Integrated monitoring systems for the land surface, boundary layer, troposphere, and lower stratosphere over the Tibetan Plateau promote the understanding of the Earth–atmosphere coupled processes and their effects on weather and climate.
The Tibetan Plateau (TP), known as the “sensible heat pump” and the “atmospheric water tower,” modifies monsoon circulations and regional energy and water cycles over Asia (Wu and Zhang 1998; Zhao and Chen 2001a; Wu et al. 2007; Xu et al. 2008b; Zhou et al. 2009). Strong ascent over the TP may transport lower-tropospheric water vapor and anthropogenic pollutants into the upper troposphere–lower stratosphere (UT–LS), which exerts an influence on the regional ozone valley (Zhou et al. 1995; Liu et al. 2003; Bian et al. 2011) and the aerosol-layer enhancements near the tropopause (Tobo et al. 2007; Vernier et al. 2015). The TP also modulates large-scale atmospheric circulations over the Northern Hemisphere and atmosphere–ocean interactions in the tropics and midlatitudes of the North Pacific (e.g., Zhao and Chen 2001b; Liu et al. 2007; Zhao et al. 2007; Nan et al. 2009; Zhao et al. 2009; Zhou et al. 2009; Duan et al. 2012). Therefore, global weather and climate research would be incomplete without considering the significant role of the TP.
Compared to other land regions in the world, observational data are scarce over the TP, owing to its high elevations, naturally harsh environmental conditions, and less-developed logistics. Thus, a few field experiments have been implemented in the data-scarce areas. For instance, the first Qinghai–Xizang Plateau Meteorology Experiment (QXPMEX) was carried out from May to August 1979 (Tao et al. 1986). This experiment promoted, for the first time, systematic research on the diurnal and seasonal variations and spatial features of the surface heat budget, the structures and evolutions of atmospheric circulation systems over the TP, and their effects on global and Asian general circulations.
In the 1990s, a longer-term field experiment was conducted over the TP with the support of the Japanese Experiment on Asian Monsoon (JEXAM). It estimated the drag coefficient Cd of surface momentum and the bulk transfer coefficient Ch of surface sensible heat (SH) and revealed seasonal and interannual variations of the surface heat budget over the TP and their relationships with rainy seasons (Chen 1999; Zhao and Chen 2000a,b). Afterward, the Second Tibetan Plateau Atmospheric Scientific Experiment (TIPEX-II) was carried out from May to August 1998. Its results showed an imbalance phenomenon of the surface heat budget, strong mesoscale convection activities, and shear-line characteristics (Chen et al. 1999). The Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME)/Tibet intensive observation conducted a plateau-scale automated weather station experiment and a mesoscale experiment of the land surface and planetary boundary layer (PBL) observations with one X-band Doppler radar at Naqu, China, from May to September 1998 (Wang 1999; Ueno et al. 2001). GAME/Tibet made progress in retrieving the land surface radiative budget, precipitation, and soil moisture from satellite remote sensing products and understanding the PBL structures, the convective rapid development, and the precipitating cloud characteristics (Wang 1999; Ueno et al. 2001; Uyeda et al. 2001; Choi et al. 2004).
Entering the twenty-first century, the Coordinated Enhanced Observing Period (CEOP) Asia–Australia Monsoon Project on the Tibetan Plateau (CAMP/Tibet), and the Tibetan Observation and Research Platform (TORP) were implemented over the central northern TP during 2002–04 (Ma et al. 2006, 2008). Their research documented regional characteristics of land surface heat and CO2 fluxes, turbulence, and the PBL (Ma et al. 2009). Under the support of the Japan International Cooperation Agency (JICA) project, a New Integrated Observational System over the Tibetan Plateau (NIOST) project (JICA/Tibet) was carried out during 2005–09 (Xu et al. 2008a; Zhang et al. 2012; Chen et al. 2011, 2013). It found diurnal variations of rainfall over the TP and effects of latent heat release on TP vortices, provided evidence of strong troposphere–stratosphere exchanges over the TP, improved the National Oceanic and Atmospheric Administration/National Centers for Environmental Prediction (NOAA/NCEP)–Oregon State University–Air Force Research Laboratory–NOAA/Office of Hydrology land surface model (Noah) on the basis of observational characteristics in the land surface energy balance, and revealed the importance of the deep PBL to the troposphere–stratosphere exchange over the TP. In the summer of 2011, an experiment of the TORP ground-based and airborne remote sensing observations was conducted over the central TP as part of the Global Change Program of China (Ma et al. 2014). This experiment found hydrothermal and momentum exchanges and moisture transports over the southeastern TP during the monsoon period, as well as land surface and atmospheric circulation variations against the background of global change. Moreover, the Tibetan Plateau: Formation–Climate–Ecosystems (TiP) program focused on a longer-term evolution of climate over the TP and its influence (Mosbrugger and Appel 2012).
