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Shuzhou Wang and Yaoming Ma

Abstract

In this study, eddy covariance flux data collected from three research stations on the Tibetan Plateau—Qomolangma for Atmospheric and Environmental Observation and Research, Nam Co for Multisphere Observation and Research, and Southeast Tibet Station for Alpine Environment Observation and Research, Chinese Academy of Sciences—are used to analyze the variation of momentum transfer coefficient (CD), heat transfer coefficient (CH), aerodynamic roughness length (z 0m), thermal roughness length (z 0h), and excess resistance to heat transfer (kB −1, where k is von Kármán’s constant and B −1 is a non-dimensional bulk parameter). The following results are found. The monthly average surface roughness, bulk transfer coefficient, and excess resistance to heat transfer at all three stations are obtained. The values of average heat bulk transfer coefficients are larger than those of average momentum bulk transfer coefficients at all three stations. The parameter kB −1 exhibits clear diurnal variations with lower values in the night and higher values in the daytime, especially in the afternoon. Negative values of kB −1 are often observed in the night for relatively smooth surfaces on the Tibetan Plateau, indicating that heat transfer efficiency may exceed that of momentum transfer.

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Chao Xu, Yaoming Ma, Jiehua Ma, Chao You, and Huijun Wang

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Dust is the major aerosol type over the Tibetan Plateau (TP), and the TP plays an important role in forming the spring dust belt across the Northern Hemisphere in the upper troposphere. Estimated spring dust mass flux (DMF) showed a significant declining trend over the TP during 2007–19. The total spring DMF across the TP (TDMFTP) was mainly affected by DMFs over the Tarim Basin, while the spring DMF across the TP in the midtroposphere was also connected with DMFs over the northwest Indian Peninsula and central Asia. Interannual variability of spring TDMFTP was strongly correlated with the North Atlantic winter sea surface temperature (SST) tripole. A cold preceding winter induced by the North Atlantic winter SST tripole over midlatitude Eurasia promotes dust activities in the subsequent spring. The North Atlantic winter SST tripole anomalies persist into the subsequent spring and induce a corresponding atmosphere response. Enhanced atmospheric baroclinicity develops over northwest China and the northern Indian Peninsula during spring, which is attributed to surface thermal forcing induced by the positive winter SST tripole phase. A strong positive North Atlantic winter SST tripole anomaly strengthens the upper-level westerly jets, enhancing airflow toward the TP midtroposphere; together, these circulation patterns cause anomalous cyclonic conditions in the lower troposphere, especially over the Tarim Basin, via the eastward propagation of a Rossby wave train. These atmospheric circulation conditions are likely to increase the frequency of dust occurrence and promote the transport of dust onto the TP.

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Yuichiro Oku, Hirohiko Ishikawa, Shigenori Haginoya, and Yaoming Ma

Abstract

The diurnal, seasonal, and interannual variations in land surface temperature (LST) on the Tibetan Plateau from 1996 to 2002 are analyzed using the hourly LST dataset obtained by Japanese Geostationary Meteorological Satellite 5 (GMS-5) observations. Comparing LST retrieved from GMS-5 with independent precipitation amount data demonstrates the consistent and complementary relationship between them. The results indicate an increase in the LST over this period. The daily minimum has risen faster than the daily maximum, resulting in a narrowing of the diurnal range of LST. This is in agreement with the observed trends in both global and plateau near-surface air temperature. Since the near-surface air temperature is mainly controlled by LST, this result ensures a warming trend in near-surface air temperature.

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Chao Xu, Yaoming Ma, Kun Yang, and Chao You

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Dust is a major component of atmospheric aerosol worldwide, greatly affecting regional and global climate. In this study dust aerosol optical depth (DAOD) and dust mass fluxes (DMF) were evaluated at different altitudes using measurements by the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) and ERA-Interim data from March through May (MAM) for the period 2007–16. Significantly higher upper-tropospheric (above ~8 km) dust loads and DMF downstream of the Tibetan Plateau (TP) relative to those over other major dust sources of the Northern Hemisphere were found during spring. A DMF magnitude of 1010 g integrated across a 2°-latitude segment during spring was estimated downstream of the TP in the upper troposphere. A dust belt can be clearly seen at altitudes higher than 6 km over the downwind direction of the TP at latitudes of around 30°–40°N, crossing the Pacific Ocean and extending to North America during spring. A pathway for transporting dust aerosols into the upper troposphere is proposed, as follows. Dust is uplifted to the midtroposphere over the source regions; then, frequent, deep, dry convection prevailing over the TP during spring can cause convective overshooting that uplifts the dust aerosols to the upper troposphere. The TP thus acts as a channel for transporting dust from the lower atmosphere to the upper troposphere, enabling the long-range zonal transport of dust around the Northern Hemisphere.

