Yingying Chen

A MULTISCALE SOIL MOISTURE AND FREEZE-THAW MONITORING NETWORK ON THE THIRD POLE

Multisphere interactions over the Tibetan Plateau directly impact its surrounding climate and environment at a variety of spatiotemporal scales. Remote sensing and modeling are expected to provide hydrometeorological data needed for these process studies, but in situ observations are required to support their calibration and validation. For this purpose, we have established a dense monitoring network on the central Tibetan Plateau to measure two state variables (soil moisture and temperature) at three spatial scales (1.0°, 0.3°, and 0.1°) and four soil depths (0–5, 10, 20, and 40 cm). The experimental area is characterized by low biomass, high soil moisture dynamic range, and typical freeze–thaw cycle. The network consists of 56 stations with their elevation varying over 4470–4950 m. As auxiliary parameters of this network, soil texture and soil organic carbon content are measured at each station to support further studies. To guarantee continuous and high-quality data, tremendous efforts have been made t...

Improving the Noah Land Surface Model in Arid Regions with an Appropriate Parameterization of the Thermal Roughness Length

Daytime land surface temperatures in arid and semiarid regions are typically not well simulated in current land surface models (LSMs). This study first evaluates the importance of parameterizing the thermal roughness length (z0h) to model the surface temperature (Tsfc) and turbulent sensible heat flux (H) in arid regions. Six schemes for z0h are implemented into the Noah LSM, revealing the high sensitivity of the simulations to its parameterization. Comparisons are then performed between the original Noah LSM and a revised version with a novel z0h scheme against observations at four arid or semiarid sites, including one in Arizona and three in western China. The land they cover is sparse grass or bare soil. The results indicate that the original Noah LSM significantly underestimates Tsfc and overestimates H in the daytime, whereas the revised model can simulate well both Tsfc and H simultaneously. The improved version benefits from the successful modeling of the diurnal variation of z0h, which the original model cannot produce.

Parameterizing soil organic carbon’s impacts on soil porosity and thermal parameters for Eastern Tibet grasslands

This study investigates the stratification of soil thermal properties induced by soil organic carbon (SOC) and its impacts on the parameterization of the thermal properties. Soil parameters were measured for alpine grassland stations and North China flux stations, with a total of 34 stations and 77 soil profiles. Measured data indicate that the topsoils of alpine grasslands contain high SOC contents than underlying soil layers, which leads to higher soil porosity values and lower thermal conductivity and bulk density values in the topsoils. However, this stratification is not evident at the lowland stations due to low SOC contents. Evaluations against measured data show that three thermal conductivity schemes used in land surface models severely overestimate the values for soils with high SOC content (i.e. topsoils of alpine grassland), but they are better for soils with low SOC content. A new parameterization is then developed to take the impacts of SOC into account. The new one can well estimate the soil thermal conductivity values in both low and high SOC content cases, and therefore, it is a potential candidate of thermal conductivity scheme to be used in land surface models.

Impacts of wind stilling on solar radiation variability in China

Solar dimming and wind stilling (slowdown) are two outstanding climate changes occurred in China over the last four decades. The wind stilling may have suppressed the dispersion of aerosols and amplified the impact of aerosol emission on solar dimming. However, there is a lack of long-term aerosol monitoring and associated study in China to confirm this hypothesis. Here, long-term meteorological data at weather stations combined with short-term aerosol data were used to assess this hypothesis. It was found that surface solar radiation (SSR) decreased considerably with wind stilling in heavily polluted regions at a daily scale, indicating that wind stilling can considerably amplify the aerosol extinction effect on SSR. A threshold value of 3.5 m/s for wind speed is required to effectively reduce aerosols concentration. From this SSR dependence on wind speed, we further derived proxies to quantify aerosol emission and wind stilling amplification effects on SSR variations at a decadal scale. The results show that aerosol emission accounted for approximately 20% of the typical solar dimming in China, which was amplified by approximately 20% by wind stilling.