To quantify uncertainties in satellite and model products of soil moisture and temperature, some regional-scale observation networks were established. For example, during 2008–13, the CEOP–Asian-Monsoon System with Ground Satellite Image Data and Numerical Simulations (AEGIS) project monitored the land surface characteristics and analyzed their linkages with convection, precipitation, and Asian monsoons by satellites, the existing ground-based flux measurements, and stable isotopes in precipitation over the southeastern TP (www.ceop-aegis.net/doku.php). The Tibetan Plateau observatory of plateau-scale soil moisture and soil temperature (Tibet-Obs) built a Naqu–Maqu–Ngari regional network in cold semiarid, cold humid, cold arid climates (Su et al. 2011). A regional-scale Soil Moisture and Temperature Monitoring Network (SMTMN) was also built on the central Tibetan Plateau (Yang et al. 2013). These field observation networks increased the understanding of regional land surface hydrological processes and errors of satellite-derived soil moisture products. In addition, an ecohydrological experiment over the Heihe River basin in 2010 provided a test bed to confirm or falsify new ideas on ecohydrology and new hypotheses on scaling (Li et al. 2013).
The aforementioned field experiments have made significant progress in promoting the scientific understanding of the Earth–atmosphere coupled system over the TP. However, the problem of scarce observations in the area is still not well solved, which has further hindered the better understanding of the local land–atmosphere coupled system and its effects. The answers to some key scientific questions are still unclear. For example, owing to the lack of plateau-scale soil moisture and PBL observation networks, there is a wide divergence in estimates of Cd and Ch over the TP (e.g., Ye and Gao 1979; Zhao and Chen 2000b; Choi et al. 2004; Gao et al. 2015; Zhang et al. 2016), which directly results in uncertainties when estimating the heat intensity and its impacts. Because of the lack of direct observations of cloud microphysical and troposphere–stratosphere exchange processes over the TP, the cloud microphysical characteristics in the formation and development of clouds and precipitation, their interactions with atmospheric environments, the UT–LS atmospheric vertical structures, and their effects on ozone variations and aerosol-layer enhancements near the tropopause are not well understood. Moreover, because of the scarce radiosonde data over the western TP, it is not known how the local atmospheric circulation systems (especially synoptic- and mesoscale systems) develop and move from the west to the east. Thus, numerical weather and climate forecast models often have poor reliability when modeling weather and climate features over the TP, including soil moisture, surface heat fluxes, surface air temperature, rainfall, PBL structures, cloud amount, and stratospheric ozone (Wang 2011; Wu and Zhou 2011; Qiu et al. 2013; Hu et al. 2014; Zheng et al. 2014, 2015a,b,c, 2016; Guo et al. 2015; Zhuo et al. 2016; Wan et al. 2017). These problems may also cause large uncertainties in reanalysis datasets and satellite products (such as air temperature, soil moisture, surface heat fluxes, and radiation) over the TP (Li et al. 2012; Wang et al. 2012; Zhu et al. 2012; Su et al. 2013; Zeng et al. 2016).
To promote Tibetan meteorological research, the Third Tibetan Plateau Atmospheric Scientific Experiment (TIPEX-III), to continue for 8–10 years, was initiated jointly by the China Meteorological Administration (CMA), the National Natural Scientific Foundation of China (NSFC), and the Chinese Academy of Sciences (CAS). A preliminary experiment was implemented in 2013, and TIPEX-III began formally in 2014.
OBJECTIVES.
The field observational objective of TIPEX-III is to constitute a three-dimensional (3D) observation system of the land surface, PBL, troposphere, and lower stratosphere over the TP. This system integrates ground-, air-, and space-based platforms based on the meteorological operational networks, the TIPEX-III network, the existing NIOST network, and the part of the TORP observation sites in China. The scientific objectives of TIPEX-III are to understand the surface heat budget, cloud microphysical characteristics, atmospheric water cycles, and troposphere–stratosphere exchange characteristics over the TP; to clarify the impacts of the changing Tibetan land–atmosphere coupled system on severe weather and climate events and atmospheric energy and water cycles; to improve the parameterization schemes of the land surface, PBL, cloud–precipitation, and troposphere–stratosphere exchange processes over the TP; and to enhance the skills of weather and climate forecast operations. The specific intensive field observational and scientific objectives of TIPEX-III are addressed as follows:
In the previous field experiments, the soil moisture and PBL networks were mainly located over the central and southeastern TP, not yet constituting a plateau-scale observation network. Meanwhile, the meteorological operational observation sites are still sparse from the western to central TP (Figs. 1a and 1b). To create plateau-scale soil moisture and PBL tower observation networks, TIPEX-III will build new sites over the data-scarce central and western areas, which may increase the understanding of temporal and spatial variations of the land surface characteristics over the TP and their interactions with atmospheric circulations. Meanwhile, the regional-scale soil moisture and PBL networks over the TP will be used to understand mesoscale spatial differences in the surface heat budget over the complex topography and landscape and their effects on mesoscale systems. These intensive observations also allow an objective evaluation of numerical models, reanalysis data, and satellite-retrieval products.