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Xuelong Chen, Zhongbo Su, Yaoming Ma, James Cleverly, and Michael Liddell

Abstract

MODIS thermal sensors can provide us with global land surface temperature (LST) several times each day, but have difficulty in obtaining information from the land surface in cloudy situations. As a result, the monthly day or night LST products [Terra monthly day LST (TMD), Terra monthly night LST (TMN), Aqua monthly day LST (AMD), and Aqua monthly night LST (AMN)] are the average LST values calculated over a variable number of clear-sky days in a month. Is it possible to derive an accurate estimate of monthly mean LST based on averaging of the multidaily overpasses of MODIS sensors? In situ ground measurements and ERA-Interim reanalyses data, both of which provide continuous information in either clear or cloudy conditions, have been used to validate the approach. Using LST measurements from 156 ground flux towers, it was found that the three mean values , , and (mean biases of 0.19, 0.59, and 0.40 K, respectively) can all provide a reliable estimate of all-sky monthly mean LST. Of the three means, we recommend the use of for monthly mean LST in climate studies as it provides the most complete coverage. When retrievals from either Terra or Aqua are not available, then either or may be used to fill the gaps. The intrinsic error in the MODIS monthly mean LST cannot be explained from monthly mean view time, view angle, and clear-sky ratio. MODIS monthly LST calculated using this approach (RMSE = 2.65, mean bias < ±1 K) will have wide applicability for climate studies and numerical model evaluation.

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Kun Yang, Jun Qin, Xiaofeng Guo, Degang Zhou, and Yaoming Ma

Abstract

To clarify the thermal forcing of the Tibetan Plateau, long-term coarse-temporal-resolution data from the China Meteorological Administration have been widely used to estimate surface sensible heat flux by bulk methods in many previous studies; however, these estimates have seldom been evaluated against observations. This study at first evaluates three widely used bulk schemes against Tibet instrumental flux data. The evaluation shows that large uncertainties exist in the heat flux estimated by these schemes; in particular, upward heat fluxes in winter may be significantly underestimated, because diurnal variations of atmospheric stability were not taken into account. To improve the estimate, a new method is developed to disaggregate coarse-resolution meteorological data to hourly according to statistical relationships derived from high-resolution experimental data, and then sensible heat flux is estimated from the hourly data by a well-validated flux scheme. Evaluations against heat flux observations in summer and against net radiation observations in winter indicate that the new method performs much better than previous schemes, and therefore it provides a robust basis for quantifying the Tibetan surface energy budget.

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Rogier van der Velde, Mhd. Suhyb Salama, Marcel D. van Helvoirt, Zhongbo Su, and Yaoming Ma

Abstract

Understanding the sources of uncertainty that cause deviations between simulated and satellite-observed states can facilitate optimal usage of these products via data assimilation or calibration techniques. A method is presented for separating uncertainties following from (i) scale differences between model grid and satellite footprint, (ii) residuals inherent to imperfect model and retrieval applications, and (iii) biases in the climatologies of simulations and retrievals. The method is applied to coarse (10 km) soil moisture simulations by the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5)–Noah regional climate model and 2.5 years of high-resolution (100 m) retrievals from the Advanced Synthetic Aperture Radar (ASAR) data collected over central Tibet. Suppression of the bias is performed via cumulative distribution function (CDF) matching. The other deviations are separated by taking the variance of the ASAR soil moisture at the coarse MM5 model grid as measure for the deviations caused by scale differences. Via decomposition of the uncertainty sources it is shown that the bias and the spatial-scale difference explain the majority (>70%) of the deviations between the two products, whereas the contribution of model–observation residuals is less than 30% on a monthly basis. Consequently, this study demonstrates that accounting for uncertainties caused by bias as well as spatial-scale difference is imperative for meaningful assimilation of high-resolution soil moisture products. On the other hand, the large uncertainties following from spatial-scale differences suggests that high-resolution soil moisture products have a potential of providing observation-based input for the subgrid spatial variability parameterizations within large-scale models.