Optimal Exploitation of AMSR-E Signals for Improving Soil Moisture Estimation Through Land Data Assimilation

Regional soil moisture can be estimated by assimilating satellite microwave brightness temperature into a land surface model (LSM). This paper explores how to improve soil moisture estimation based on sensitivity analysis when assimilating Advanced Microwave Scanning Radiometer for the Earth Observing System brightness temperatures. By assimilating a lower and a higher frequency combination, the land data assimilation system (LDAS) used in this paper estimates first model parameters in a calibration pass and then estimates soil moisture in an assimilation pass. The ground truth of soil moisture was collected at a soil moisture network deployed in a semiarid area of Mongolia. Analyzed are the effects of assimilating different polarizations, frequencies, and satellite overpass times on the accuracy of the estimated soil moisture. The results indicate that assimilating the horizontal polarization signal underestimates soil moisture and assimilating the daytime signal overestimates soil moisture. The former is due to improper parameter estimation perhaps caused by high sensitivity of the horizontal polarization to land surface heterogeneity, and the latter is due to the effective soil temperature for microwave emission in the daytime being close to the one at a soil depth of several centimeters but not to the surface skin temperature simulated in the LSM. Therefore, assimilating the nighttime vertical polarizations in the LDAS is recommended. A further analysis shows that assimilating different frequency combinations produces different soil moisture estimates, and none is always superior to the others, because different frequency signals may be contaminated by varying clouds and/or water vapor with different degrees. Thus, an ensemble estimation based on frequency combinations was proposed to filter off, to some extent, the stochastic frequency-dependent biases. The ensemble estimation performs more robust when driven by different forcing data.

Quantifying evaporation and its decadal change for Lake Nam Co, central Tibetan Plateau

Most lakes in the interior Tibetan Plateau have expanded rapidly since the late 1990s. Because of a lack of observations, lake water balances and their changes are far from well understood. Evaporation is a component of the lake water balance, and this study quantifies its magnitude, decadal change, and its contribution to the water balance changes in Lake Nam Co, one of the largest lakes on the Tibetan Plateau (with an area of approximately 2000 km2 and a mean depth of approximately 40 m). The lake temperature and the evaporation are simulated by the Flake model. The simulation results are validated against observed lake temperature profile from 2013 and Moderate Resolution Imaging Spectroradiometer lake surface temperature data from 2000 to 2014. The simulated latent heat flux and sensible heat flux are validated against Bowen ratio-derived estimates for 2013. Based on the validated simulation results, the long-term mean annual evaporation is approximately 832 ± 69 mm, and this value is much less than the potential evaporation estimated using the Penman-Monteith equation. The annual evaporation from 1980 to 2014 displays a complex decadal oscillation, mainly due to the changes in energy-related terms (air temperature and radiation). The mean lake evaporation since the late 1990s is greater than previous periods; thus, this change in evaporation has suppressed the recent expansion of Nam Co.

An Algorithm Based on the Standard Deviation of Passive Microwave Brightness Temperatures for Monitoring Soil Surface Freeze/Thaw State on the Tibetan Plateau

The land surface on the Tibetan Plateau (TP) experiences diurnal and seasonal freeze/thaw processes that play important roles in the regional water and energy exchanges, and passive microwave satellites provide opportunities to detect the soil state for this region. With the support of three soil moisture and temperature networks on the TP, a dual-index microwave algorithm with Advanced Microwave Scanning Radiometer-Earth Observing System data is developed for the detection of soil surface freeze/thaw state. One index is the standard deviation index (SDI) of brightness temperature (TB), which is defined as the standard deviation of horizontally polarized brightness temperatures at 6.9, 10.7, 18.7, 23.8, 36.5, and 89.0 GHz. It is the major index and is used to reflect the reduction of liquid water content after soils get frozen. The other index is the 36.5-GHz vertically polarized brightness temperature (TB36.5V), which islinearly correlated with ground temperature. The threshold values of the two indices (SDI and TB36.5V) are determined with one grid from the network located in a semiarid climate, and the algorithm is validated with other grids from the same network. Further validations are conducted based on the other two networks located in different climates (semihumid and arid, respectively). Results show that the classification accuracy using this algorithm is more than 90% for the semihumid and semiarid regions, and misclassifications mainly occur at the transition period between unfrozen and frozen seasons. Nevertheless, the algorithm has limited capability in identifying the soil surface freeze/thaw state in the arid region because the microwave signals can penetrate deep dry soils and thus embody the bulk information beyond the surface layer.