The previous field experiments only utilized the X-band Doppler radar for probing large precipitating cloud particles, lacking direct observations of cloud microphysical features. TIPEX-III will carry out intensive observations over key areas with frequent cloud activities by integrating measurements of cloud radars, aircraft campaigns, and radiosondes, as well as regional-scale land surface and PBL observation networks. These data will help to understand the microphysical characteristics of cloud development and interactions between clouds, surface heating, and atmospheric environments and to improve parameterization schemes of cloud microphysical processes.
The previous field experiments also lacked plateau-scale observations of troposphere–stratosphere exchange processes. TIPEX-III will conduct intensive observation tasks for water vapor, aerosol, and ozone in the troposphere and lower stratosphere by balloonborne package instruments and ground-based remote sensing measurements. These data will help to clarify the UT–LS characteristics and the mechanisms for ozone variations and aerosol-layer enhancements near the tropopause; to validate satellite-retrieval profiles of water vapor, aerosol, and ozone, especially in the UT–LS; and to improve parameterization schemes of the lower-stratospheric physical and chemical processes over the TP.
Only a few of the field experiments (such as the QXPMEX and TIPEX-II) executed intensive tropospheric radiosonde observations over the data-scarce western TP for about one year, not constituting a longer-term plateau-scale radiosonde observation network. To constitute such an observation network from the western to eastern TP, TIPEX-III will build new radiosonde stations over the western TP (Fig. 1b). These stations will gradually become part of the CMA operational observation system. The intensive observational data can be used to monitor the evolution of atmospheric circulation systems and water cycles from the west to the east and may be applied in services of weather and climate forecasts.
The importance of the summer TP sensible heat pump and atmospheric water tower to downstream monsoon rainfall has been documented (Wu and Zhang 1998; Xu et al. 2008b). However, their maintaining mechanisms and effects on the local atmospheric circulation systems, as well as extreme weather and climate events over the downstream area and a larger area of the Northern Hemisphere, are still not clearly understood. In particular, the dominant control of the South Asian monsoon by thermodynamic functions over the TP is controversial. Boos and Kuang (2010) believed that the uplift of a narrow orography of the Himalayas and adjacent mountain ranges, instead of surface heating over the TP, plays a more important role in the South Asian monsoon climate. TIPEX-III will deeply investigate mechanisms responsible for the effects of the TP thermodynamic functions on the local atmospheric circulation systems and water cycles and extreme weather and climate events in the downstream area, as well as the Northern Hemisphere, by multiple observational datasets and numerical simulations. TIPEX-III will also probe a way to enhance the skills of weather and climate forecasts.
(a) Distribution of 46 newly built (black dots) and CMA operational (red dots) sites for monitoring soil moisture and temperature (boxes represent regional-scale soil moisture and temperature observation networks); (b) distribution of PBL sites (purple stars), a regional PBL network near Naqu (blue solid box), newly built (black dots) and CMA operational (red dots) radiosonde sites, observation areas for cloud–precipitation physical processes (black dashed boxes), new observation sites for ozone, water vapor, and aerosol in the UT–LS (blue hollow triangles), and CMA operational observation sites for atmospheric composition at the land surface (black solid triangles) (the thick, black dashed line indicates the line of aircraft flight from Golmud to Naqu in the summer of 2014); and (c) distribution of 33 sites of the regional-scale soil moisture and temperature observation network over Naqu.
Citation: Bulletin of the American Meteorological Society 99, 4; 10.1175/BAMS-D-16-0050.1
Compared to the previous experiments, the unique feature of TIPEX-III is that it will greatly expand the surface and tropospheric meteorological operational observation networks over the traditionally data-scarce central and western parts of the TP and promote integrated observations of multiple physical processes from the land surface to the lower stratosphere. Ultimately, the implementation of these observation networks could increase the understanding of the Earth–atmosphere coupled system over the TP and its effects on extreme weather events and regional climate variability and could also improve weather and climate forecast operations.
EXPERIMENTAL AREA AND NETWORK CONFIGURATION.
Considering the tremendous heterogeneity in terrain elevations and land-cover types, the distributions of the existing CMA meteorological operational observation stations, and the logistical challenges of maintaining observation sites, TIPEX-III comprises a plateau-scale intensive observation network, as well as regional-scale dense networks (Fig. 1). With a particular focus on understanding the complex interactions between the land surface, PBL, and cloud microphysical processes over the TP, TIPEX-III also integrates intensive observations over a few areas with active convection using the regional-scale land surface and PBL observation networks, ground-based radars, and airborne campaigns.
Intensive observations of land surface and PBL characteristics.
