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Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2013)

The objective of this research is to gain a better understanding of the biophysical processes, vegetation dynamics, and climate interactions on global and regional scales. Sahel is the major focus region for this research. Our methodology is based on the analysis of observations, as well as simulations by the National Centers for Environmental Prediction (NCEP) Atmospheric General Circulation Model (AGCM) coupled with the Simplified Simple Biosphere model version 2 (SSiB2) and an offline version of the SSiB4 coupled with a Dynamic Global Vegetation Model (DGVM) Top-down Representation of Interactive Foliage and Flora Including Dynamics (TRIFFID).

We first examine the impact of vegetation biophysical processes (VBP) on climate from a long temporal scale point of view, specifically, the climate variability on inter-annual and inter-decadal time scales in the past six decades by using the NCEP AGCM coupled with two different land surface parameterizations: SSiB2 and the two-layer soil model. At the inter-annual time scale, the simulation with VBP decreases the root mean squared error by about 65%. Moreover, on inter-decadal time scale, VBP corrects the wet or dry biases over West Africa, South Africa, Amazon, and East Asia, through changing surface energy balances and the partitioning of surface latent and sensible heat fluxes, as well as changing atmospheric circulation and moisture flux convergence.

In the second part, we systematically investigate the climate impact of large-scale land use land cover change (LULCC), and identify the mechanisms that control the response of climate to LULCC, by using the most recent LULCC data in an "idealized but realistic" way for the past six decades. LULCC leads to an increase in albedo and decrease in evaporation. The albedo effect (cooling) and evaporation effect (warming) compete with each other, resulting in warmer surface temperatures at tropics, and cooler surface temperatures at middle latitude. Over global land, the LULCC amplifies surface warming (0.11K over global land and 0.43K over degraded area respectively). LULCC cause a precipitation reduction globally (-0.15mm/day over global land and -0.35mm/day over degraded areas), with strongest signals over monsoon regions, resulting from evaporation reduction and less convergence from monsoon convergence zones.

Finally, I systematically investigate how climate variability and anomalies in West Africa affect the regional terrestrial ecosystem, including spatial distribution and temporal variations of plant functional types' (PFT) and other vegetation characteristics, though biophysical and photosynthesis processes at different scales. The offline SSiB4/TRIFFID model is used in this study. The results show that the simulated PFT's spatial distribution and total leaf area index (LAI) correspond well to climate variability and are consistent with satellite derived vegetation conditions. The simulated inter-decadal variability in vegetation conditions is consistent with the Sahel drought in the 1970s and the 1980s and partial recovery in the 1990s and the 2000s. The vegetation characteristics simulated by SSiB4/TRIFFID responds primarily to air temperature, soil moisture and radiative fluxes.

    Cover page: Global and Sahel Regional Biophysical Processes, Vegetation Dynamics, and Climate Interactions
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2016)
    The population of the Sahel region of West Africa has approximately doubled in the past 50 years, and could potentially double again by the middle of this century. This has led to the northward expansion of agricultural areas at the expense of natural savanna, leading to widespread land use -land cover change (LULCC). Because there is strong evidence of significant surface-atmosphere coupling in this region, one of the main goals of the West African Monsoon Modeling and Evaluation project phase II is to provide basic understanding of LULCC on the regional climate, and to evaluate the sensitivity of the seasonal variability of the West African Monsoon to LULCC. The prescribed LULCC is based on the changes from 1950 through 1990, representing a maximum feasible degradation scenario in the past half century. It is applied to 5 state of the art global climate models (GCMs) over a 6-year simulation period. Multiple GCMs are used because the magnitude of the impact of LULCC depends on model-dependent coupling strength between the surface and the overlying atmosphere, the magnitude of the surface biophysical changes, and how the key processes linking the surface with the atmosphere are parameterized within a particular model framework. Land cover maps and surface parameters may vary widely among models; therefore a special effort was made to impose consistent biogeophysical responses of surface parameters to LULCC using a simple experimental setup. The prescribed LULCC corresponds to degraded vegetation conditions, which mainly cause increases in the Bowen ratio and decreases in the surface net radiation, and result in a significant reduction in surface evaporation (upwards of 1 mm day−1 over a large part of the Sahel). This, in turn, mainly leads to less moisture convergence and precipitation over the LULCC zone. The overall impact is a rainfall reduction with every model, which ranges across models from 4 to 25 % averaged over the Sahel, and a southward shift of the rainfall peak in three of the five models which evokes a precipitation dipole pattern which is consistent with the observed pattern for dry climate anomalies over this region. The African Easterly Jet shifts equator-ward, although the strength of this change varies considerably among the models. In most of the models, the main factor causing diabatic cooling of the upper troposphere and enhanced subsidence over the region of LULCC is the reduction of convective heating rates linked to reduced latent heat flux and moisture flux convergence. In broad agreement with previous studies, the impact of degradation on the regional climate is found to vary among the different models, however, the signal is stronger and more consistent between the models here than in previous inter-comparison projects. This is likely related to our emphasis on prioritizing a consistent impact of LULCC on the surface biophysical properties.
      Cover page: The regional impact of Land-Use Land-cover Change (LULCC) over West Africa from an ensemble of global climate models under the auspices of the WAMME2 project
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2016)
      The second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II) is designed to improve understanding of the possible roles and feedbacks of sea surface temperature (SST), land use land cover change (LULCC), and aerosols forcings in the Sahel climate system at seasonal to decadal scales. The project's strategy is to apply prescribed observationally based anomaly forcing, i.e., "idealized but realistic" forcing, in simulations by climate models. The goal is to assess these forcings' effects in producing/amplifying seasonal and decadal climate variability in the Sahel between the 1950s and the 1980s, which is selected to characterize the great drought period of the last century. This is the first multi-model experiment specifically designed to simultaneously evaluate such relative contributions. The WAMME II models have consistently demonstrated that SST forcing is a major contributor to the 20th century Sahel drought. Under the influence of the maximum possible SST forcing, the ensemble mean of WAMME II models can produce up to 60% of the precipitation difference during the period. The present paper also addresses the role of SSTs in triggering and maintaining the Sahel drought. In this regard, the consensus of WAMME II models is that both Indian and Pacific Ocean SSTs greatly contributed to the drought, with the former producing an anomalous displacement of the Intertropical Convergence Zone (ITCZ) before the WAM onset, and the latter mainly contributes to the summer WAM drought. The WAMME II models also show that the impact of LULCC forcing on the Sahel climate system is weaker than that of SST forcing, but still of first order magnitude. According to the results, under LULCC forcing the ensemble mean of WAMME II models can produces about 40% of the precipitation difference between the 1980s and the 1950s. The role of land surface processes in responding to and amplifying the drought is also identified. The results suggest that catastrophic consequences are likely to occur in the regional Sahel climate when SST anomalies in individual ocean basins and in land conditions combine synergistically to favor drought.
        Cover page: West African monsoon decadal variability and surface-related forcings: second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II)
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