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.

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.

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.

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.

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.

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.

Abstract

This study investigates the capability of the dynamic downscaling method (DDM) in a North American regional climate study using the Eta/Simplified Simple Biosphere (SSiB) Regional Climate Model (RCM). The main objective is to understand whether the Eta/SSiB RCM is capable of simulating North American regional climate features, mainly precipitation, at different scales under imposed boundary conditions. The summer of 1998 was selected for this study and the summers of 1993 and 1995 were used to confirm the 1998 results. The observed precipitation, NCEP–NCAR Global Reanalysis (NNGR), and North American Regional Reanalysis (NARR) were used for evaluation of the model’s simulations and/or as lateral boundary conditions (LBCs). A spectral analysis was applied to quantitatively examine the RCM’s downscaling ability at different scales.

The simulations indicated that choice of domain size, LBCs, and grid spacing were crucial for the DDM. Several tests with different domain sizes indicated that the model in the North American climate simulation was particularly sensitive to its southern boundary position because of the importance of moisture transport by the southerly low-level jet (LLJ) in summer precipitation. Among these tests, only the RCM with 32-km resolution and NNGR LBC or with 80-km resolution and NARR LBC, in conjunction with appropriate domain sizes, was able to properly simulate precipitation and other atmospheric variables—especially humidity over the southeastern United States—during all three summer months—and produce a better spectral power distribution than that associated with the imposed LBC (for the 32-km case) and retain spectral power for large wavelengths (for the 80-km case). The analysis suggests that there might be strong atmospheric components of high-frequency variability over the Gulf of Mexico and the southeastern United States.

Abstract

Land-use and land-cover change (LULCC) is one of the most important forcings affecting climate in the past century. This study evaluates the global and regional LULCC impacts in 1950–2015 by employing an annually updated LULCC map in a coupled land–atmosphere–ocean model. The difference between LULCC and control experiments shows an overall land surface temperature (LST) increase by 0.48 K in the LULCC regions and a widespread LST decrease by 0.18 K outside the LULCC regions. A decomposed temperature metric (DTM) is applied to quantify the relative contribution of surface processes to temperature changes. Furthermore, while precipitation in the LULCC areas is reduced in agreement with declined evaporation, LULCC causes a southward displacement of the intertropical convergence zone (ITCZ) with a narrowing by 0.5°, leading to a tripole anomalous precipitation pattern over the warm pool. The DTM shows that the temperature response in LULCC regions results from the competing effect between increased albedo (cooling) and reduced evaporation (warming). The reduced evaporation indicates less atmospheric latent heat release in convective processes and thus a drier and cooler troposphere, resulting in a reduction in surface cooling outside the LULCC regions. The southward shift of the ITCZ implies a northward cross-equatorial energy transport anomaly in response to reduced latent/sensible heat of the atmosphere in the Northern Hemisphere, where LULCC is more intensive. Tropospheric cooling results in the equatorward shift of the upper-tropospheric westerly jet in both hemispheres, which, in turn, leads to an equatorward narrowing of the Hadley circulation and ITCZ.

Abstract

Soil moisture observations in sites with natural vegetation were made for several decades in the former Soviet Union at hundreds of stations. In this paper, the authors use data from six of these stations from different climatic regimes, along with ancillary meteorological and actinometric data, to demonstrate a method to validate soil moisture simulations with biosphere and bucket models. Some early and current general circulation models (GCMS) use bucket models for soil hydrology calculations. More recently, the Simple Biosphere Model (SiB) was developed to incorporate the effects of vegetation on fluxes of moisture, momentum, and energy at the earth's surface into soil hydrology models. Until now, the bucket and SiB have been verified by comparison with actual soil moisture data only on a limited basis. In this study, a Simplified SiB (SSIB) soil hydrology model and a 15-cm bucket model are forced by observed meteorological and actinometric data every 3 h for 6-yr simulations at the six stations. The model calculations of soil moisture are compared to observations of soil moisture, literally “ground truth,” snow cover, surface albedo, and net radiation” and with each other.

For three of the stations, the SSIB and 15-cm bucket models produce good simulations of seasonal cycles and interannual variations of soil moisture. For the other three stations, there are large errors in the simulations by both models. Inconsistencies in specification of field capacity may be partly responsible. There is no evidence that the SSiB simulations are superior in simulating soil moisture variations. In fact, the models are quite similar since SSiB implicitly has a bucket embedded in it. One of the main differences between the models is in the treatment of runoff due to melting snow in the spring-SSiB incorrectly puts all the snowmelt into runoff. While producing similar soil moisture simulations, the models produce very different surface latent and sensible beat fluxes, which would have large effects on GCM simulations.

Abstract

A global and seasonal assessment of regions of the earth with strong climate–vegetation biophysical process (VBP) interactions is provided. The presence of VBP and degree of VBP effects on climate were assessed based on the skill of simulations of observed global precipitation by two general circulation models of the atmosphere coupled to three land models with varying degrees of complexity in VBP representation. The simulated VBP effects on precipitation were estimated to be about 10% of observed precipitation globally and 40% over land; the strongest impacts were in the monsoon regions. Among these, VBP impacts were highest on the West African, South Asian, East Asian, and South American monsoons. The specific characteristics of vegetation–precipitation interactions in northern high latitudes were identified. Different regions had different primary impact season(s) depending on regional climate characteristics and geographical features. The characteristics of VBP effects on surface energy and water balance as well as their interactions were also analyzed. The VBP-induced change in evaporation was the dominant factor in modulating the surface energy and water balance. The land–cloud interaction had substantial effects in the feedback. Meanwhile, the monsoon regions, midlatitudes lands, and high-latitude lands each exhibited quite different characteristics in circulation response to surface heating changes. This study is the first to compare simulations with observations to identify and assess global seasonal mean VBP feedback effects. It is concluded that VBPs are a major component of the global water cycle.