TIPEX-III has established a soil observation network consisting of 46 sites in the central and western TP (Fig. 1a). Consistent with the operational observations of the CMA, at each site, the measurement system measures soil water content (Table 1). All of these sites have been operating since September 2015. Meanwhile, TIPEX-III has also built two regional-scale soil moisture and temperature observation networks over Geji (near Shiquanhe), China, of the western bare soil with few features, and Naqu, of the central alpine steppe (Table 1 and Fig. 1a). The regional network consists of 33 sites over Naqu (Fig. 1c), which began operating in August 2015, and 17 sites over Geji (not shown), which began operating in December 2016.
Measurements of soil moisture and temperature and PBL elements.
For the PBL observations over the TP, TIPEX-III includes a plateau-scale network and a regional-scale network. The former consists of 10 multilayer towers (with distance between towers of ∼500 km) at Shiquanhe, Gaize, Naqu, Linzhou, Linzhi, Tuotuohe, Maqu, Litang, Dali, and Wenjiang, China (Fig. 1b and Table 1), and helps to study general patterns of the surface heat fluxes and PBL structures over the TP. The regional-scale network is located within an area of 300 km × 200 km near Naqu, a main source region of mesoscale low pressure and convection systems over the TP. This network consists of six additional sites at Bange, Namucuo, Anduo, Nierong, Jiali, and Biru, China, and contributes to integrated research on the high-resolution land surface and PBL processes over the central TP and their effects on mesoscale systems. These observations have been conducted at Shiquanhe, Namucuo, Naqu, Anduo, Linzhi, Litang, Dali, and Wenjiang from July 2014 to December 2016; at Bange, Biru, Jiali, and Nierong from July 2014 to March 2016; and at Linzhou from August 2015 to December 2016.
Intensive routine radiosonde observations of tropospheric atmospheric profiles.
Using the Vaisala portable radiosonde systems (Table 2), TIPEX-III conducted the intensive routine radiosonde observations at Shiquanhe, Gaize, and Shenzha stations in the western TP (Fig. 1b) at 0800, 1400, and 2000 Beijing standard time (BST) each day from 8 July to 31 August 2014 (BST = UTC + 8 h). Meanwhile, routine automatic sounding systems (Table 2) were newly built at these three stations and have carried out intensive observations at 0800 and 2000 BST each day since November 2014. After assessment of their performance, these automatic sounding stations are to become part of the CMA operational observation systems, which will ultimately constitute a long-term plateau-scale sounding observation network. Moreover, TIPEX-III will also include intensive radiosonde observations at Gongshan (27.75°N, 98.67°E), Jinchuan (31.48°N, 102.07°E), and Litang (30°N, 100.27°E) stations on the southeastern slope of the TP (Fig. 1b), a key area for gauging water vapor transports from the Indian Ocean to East Asia.
Sounding system for the atmospheric profiles.
Intensive observations of tropospheric cloud–precipitation physical characteristics.
TIPEX-III combines ground-based radars (Table 3), aircraft campaigns (Table 4), and the CMA operational Doppler radar systems for measuring physical properties of cloud and precipitation in the central (Naqu) and southeastern TP (Linzhi and Daocheng) (Fig. 1b). A primary goal of these observations is to explore the cloud microphysical characteristics and their relationships with convective precipitation systems and atmospheric environments. In July and August 2014, a campaign using the ground-based radars and airborne instruments was conducted within a 200 km × 200 km area of Naqu (Fig. 1b). A follow-up field campaign using ground-based radars was carried out at Naqu and Linzhi from 15 July to 31 August 2015 and at Daocheng from 1 July to 31 August 2016.
Ground-based radar measurements of cloud–precipitation physical features.
Airborne measurements of cloud–precipitation physical features.
Intensive observations of troposphere–lower-stratosphere ozone, aerosol, and water vapor profiles.
Strong transports of air masses from the troposphere to the lower stratosphere appear over the southeastern TP and weaken toward the north and west (Tao et al. 1986; Cong et al. 2002). TIPEX-III includes plateau-scale intensive measurements for vertical profiles of ozone, aerosol, and water vapor at Shiquanhe, Lhasa, Linzhi, Tuotuohe, Mangya, Golmud, and Xining meteorological stations (Fig. 1b). Using balloonborne package instruments and ground-based remote sensing instruments (Table 5), two observational missions were separately implemented at Linzhi from 6 June to 31 July 2014 and at Shiquanhe, Lhasa, and Golmud during the period from May to September 2016.
Balloonborne and ground-based measurements of ozone, aerosol, and water vapor in the troposphere and stratosphere.
PRELIMINARY ACHIEVEMENTS OF TIPEX-III.
The implementation of TIPEX-III has enhanced the monitoring capability for the land surface, PBL, troposphere, and lower stratosphere over the TP. It has also promoted the understanding of their features, physical processes, and effects and improved the capability of weather and climate models. Noticeable progress has been achieved in research and data applications.
The new TIPEX-III data revealed a possible overestimation of the bulk transfer coefficient of sensible heat over the TP in previous studies, a larger plateau-scale heterogeneity in latent heat flux than in sensible heat flux, and the linkages of surface heat fluxes to Asian monsoon activities.
