph and Terra-Moderate Resolution Imaging Spectroradiometer (MODIS) observation. Research results suggest that after being treated by the variational technique with respect to sunphotometer observations, the Terra-MODIS remote sensing aerosol data are remarkably improved, thus for the first time revealing features of the influence of aerosols and pollution emissions of Beijing and its adjoining areas (Hebei, Shandong, etc. provinces). The regional impact features of aerosols are related with the peripheral U-shape topography of Beijing. Analyses with Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT-4) and meteorological data in the case studied confirm the pollutants diffusion process along the trajectory from the sources in the south-west region, and the regional aerosol impact features.

Structures of convection and turbulent kinetic energy in boundary layer over the southeastern edge of the Tibetan Plateau

Based on a comprehensive analysis on Sonic Anemometer and gradient data, wind profile radar (WPR) and GPS sounding data of March–August 2008 from the boundary layer (BL) tower observation system at Dali on the southeastern edge of Tibetan Plateau (TP), it is found that the strengths of turbulent kinetic energy (TKE), buoyancy term and shear term depend on vegetation cover in association with local stability and thermodynamic condition. Strong kinetic turbulence appears when near surface layer in neutral condition with the large contribution from shear term. In an unstable condition within near surface layer, the atmospheric turbulent motion is mainly thermal turbulence, as buoyancy term is obviously larger than shear term. Under a stable condition the intermittent turbulence is accompanied by weak shear and buoyancy term, and TKE is significantly less than neutral or instable condition. The study also presents that the buoyancy term contribution at Nyingchi station in the southern slopes of the TP large topography in spring is significantly larger than that at Dali over the southeastern TP edge, reflecting that the thermal turbulence makes an important contribution to convection activity in the southern slopes of TP. Dali station is located in complex terrain with mountain and valley leading to larger kinetic turbulence. From the perspective of interaction of turbulence-convection in different scales, the study revealed that the height of convective boundary layer (CBL) could reach up to 1500–2000 m. TKE, shear term, and buoyancy term in near surface layer have the notable correlations with BL height and local vertical motion. The daytime thermodynamic turbulence effect of heat flux and buoyancy term has an obvious impact on the height of CBL, whereas mechanical turbulence only exerts a less impact. Mechanical turbulence in near surface layer has a significant impact on vertical motion especially in the forenoon with impacting height of 2500–3000 m. The peaks in diurnal variations of shear term and buoyancy term correspond to the high instable periods, especially in summer forenoon. Our observation analysis characterized the convection activity triggered by TKE source and their interaction in the southeastern TP edge.