Search Results

You are looking at 1 - 10 of 30 items for

  • Author or Editor: Yongkang Xue x
  • All content x
Clear All Modify Search
Yongkang Xue

Abstract

This is an investigation of the impact of and mechanisms for biosphere feedback in the northeast Asian grassland on the regional climate. Desertification in the Inner Mongolian grassland has dramatically increased during the past 40 years. The Center for Ocean-Land-Atmosphere Studies atmospheric general circulation model, which includes a biosphere model, was used to test the impact of this desertification. In the grassland experiment, areas of Mongolia and Inner Mongolia were specified as grassland. In the desertification experiment, these areas were specified as desert. Each experiment consists of six integrations with different atmospheric initial conditions and different specifications of the extent of the desertification area. All integrations were 90 days in length, beginning in early June and continuing through August, coincident with the period of the East Asian summer monsoon.

The desertification had a significant impact on the simulated climate. During the past 40 years, the observed rainfall has decreased in northern and southern China but increased in central China, and the Inner Mongolian grassland and northern China have become warmer. The simulated rainfall and surface temperature differences between the desertification integrations and the grassland integrations are consistent with these observed changes.

The water balance and surface energy balance were altered by the desertification. The reduction in evaporation in the desertification experiment dominated the changes in the local surface energy budget. The reduction in convective latent beating above the surface layer enhanced sinking motion (or weakened rising motion) over the desertification area and over the adjacent area to the south. Coincidentally, the monsoon circulation was weakened and the rainfall was reduced.

Full access
Yongkang Xue and Jagadish Shukla

Abstract

This is a general circulation model sensitivity study of the physical mechanisms of the effects of desertification on the Sahel drought. The model vegetation types were changed in the prescribed desertification area, which led to changes in the surface characteristics. The model was integrated for three months (June, July, August) with climatological surface conditions (control) and desertification conditions (anomaly) to examine the summer season response to the changed surface conditions. The control and anomaly experiments consisted of five pairs of integrations with different initial conditions and / or sea surface temperature boundary conditions.

In the desertification experiment, the moisture flux convergence and rainfall were reduced in the test area and increased to the immediate south of this area. The simulated anomaly dipole pattern was similar to the observed African drought patterns in which the axis of the maximum rainfall shifts to the south. The circulation changes in the desertification experiment were consistent with those observed during sub-Saharan dry years. The tropical easterly jet was weaker and the African easterly jet was stronger than normal. Further, in agreement. with the observations, the easterly wave disturbances were reduced in intensity but not in number. Descending motion dominated the desertification area. The surface energy budget and hydrological cycle were also changed substantially in the anomaly experiment.

Full access
Yongkang Xue and Jagadish Shukla

Abstract

A numerical experiment was performed to explore the nature of and mechanisms for the effect of large-scale afforestation in the sub-Saharan area on the climate. This sensitivity study, which consists of several short-term integrations of a climate model, suggests that afforestation would enhance the rainfall in the region and would have the largest impact during dry years. While the rainfall increased in the afforestation area, it decreased to the south of that region. It was found that this land surface change altered the surface energy balance and induced a circulation change that led to a change in rainfall. The influences of different vegetation species and the extent of the afforestation area on the rainfall were tested and are discussed. Reducing the afforestation area by about 50% still resulted in a positive simulated rainfall anomaly. A detailed analysis of the surface energy balance is presented. A comparison between the effects of afforestation and desertification is also made.

Full access
Yongkang Xue and Jagadish Shukla

Abstract

A general circulation model sensitivity study was carried out to investigate the influence of global sea surface temperature (SST) on Sahel rainfall. This study was inspired by the impressive model simulations of Sahel rainfall reported by Folland et al. and Rowell et al. The model was integrated from June through September with three different atmospheric initial conditions and four years (1950, 1958, 1983, and 1984) of SST. In three out of four cases (1950, 1983, 1984), the area-averaged simulated rainfall anomaly was consistent with the observations. However, the model’s internal variability was rather large. The simulated anomalies had relatively larger sensitivity to the initial conditions in this study than those in a desertification study performed previously by the authors. This model failed to simulate the rainfall anomaly for 1958. Additional model experiments are needed to establish the role of SST variation in determining the Sahel rainfall variation.