The plateau-scale heating contrasts exert a significant effect on the development of mesoscale convection and circulation systems over the eastern TP (Sugimoto and Ueno 2010). However, previous estimates of Cd, Ch, and surface heat fluxes show large uncertainties over the TP. The new data allowed the plateau-scale differences in Cd, Ch, and surface heat fluxes to be reexamined. It was found that a new estimate of SH is 18 W m−2 in the central TP (with a range between 5 and 40 W m−2) and 56 W m−2 at the western TP (with a variation between 40 and 70 W m−2) (Fig. 2a). The spatial difference in SH is larger on the plateau scale (between the west and central parts) than on the regional scale (in the Naqu regional network), and surface latent heat flux (LH) has a larger plateau-scale difference than SH (Fig. 2b). The new estimate of Ch (from 2 × 10−3 to 4 × 10−3) shows the smaller plateau-scale heterogeneity relative to Cd (from 3 × 10−3 to 11 × 10−3) (Table 6) and is smaller than previous estimates (>4 × 10−3) (e.g., Ye and Gao 1979; Yang et al. 2011). Based on the larger values of Ch, the estimated July–August-mean intensity of SH is 60–80 (150–190) W m−2 over the central (western) TP by Ye and Gao (1979) and 50–60 (75–90) W m−2 by Yang and Guo (2011), remarkably larger compared to the new estimate. This result indicates that SH has been possibly overestimated by the previous studies when calculating SH using the bulk transfer method, which could lead to an incorrect understanding of the TP SH intensity. Therefore, the role of summer SH over the TP in local thermal convective formation and development, and the uncertainties in model land surface processes, must be reestimated. Moreover, it was also found that in the numerical forecast models, the overestimated effect of summer SH over the TP may be reduced by improving the parameterization scheme of land surface processes for bare soil (Zhuo et al. 2016).
(a) Daily mean time series of SH (W m−2) at the Shiquanhe site (dashed line) and a regional mean of SH over six sites of the Naqu regional network (solid line) from 1 to 31 Aug 2014. Bars indicate the maximum and minimum values of SH among the six sites. (b) As in (a), but for LH.
Citation: Bulletin of the American Meteorological Society 99, 4; 10.1175/BAMS-D-16-0050.1
The median values of Cd (×10−3) and Ch (×10−3) for the 11 TIPEX-III sites under neutral conditions (Wang et al. 2016).
The data diagnosis of TIPEX-III further revealed a plateau-scale difference in diurnal variations of surface heat fluxes over the TP and their linkages with the South Asian monsoon. SH shows a larger diurnal variation in the west than in the middle and east, but LH does not show remarkable diurnal variations (Wang et al. 2016). When the strong warm moist southerly wind in front of the South Asian monsoon trough prevails over the southeastern TP, the diurnal variations of local SH and downward shortwave radiation are weaker (Li et al. 2016), which suggests the feedback of the Asian monsoon on the TP heat source. Previous studies paid more attention to the impacts of the TP heating on the Asian monsoon (e.g., Wu and Zhang 1998; Zhao and Chen 2001a,b; Liu et al. 2007; Zhou et al. 2009; Duan et al. 2012) or the effects of extratropical atmospheric planetary-scale waves over the Northern Hemisphere on the TP heating (Zhao et al. 2009; Cui et al. 2015). Thus, this feedback of the Asian monsoon provides a new insight for understanding reasons for the TP heating variations.
The new TIPEX-III observations uncovered the characteristics of cloud, its radiative effect, and raindrop size distribution and the importance of warm rain processes in the formation and development of cloud and precipitation.
Cloud diurnal variation and warm rain process.
The lack of direct observations for cloud physical processes hinders the understanding of the cloud microphysical characteristics over the TP and their roles in local cloud development. The TIPEX-III intensive observations revealed the diurnal variation of cloud over the TP. It was found that convective cloud and precipitation exhibits a distinct diurnal variation over the central TP. The strong convective precipitation begins at noon (Chang and Guo 2016), corresponding well to the peak of the local sensible heat flux (Wang et al. 2016), which implies an influence of surface heating on the diurnal variation of convection and precipitation. Convective precipitation then gradually turns to stratiform precipitation. For the stratiform cloud, the dominant cloud particles are raindrop-sized supercooled water with fewer ice particles (Fig. 3a), which indicate a warm rain process. This process can generate heavier precipitation over the precipitation centers of weak convection systems than the cold rain processes (Gao et al. 2016).
(a) Cloud particle images sampled by airborne cloud particle imager (CPI) and two-dimensional stereo (2DS) for precipitating cumulus clouds at temperatures between −2.5° and −3.5°C at Naqu on 21 Jul 2014 (see Table 4); (b) the mean raindrop size distribution of 112 rainfall events near Naqu in Jul and Aug 2014, and the fitted Marshall–Palmer (MP) (with N0 = 8,000 m−3 mm−1 and R = 1.16 mm h−1) and Γ (with N0 = 17,349.34 m−3, µ = 4.03, and Λ = 6.95 mm−1) RSDs (Chang and Guo 2016); and (c) the relationship between concentration (L−1 µm−1) and diameter (µm) of cloud particles at Naqu on 21 Jul 2014, with a 2DS, a fast cloud droplet probe (FCDP), and a high-volume precipitation spectrometer (HVPS) (see Table 4).