Evaluation of Precipitable Water Vapor from Four Satellite Products and Four Reanalysis Datasets against GPS Measurements on the Southern Tibetan Plateau

AbstractThe southern Tibetan Plateau (STP) is the region in which water vapor passes from South Asia into the Tibetan Plateau (TP). The accuracy of precipitable water vapor (PWV) modeling for this region depends strongly on the quality of the available estimates of water vapor advection and the parameterization of land evaporation models. While climate simulation is frequently improved by assimilating relevant satellite and reanalysis products, this requires an understanding of the accuracy of these products. In this study, PWV data from MODIS infrared and near-infrared measurements, AIRS Level-2 and Level-3, MERRA, ERA-Interim, JRA-55, and NCEP final reanalysis (NCEP-Final) are evaluated against ground-based GPS measurements at nine stations over the STP, which covers the summer monsoon season from 2007 to 2013. The MODIS infrared product is shown to underestimate water vapor levels by more than 20% (1.84 mm), while the MODIS near-infrared product overestimates them by over 40% (3.52 mm). The AIRS PWV pr...

The role of cloud height and warming in the decadal weakening of atmospheric heat source over the Tibetan Plateau

The warming over the Tibetan Plateau (TP) is very significant during last 30 years, but the thermal forcing has been weakened. The thermal weakening is attributed mainly to the enhancement of the TOA (top of atmosphere) outgoing radiation. This enhancement is opposite to the greenhouse-gas-induced weakening of the global mean TOA outgoing radiation and is also unable to be explained by the observed decrease of total cloud cover. This study presents the importance of cloud height change and the warming over the TP in modulating the TOA radiation budget and thus the thermal forcing during spring and summer. On the basis of surface observations and satellite radiation data, we found that both the TOA outgoing shortwave radiation and longwave radiation were enhanced during this period. The former enhancement is due mainly to the increase of low-level cloud cover, which has a strong reflection to shortwave radiation, especially in summer. The latter enhancement is caused mainly by the planetary warming, and it is further enhanced by the decrease of total cloud cover in spring, as clouds extinguish outgoing longwave radiation emitted from the land surface. Therefore, the radiative cooling enhancement and thus the thermal weakening over the TP is a response of the earth-atmosphere system to the unique change of cloud cover configuration and the rapid warming of the land surface. However, these trends in cloud cover and TOA outgoing radiation are not well represented in four reanalyses.

Evaluation of SMAP, SMOS, and AMSR2 soil moisture retrievals against observations from two networks on the Tibetan Plateau

Two soil moisture and temperature monitoring networks were established in the Tibetan Plateau (TP) during recent years. One is located in a semi-humid area of central TP and consists of 56 soil moisture and temperature measurement (SMTM) stations, the other is located in a semi-arid area of southern TP and consists of 21 SMTM stations. In this study, the station data are used to evaluate soil moisture retrievals from three microwave satellites, i.e. the SMAP of NASA, the SMOS of ESA, and the AMSR2 of JAXA. It is found that the SMAP retrievals tend to underestimate soil moisture in the two TP networks, mainly due to the negative biases in the effective soil temperature that is derived from a climate model. However, the SMAP product well captures the amplitude and temporal variation of the soil moisture. The SMOS product performs well in Naqu network with acceptable error metrics, but fails to capture the temporal variation of soil moisture in Pali network. The AMSR2 products evidently exaggerate the temporal variation of soil moisture in Naqu network, but dampen it in Pali network, suggesting its retrieval algorithm needs further improvements for the TP.