Full access
Xia Zhang, Shu Fen Sun, and Yongkang Xue

Abstract

Proper simulation of soil freezing and thawing processes is an important issue in cold region climate studies. This paper reports on a frozen soil parameterization scheme for cold region studies that includes water flow and heat transfer in soil with water phase change. The mixed-form Richards’ equation is adopted to describe soil water flow affected by thermal processes in frozen soil. In addition, both liquid water and ice content have been taken into account in the frozen soil hydrologic and thermal property parameterization. To solve the complex nonlinear equation set and to ensure water conservation during simulation of complex phase change processes, efficient computational procedures have been designed and a new modified Picard iteration scheme is extended to solve the mixed-form Richards’ equation with phase change. The frozen soil model was evaluated using observational data from the field station at Rosemount, Minnesota, and the Tibet D66 site. The results show that the model is capable of providing good simulations of the evolution of temperature and liquid water content in frozen soil. Comparisons of simulation results from sensitivity studies indicate that there is a maximum difference of about 50 W m−2 in sensible and ground heat fluxes with and without the inclusion of the effect of ice content on matric potential and that using the exponential relationship between hydraulic conductivity and ice content produces realistic results.

Full access
Yongkang Xue, Kuo-Nan Liou, and Akira Kasahara

Abstract

A 2-D zonally averaged, time-dependent climate model has been developed to study the biogeophysical feedback for the climate of Africa. A numerical scheme has been specifically designed for the model to ensure the conservation of mass, momentum, energy, and water vapor. A control experiment has been carried out in which the solar zenith angle was varied from 15 June to 30 July. The simulated results are presented using averages over the last 30 days. The simulated temperature, humidity, and winds for July mean conditions compare reasonably, well with zonally averaged, observed values.

A vegetation layer has been incorporated in the present 2-D climate model. Using the coupled climate-vegetation model, we performed two tests involving the removal and expansion of the Sahara Desert. Results show that variations in the surface conditions produce a significant feedback to the climate system. The feedback from the land surface to the atmosphere affects not only precipitation and cloud cover, but also temperature, radiation budgets, and wind fields. The simulation responses to the temperature and zonal wind in the case of an expanded desert agree with the climatological data for African dry years.

Perturbed simulations have also been performed by changing the albedo only, without allowing the variation in the vegetation layer. In this case, the model is unable to reproduce the observed temperature, humidity, and wind fields over the African continent for both dry and wet years. We show that the variation in latent heat release is significant and is related to changes in the vegetation cover in a number of ways. As the desert is expanded, the decrease in latent heat is much larger than the increase in sensible heat generated by the hot surface. The specific humidity in the atmosphere decreases due to less evaporation from the ground and a reduction in the horizontal convergence of water vapor transport. As a result, precipitation and cloud cover are reduced.

Full access
Devdutta S. Niyogi, Yongkang Xue, and Sethu Raman

Abstract

A statistical–dynamical study was performed on the role of hydrometeorological interactions in the midlatitudes and the semiarid Tropics. For this, observations from two field experiments, the First International Satellite Land Surface Climatology Project Field Experiment (FIFE) and the Hydrological Atmospheric Pilot Experiment (HAPEX)–Sahel, representative of the midlatitudes and the semiarid tropical conditions, and simulated results from a land surface model, Simplified Simple Biosphere (SSiB) model were statistically analyzed for direct and interaction effects. The study objectives were to test the hypothesis that there are significant differences in the land surface processes in the semiarid tropical and midlatitudinal regimes and to identify the nature of the differences in the evapotranspiration exchanges for the two biogeographical domains. Results suggest there are similarities in the direct responses but the interactions or the indirect feedback pathways could be very different. The arid tropical regimes are dominated through vegetative pathways (via variables such leaf area index, stomatal resistance, and vegetal cover); the midlatitudes show soil wetness (moisture)–related feedback. In addition, for the midlatitudinal case, the vegetation and the soil surface acted in unison, leading to more interactive exchanges between the vegetation and the soil surface. The water-stressed semiarid tropical surface, on the other hand, showed response either directly between the vegetation and the atmosphere or between the soil and the atmosphere with very little interaction between the vegetation and the soil variables. Thus, the semiarid Tropics would require explicit bare ground and vegetation fluxes consideration, whereas the effective (combined vegetation and soil fluxes) surface representation used in various models may be more valid for the midlatitudinal case. This result also implied that with higher resource (water) availability the surface invested more in the surrounding environment. On the other hand, with poor resource availability (such as water stress in the tropical site), the surface components retain individual resources without sharing.