Citation: Bulletin of the American Meteorological Society 99, 4; 10.1175/BAMS-D-16-0050.1
Characteristics of raindrop size distribution.
The new observational results indicate that the raindrop size distribution (RSD) over the TP is wider during the day than at night, with the widest RSD in the late afternoon (Chang and Guo 2016). Moreover, the RSD is wider over the TP compared to heavy rainfall over the downstream plains, and the concentration of cloud droplets is much lower compared to over clean oceans. The RSD varies between 102 and 103 m−3 mm−3 over the TP when the precipitation particle radius is <1 mm (Fig. 3b) and shows a Γ distribution when the radius is <2 mm. The larger raindrops (with a size of 10 µm) in the shallow convection over the TP (Fig. 3c) may enhance collision–coalescence processes, producing light rain, even though the concentration of larger raindrops is relatively low. This phenomenon is quite different from the plain of China where light rain is often suppressed by high aerosol loading. Because the atmospheric environment is relatively clean over the TP, the study of the formation and development of local cloud and precipitation could help to improve understanding of the effects of aerosols in a polluted atmospheric environment.
Vertical structures of cloud radiative effect.
The TIPEX-III analysis with the CloudSat–CALIPSO products indicated a salient difference in the vertical structures of both shortwave and longwave cloud radiative effects (CREs) over the TP from the adjacent regions. This difference is characterized by the deeper shortwave CRE heating and longwave CRE cooling layers in the troposphere and the maximum values of the CRE heating and cooling in the lower layers over the TP (Yan et al. 2016). A strong cooling layer of net CRE appears in the upper troposphere, and a shallow but strong heating layer appears in the lower layer. However, more general and precise information related to their full diurnal cycles and averages is needed to combine with the geostationary satellites and ground-based observations.
The TIPEX-III observations and analyses revealed the contributions of regional photochemistry and long-range ozone transport to the lower-tropospheric ozone and the relative importance of heterogeneous chemical processes near the tropopause and convective transports to the UT–LS ozone.
Because of a lack of direct observational evidence for contributions of regional photochemistry reactions and horizontal and vertical transports to tropospheric and lower-stratospheric ozone variations, the intensive observations of TIPEX-III helped to improve understanding of the contributions of these processes. It was found that, over the southeastern TP, the ozone concentration is lower in the middle and lower troposphere compared to the South Asian monsoon region, such as New Delhi, India (Fig. 4b). This lower-tropospheric low ozone over the TP could be mainly attributed to the lower anthropogenic pollution emissions through photochemical ozone production in the area. Over the northeastern TP, the lower-tropospheric ozone maximum could be attributed to both the regional photochemistry processes and the long-range ozone transport from East Asia, Europe, and Africa during the summer (Zhu et al. 2016). Especially in June, the contribution of the regional photochemistry process is almost half of that of the horizontal transport.
(a) The vertical profiles of averaged temperature (°C; black) over 31 observations, averaged ozone concentration (mPa; red) over 31 ozonesondes, and averaged water vapor concentration (ppm; blue) over 11 observations at Linzhi in Jun and Jul 2014. (b) The vertical profile of ozone concentration difference (mPa) between Linzhi and New Delhi (28.3°N, 77.07°E), in which the ozone concentration in New Delhi comes from Saraf and Beig (2004). (c) Variations of the polar tropopause height as a function of time at Gaize from 8 Jul to 31 Aug 2014 (Hong et al. 2016). (d) As in (c), but for the tropical tropopause height (Hong et al. 2016).
Citation: Bulletin of the American Meteorological Society 99, 4; 10.1175/BAMS-D-16-0050.1
Previous studies did not explicitly indicate the relative importance of upward transport from the lower-tropospheric low ozone concentration or in situ photochemical processes caused by anthropogenic pollution emissions to the UT–LS ozone valley during the summer. The intensive profile observations of the TIPEX-III campaign over the southeastern TP revealed a mean temperature of >−78.15°C (the threshold of the highest temperature required for the formation of polar stratospheric clouds) and a low water vapor concentration near the tropopause (Fig. 4a), which indicates a dehydration process induced by a cold temperature trap. In such an environment, phenomena such as polar stratospheric clouds are not likely to occur over the TP, and the heterogeneous chemical reaction of depleting ozone is weak over the TP. Thus, the in situ heterogeneous chemical processes near the tropopause might not be a dominant mechanism responsible for the formation of the UT–LS ozone valley during the summer.
