Fanglin Sun

A 3-year dataset of sensible and latent heat fluxes from the Tibetan Plateau, derived using eddy covariance measurements

The Tibetan Plateau (TP) has become a focus of strong scientific interest due to its role in the global water cycle and its reaction to climate change. Regional flux estimates of sensible and latent heat are important variables for linking the energy and hydrological cycles at the TP’s surface. Within this framework, a 3-year dataset (2008–2010) of eddy covariance measured turbulent fluxes was compiled from four stations on the TP into a standardised workflow: corrections and quality tests were applied using an internationally comparable software package. Second, the energy balance closure (CEB) was determined and two different closure corrections applied. The four stations (Qomolangma, Linzhi, NamCo and Nagqu) represent different locations and typical land surface types on the TP (high altitude alpine steppe with sparse vegetation, a densely vegetated alpine meadow, and bare soil/gravel, respectively). We show that the CEB differs between each surface and undergoes seasonal changes. Typical differences in the turbulent energy fluxes occur between the stations at Qomolangma, Linzhi and NamCo, while Nagqu is quite similar to NamCo. Specific investigation of the pre-monsoon, the Tibetan Plateau summer monsoon, post-monsoon and winter periods within the annual cycle reinforces these findings. The energy flux of the four sites is clearly influenced by the Tibetan Plateau monsoon. In the pre-monsoon period, sensible heat flux is the major energy source delivering heat to the atmosphere, whereas latent heat flux is greater than sensible heat flux during the monsoon season. Other factors affecting surface energy flux are topography and location. Land cover type also affects surface energy flux. The energy balance residuum indicates a typically observed overall non-closure in winter, while closure (or ‘turbulent over-closure’) is achieved during the Tibetan Plateau summer monsoon at the Nagqu site. The latter seems to depend on ground heat flux, which is higher in the wet season, related not only to a larger radiation input but also to a thermal decoupling of dry soils. Heterogeneous landscape modelling using a MODIS product is introduced to explain energy non-closure.

Uncertainty in atmospheric profiles and its impact on modeled convection development at Nam Co Lake, Tibetan Plateau

This work investigates the influence of atmospheric temperature and relative humidity profiles obtained from radio soundings, NCEP-I and ERA-Int reanalysis and GFS-FNL analysis data on the simulated evolution of clouds and convection at Nam Co Lake on the Tibetan Plateau. In addition to differences in moisture, the initial atmospheric profiles exhibit considerable differences in near-surface temperatures that affect vertical stability. Our analysis is carried out during 2 days in summer 2012 using a 2-D high-resolution modeling approach with a fully interactive surface model so that surface fluxes react to changes in cloud cover. Modeled convection for the radio-sounding profile compares reasonably well with weather observations for the first day, but less well for the second day, when large-scale synoptic effects, not included in the model, become more important. The choice of vertical profile information leads to strongly differing convection development, translating into modifications of the surface energy balance and of the energy and water cycle for the basin. There are strong differences spanning one order of magnitude in the generated precipitation between the model simulations driven by different vertical profiles. This highlights the importance of correct and high-resolution vertical profiles for model initialization.

An analysis on the influence of spatial scales on sensible heat fluxes in the north Tibetan Plateau based on Eddy covariance and large aperture scintillometer data

The influence of spatial scales on surface fluxes is an interesting but not fully investigated question. This paper presents an analysis on the influence of spatial scales on surface fluxes in the north Tibetan Plateau based on eddy covariance (EC) and large aperture scintillometer (LAS) data at site Nagqu/BJ, combined with the land surface temperature (LST) and normalized difference vegetation index (NDVI) of moderate-resolution imaging spectroradiometer (MODIS). The analysis shows that sensible heat fluxes calculated with LAS data (H_LAS) agree reasonably well with sensible heat fluxes calculated with EC data (H_EC) in the rain and dry seasons. The difference in their footprints due to the wind direction is an important reason for the differences in H_EC and H_LAS. The H_LAS are statistically more consistent with H_EC when their footprints overlap than when their footprints do not. A detailed analysis on H_EC and H_LAS changes with net radiation and wind direction in rain and dry season indicates that the spatial heterogeneity in net radiation created by clouds contributes greatly to the differences in H_EC and H_LAS in short-term variations. A significant relationship between the difference in footprint-weighted averages of LST and difference in H_EC and H_LAS suggests that the spatial heterogeneity in LST at two spatial scales is a reason for the differences in H_EC and H_LAS and that LST has a positive correlation with the differences in H_EC and H_LAS. A significant relationship between the footprint-weighted averages of NDVI and the ratio of sensible heat fluxes at two spatial scales to net radiation (H/Rn) in the rain season supports the analysis that the spatial heterogeneity in canopy at two spatial scales is another reason for differences in H_EC and H_LAS and that canopy has a negative correlation with (H/Rn). An analysis on the influence of the difference in aerodynamic roughness lengths at two spatial scales on sensible heat fluxes shows that the influence is greater in the dry season and smaller in the rain season because the ratio of z0m_LAS to z0m_EC is big in the dry season and is close to 1.0 in the rain season. This study on spatial scales on surface fluxes in the Tibetan Plateau will be helpful in analyzing and understanding its influence on climate.

Mechanism of Daytime Strong Winds on the Northern Slopes of Himalayas, near Mount Everest: Observation and Simulation

AbstractThe seasonal variability of strong afternoon winds in a northern Himalayan valley and their relationship with the synoptic circulation were examined using in situ meteorological data from March 2006 to February 2007 and numerical simulations. Meteorological observations were focused on the lower Rongbuk valley, on the north side of the Himalayas (4270 m MSL), where a wind profile radar was available. In the monsoon season (21 May–4 October), the strong afternoon wind was southeasterly, whereas it was southwesterly in the nonmonsoon season. Numerical simulations were performed using the Weather Research and Forecasting Model to investigate the mechanism causing these afternoon strong winds. The study found that during the nonmonsoon season the strong winds are produced by downward momentum transport from the westerly winds aloft, whereas those during the monsoon season are driven by the inflow into the Arun Valley east of Mount Everest. The air in the Arun Valley was found to be colder than that of...

Observation of Strong Winds on the Northern Slopes of Mount Everest in Monsoon Season

ABSTRACT An analysis of the local atmospheric circulation in a northern Himalayan valley in the region of Mount Everest is presented. Data were collected using an automatic weather station over a one-year period in 2014. A ground-based wind profiler radar (WPR) and an in situ GPS radiosonde (RS) were also employed. This study focuses on the characteristics of afternoon strong wind events in the downstream of Rongbuk Valley. We found that: (1) The occurrence of the southwesterly wind during non-monsoon was in good consistency with high values of westerly wind at high levels over this region and confirmed to be driven by the strong westerly jet aloft. (2) The strong afternoon wind in monsoon season has a persistent southeasterly direction, which differs from the prevailing direction of the strong wind in non-monsoon. This flow was found to be independent of the wind aloft and was strongly seasonal, developing at Qomolangma Station (QOMS) when the subtropical jet stream had moved northward and was most stable and strongest in the early monsoon season but before the rainy season. (3) The southeasterly wind in monsoon is colder than local air, suggesting that it is driven by a strong thermal gradient from the Arun Valley to QOMS. Our results contribute to improving our knowledge of local circulation patterns in the Himalayas, and also to gaining a detailed understanding of the mountain chains role in both the monsoon system and regional transport of atmospheric pollutants.