Full access
Hsi-Yen Ma, Heng Xiao, C. Roberto Mechoso, and Yongkang Xue

Abstract

This study examines the sensitivity of the global climate to land surface processes (LSP) using an atmospheric general circulation model both uncoupled (with prescribed SSTs) and coupled to an oceanic general circulation model. The emphasis is on the interactive soil moisture and vegetation biophysical processes, which have first-order influence on the surface energy and water budgets. The sensitivity to those processes is represented by the differences between model simulations, in which two land surface schemes are considered: 1) a simple land scheme that specifies surface albedo and soil moisture availability and 2) the Simplified Simple Biosphere Model (SSiB), which allows for consideration of interactive soil moisture and vegetation biophysical process. Observational datasets are also employed to assess the extent to which results are realistic.

The mean state sensitivity to different LSP is stronger in the coupled mode, especially in the tropical Pacific. Furthermore, the seasonal cycle of SSTs in the equatorial Pacific, as well as the ENSO frequency, amplitude, and locking to the seasonal cycle of SSTs, is significantly modified and more realistic with SSiB. This outstanding sensitivity of the atmosphere–ocean system develops through changes in the intensity of equatorial Pacific trades modified by convection over land. The results further demonstrate that the direct impact of land–atmosphere interactions on the tropical climate is modified by feedbacks associated with perturbed oceanic conditions (“indirect effect” of LSP). The magnitude of such an indirect effect is strong enough to suggest that comprehensive studies on the importance of LSP on the global climate have to be made in a system that allows for atmosphere–ocean interactions.

Full access
Hyun-Suk Kang, Yongkang Xue, and G. James Collatz

Abstract

This study assesses the impact of two different remote sensing–derived leaf area index (RSLAI) datasets retrieved from the same source (i.e., Advanced Very High Resolution Radiometer measurements) on a general circulation model’s (GCM) seasonal climate simulations as well as the mechanisms that lead to the improvement in simulations over several regions. Based on the analysis of these two RSLAI datasets for 17 yr from 1982 to 1998, their spatial distribution patterns and characteristics are discussed. Despite some disagreements in the RSLAI magnitudes and the temporal variability between these two datasets over some areas, their effects on the simulation of near-surface climate and the regions with significant impact are generally similar to each other. Major disagreements in the simulated climate appear in a few limited regions.

The GCM experiment using the RSLAI and other satellite-derived land surface products showed substantial improvements in the near-surface climate in the East Asian and West African summer monsoon areas and boreal forests of North America compared to the control experiment that used LAI extrapolated from limited ground surveys. For the East Asia and northwest U.S. regions, the major role of RSLAI changes is in partitioning the net radiative energy into latent and sensible heat fluxes, which results in discernable warming and decrease of precipitation due to the smaller RSLAI values compared to the control. Meanwhile, for the West African semiarid regions, where the LAI difference between RSLAI and control experiments is negligible, the decrease in surface albedo caused by the high vegetation cover fraction in the satellite-derived dataset plays an important role in altering local circulation that produces a positive feedback in land/atmosphere interaction.

Full access
Maheswor Shrestha, Lei Wang, Toshio Koike, Yongkang Xue, and Yukiko Hirabayashi

Abstract

In this study, a distributed biosphere hydrological model with three-layer energy-balance snow physics [an improved version of the Water and Energy Budget–based Distributed Hydrological Model (WEB-DHM-S)] is applied to the Dudhkoshi region of the eastern Nepal Himalayas to estimate the spatial distribution of snow cover. Simulations are performed at hourly time steps and 1-km spatial resolution for the 2002/03 snow season during the Coordinated Enhanced Observing Period (CEOP) third Enhanced Observing Period (EOP-3). Point evaluations (snow depth and upward short- and longwave radiation) at Pyramid (a station of the CEOP Himalayan reference site) confirm the vertical-process representations of WEB-DHM-S in this region. The simulated spatial distribution of snow cover is evaluated with the Moderate Resolution Imaging Spectroradiometer (MODIS) 8-day maximum snow-cover extent (MOD10A2), demonstrating the model’s capability to accurately capture the spatiotemporal variations in snow cover across the study area. The qualitative pixel-to-pixel comparisons for the snow-free and snow-covered grids reveal that the simulations agree well with the MODIS data to an accuracy of 90%. Simulated nighttime land surface temperatures (LST) are comparable to the MODIS LST (MOD11A2) with mean absolute error of 2.42°C and mean relative error of 0.77°C during the study period. The effects of uncertainty in air temperature lapse rate, initial snow depth, and snow albedo on the snow-cover area (SCA) and LST simulations are determined through sensitivity runs. In addition, it is found that ignoring the spatial variability of remotely sensed cloud coverage greatly increases bias in the LST and SCA simulations. To the authors’ knowledge, this work is the first to adopt a distributed hydrological model with a physically based multilayer snow module to estimate the spatial distribution of snow cover in the Himalayan region.

Full access