Another mechanism, strong convective transports of the lower-tropospheric low ozone concentration toward the UT–LS, may have had a greater influence. Over the southeastern TP, the upward transports are closely associated with the strongest ascending motion. Under the influence of the plateau surface thermodynamic forcing, the upward transports are stronger in the daytime and summer. The tropopause fold is also a favorable structure for cross-tropopause exchanges. It is known as a vertical intrusion of the dynamical tropopause into the troposphere and is accompanied by the normal tropopause break and two tropopause heights (i.e., the polar tropopause height and the tropical tropopause height). The intensive observations of TIPEX-III revealed that the late rainy season is an important period for troposphere–stratosphere exchanges over the TP (Hong et al. 2016). In this season, the tropopause fold is frequently observed over the western TP. The polar tropopause occurs during the entire rainy season, but its height decreases from the early to late rainy season (Fig. 4c); while the tropical tropopause is mainly observed in the late rainy season (Fig. 4d), which is possibly associated with weaker upper-tropospheric jet streams over the western TP. The frequent occurrence of the tropopause fold favors the cross-tropopause exchange of air masses with different ozone concentrations between the troposphere and stratosphere. Meanwhile, the lower-tropospheric pollutants over the TP and its surrounding areas may have also been transported to the UT–LS, which could further affect the chemical processes of the UT–LS, as well as ozone concentrations. Thus, the effects of the increasing aerosol concentration near the tropopause over South Asia on chemical processes and ozone should be closely examined.
The TIPEX-III analyses provided a new insight into the effects of the TP thermodynamic functions on local atmospheric water maintenance and vortex movement, downstream rainfall and haze events, and Northern Hemispheric continent temperature and rainfall.
The physical diagnoses and numerical simulations of TIPEX-III gave a maintenance mechanism responsible for the TP atmospheric water tower. When the TP surface heating draws the warm moist air masses from the tropical Indian Ocean toward the plateau, they travel along the southern slope of the TP and form two ladders of the conditional instability of the second kind (CISK) processes with two couplings of apparent heat source–moisture sink over the TP southern slope and main platform, respectively. This feature enforces the convergence of low-level low pressure, and convection systems over the platform and favors the formation and development of local cloud and precipitation (Fig. 5a). In this way, the TP atmospheric water tower is maintained (Xu et al. 2014).
(a) A diagram of the mechanism for sustaining the atmospheric water tower over the TP, in which Q1 and Q2 are the apparent heat source and moisture sink, the CISK(1) and CISK(2) indicate the first and second ladder of CISK-like processes on the TP slope and main platform, and ∇·V is the horizontal convergence/divergence of the air mass (Xu et al. 2014). (b) The root-mean-square difference of 24-h forecast rainfall (mm day−1) between experiments with and without intensive observational data over the western TP during 1 Jun–31 Aug 2015, in which black dots are the rain gauge stations. (c) The three cross sections of the regressed vertical circulation (black) and eddy temperature (°C; color shading) against the interannual component of a summer (Jun–Aug) TP surface air temperature index along 85°E, 55°N, and 175°W using the 1948–2015 NCEP–NCAR reanalysis, in which black shaded areas denote terrain (Liu et al. 2017). (d) Summer 850-hPa wind anomalies (m s−1) forced by an increased TP heating in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 3 (CAM3), with prescribed monthly SST (CAM3_Tree minus CAM3_Bare), in which CAM3_Tree and CAM3_Bare have a surface type of broadleaf evergreen tropical tree and bare soil (corresponding to lower and higher surface albedo, respectively) over the TP (Liu et al. 2017) and the last 10-yr outputs of a 20-yr model integration are analyzed. (e) As in (d), but for rainfall (mm day−1; black dots represent significance at the 90% confidence level). (f) As in (e), but for surface air temperature (°C).
Citation: Bulletin of the American Meteorological Society 99, 4; 10.1175/BAMS-D-16-0050.1
The analyses of TIPEX-III also revealed new influence mechanisms for downstream extreme weather and climate events. It was found that when the extratropical westerlies climb over the TP, they descend on the lee side of the TP, which favors a near-surface “harbor” with weak wind over central eastern China, followed by the accumulation of local air pollutants and the occurrence of extreme haze events (Xu et al. 2016). Moreover, the atmospheric heating intensity over the TP affects the movement of low-level vortices from the central to eastern TP (Li et al. 2014). In the developing stages of these vortices, the vertical structure of the heat source may determine their intensity and movement direction. The vortices moving away from the plateau usually trigger heavy rainfall to the east of the TP and even give rise to disastrous rainfall events over downstream areas.
These results provide valuable information about the skills of downstream weather and climate forecasts. For example, improving the land surface and PBL processes over the TP in numerical forecast models can reduce an overestimated SH effect and a cold bias in the land surface temperature over the TP, so the models can better capture the characteristics of precipitation over central eastern China and the coasts (Zhuo et al. 2016). Moreover, TIPEX-III also uncovered that when the new intensive radiosonde data at Shiquanhe, Gaize, and Shenzha stations of the western TP are assimilated in the mesoscale Weather Research and Forecasting (WRF) Model system with three-dimensional variational data assimilation, the forecast skill of rainfall in the TP and adjacent areas is remarkably enhanced, with a decrease in the root-mean-square error of the 24-h forecast rainfall by 11% over the TP (Fig. 5b).