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Xuelong Chen, Zhongbo Su, Yaoming Ma, Kun Yang, Jun Wen, and Yu Zhang

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Roughness height for heat transfer is a crucial parameter in the estimation of sensible heat flux. In this study, the performance of the Surface Energy Balance System (SEBS) has been tested and evaluated for typical land surfaces on the Tibetan Plateau on the basis of time series of observations at four sites with bare soil, sparse canopy, dense canopy, and snow surface, respectively. Both under- and overestimation at low and high sensible heat fluxes by SEBS was discovered. Through sensitivity analyses, it was identified that these biases are related to the SEBS parameterization of bare soil’s excess resistance to heat transfer (kB −1, where k is the von Kármán constant and B −1 is the Stanton number). The kB −1 of bare soil in SEBS was replaced. The results show that the revised model performs better than the original model.

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Lei Zhong, Zhongbo Su, Yaoming Ma, Mhd. Suhyb Salama, and José A. Sobrino

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Variations of land surface parameters over the Tibetan Plateau have great importance on local energy and water cycles, the Asian monsoon, and climate change studies. In this paper, the NOAA/NASA Pathfinder Advanced Very High Resolution Radiometer (AVHRR) Land (PAL) dataset is used to retrieve the land surface temperature (LST), the normalized difference vegetation index (NDVI), and albedo, from 1982 to 2000. Simultaneously, meteorological parameters and land surface heat fluxes are acquired from the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) dataset and the Global Land Data Assimilation System (GLDAS), respectively. Results show that from 1982 to 2000 both the LST and the surface air temperature increased on the Tibetan Plateau (TP). The rate of increase of the LST was 0.26±0.16 K decade−1 and that of the surface air temperature was 0.29 ± 0.16 K decade−1, which exceeded the increase in the Northern Hemisphere (0.054 K decade−1). The plateau-wide annual mean precipitation increased at 2.54 mm decade−1, which indicates that the TP is becoming wetter. The 10-m wind speed decreased at about 0.05±0.03 m s−1 decade−1 from 1982 to 2000, which manifests a steady decline of the Asian monsoon wind. Due to the diminishing ground–air temperature gradient and subdued surface wind speed, the sensible heat flux showed a decline of 3.37 ± 2.19 W m−2 decade−1. The seasonal cycle of land surface parameters could clearly be linked to the patterns of the Asian monsoon. The spatial patterns of sensible heat flux, latent heat flux, and their variance could also be recognized.

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Xuelong Chen, Bojan Škerlak, Mathias W. Rotach, Juan A. Añel, Zhonbgo Su, Yaoming Ma, and Maoshan Li

Abstract

The planetary boundary layer (PBL) over the Tibetan Plateau (with a mean elevation about 4 km above sea level) reaches an unmatched height of 9515 m above sea level. The proximity of this height to the tropopause facilitates an exchange between the stratosphere and the boundary layer. However, the underlying mechanisms responsible for this unique PBL have remained uncertain. Here, the authors explore these mechanisms and their relative importance using measurements of the PBL, the associated surface fluxes, and single-column and regional numerical simulations, as well as global reanalysis data. Results indicate that the dry conditions of both ground soil and atmosphere in late winter cannot explain the special PBL alone. Rather, the results from a single-column model demonstrate the key influence of the stability of the free atmosphere upon the growth of extremely deep PBLs over the Tibetan Plateau. Simulations with the numerical weather prediction model Consortium for Small-Scale Modelling (COSMO) exhibit good correspondence with the observed mean PBL structure and realistic turbulent kinetic energy distributions throughout the PBL. Using ERA-Interim, the authors furthermore find that weak atmospheric stability and the resultant deep PBLs are associated with higher upper-level potential vorticity (PV) values, which in turn correspond to a more southerly jet position and higher wind speeds. Upper-level PV structures and jet position thus influence the PBL development over the Tibetan Plateau.

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