TIPEX-III demonstrated an important modulation of the TP heating on large-scale atmospheric waves, teleconnections, and climates. Surface sensible heat and atmospheric latent heating over the TP could enhance the meridional circulation of the Asian summer monsoon, produce eastward-propagating Rossby waves along the extratropical westerlies, and modify large-scale climates over the Northern Hemisphere (Wu et al. 2016). It was also discovered that during summer, a strong surface heating over the TP could trigger a Northern Hemispheric extratropical teleconnection like the Asia–Pacific Oscillation (APO) (Liu et al. 2015, 2017), with an increased tropospheric temperature and strengthened ascent over Asia and a decreased tropospheric temperature and strengthened descent over the central eastern North Pacific (Fig. 5c). It has been known that associated with APO are significant anomalies in lows over the Asian–African monsoon region and subtropical anticyclones over the North Pacific and Atlantic, as well as surface air temperature and rainfall over Africa, South Asia, East Asia, and extratropical North America (Zhao et al. 2012). TIPEX-III further revealed that these APO-related anomalies in atmospheric circulation, rainfall, and temperature could be also forced by an individual surface heating change over the TP (Figs. 5d–f), which suggests the importance of the TP forcing to global-scale climate anomalies, which should be further explored.
SUMMARY AND DISCUSSION.
Meteorological observational data are scarce over the TP. Although a few field experiments over the TP have made significant progress, they have mostly focused on land surface and PBL observations. Routine surface and tropospheric observation stations are still sparse over the western TP. There is a lack of direct observations of cloud microphysical and troposphere–stratosphere exchange characteristics. All of these hinder the better understanding of the TP Earth–atmosphere coupled system and its effects.
To promote Tibetan meteorological research, in 2013 the CMA, NSFC, and CAS jointly initiated TIPEX-III for a duration of 8–10 years. It has been implemented as an integrated observation of the land surface, PBL, troposphere, and lower stratosphere by coordinating ground-, air-, and space-based facilities from the CMA operational networks and previous scientific experiment observation networks in China. The implementation of TIPEX-III has greatly expanded the CMA operational observation networks and provided important support for the NSFC comprehensive research program during 2013–22 entitled “The Earth–atmosphere coupled system over the Tibetan Plateau and its global climate effects.”
TIPEX-III has achieved noticeable progress in research and data application. Its preliminary achievements provide a special reference to advance the understanding of the plateau land–atmosphere coupled system, as well as new insight into how to improve model physical parameterization schemes and enhance the skills of weather and climate forecasts. In the future, TIPEX-III will include further field experiments and comprehensively study interactions between tropospheric cloud–precipitation physical processes, surface heating, and atmospheric environments over the TP. It will involve further examination of the vegetated surface aerodynamic conductance in land surface models and the importance of the heat source intensity over the TP to monsoon onsets and global and regional extreme weather and climate events; improve parameterization schemes of the land surface, PBL, cloud–precipitation, and troposphere–stratosphere exchange processes over the TP; and validate assimilated and remote sensing products of soil temperature and moisture and atmospheric ozone, aerosol, and water vapor.
Equally importantly, TIPEX-III aims to promote scientific exchanges and collaborations with international research communities and broader organizations. Coordinated observational experiments in the countries neighboring the TP are especially encouraged. Scientists from international communities are invited to participate in the follow-up field campaigns and to use the TIPEX-III data in their research. Validated TIPEX-III datasets will be open to the domestic and international scientific communities after a data-protection period of one year (starting from the dates of completion of the data quality control or the product generation). A dedicated data archival center of the CMA, the National Meteorological Information Center (NMIC), is responsible for collecting the raw and processed datasets and distributing them to users. The website for downloading the TIPEX-III data (http://data.cma.cn/tipex) has been preliminarily built and will be further improved. Users are also encouraged to directly contact the corresponding authors of this paper to obtain data and further details.
ACKNOWLEDGMENTS
We thank all members of the TIPEX-III lead group, expert group, implementation group, and management office for their dedication and constructive suggestions. We thank the meteorologists of the Meteorological Bureau of Tibet Autonomous Region for their participation in field experiments. We thank Dr. Yunchang Cao of the Meteorological Observation Center of the CMA for his participation in the construction of automatic radiosonde stations; Drs. Zheng Ruan, Zhiqun Hu, Zehu Cui, and Wei Zhuang of the Chinese Academy of Meteorological Sciences (CAMS) for their participation in ground-based radar observations; and Yang Gao and Delong Zhao of the Beijing Weather Modification Office and Dr. Guangxian Lu of CAMS for their participation in aircraft campaigns. This work is jointly sponsored by the Third Tibetan Plateau Atmospheric Experiment (GYHY201406001) and NSFC (91437218, 91537211, and 91637312).
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