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Open Access Publications from the University of California

Scholarly Works (13 results)

  • Thesis
  • Peer Reviewed
UC Santa Barbara Electronic Theses and Dissertations (2023)

Annual area burned has increased in California over the past three decades as a result of rising temperatures and a greater atmospheric demand for moisture, a trend that is projected to continue throughout the 21st century as a result of climate change. However, the impacts of climate on the size, severity, and seasonality of wildfire activity are strongly influenced by ecosystem, predominant vegetation types, weather patterns, topography, and human activity. The individual wildfire burned perimeter and ecoregion-level spatial scales adopted for this research increases the amount of local information, as well as the resolution with which fire and land managers can implement strategies and counter measures when addressing issues related to climate change. This research combines 18 years of wildfire burned area perimeter maps, high resolution land surface modeling, a suite of remote sensing datasets, and nearly a century of statistically downscaled climatological data in an effort to quantify the impact of wildfire on local meteorology, as well as the influence of climate change on extreme fire weather. This dissertation investigates the bi-directional climate-wildfire feedback system in California through a detailed examination of the impacts of wildfire on the surface energy balance, an assessment of the influence of wildfire burn severity on the five-year postfire trajectory of three biophysical variables, and the quantification of the impacts of climate change on extreme fire weather. Results indicate that the largest changes to net radiation in the four months following ignition were primarily caused by decreases in latent heat flux following wildfire-induced vegetation removal. This vegetation removal, coupled with the lowering of albedo from ash deposition, also contributed to decreases in sensible heat flux. Further, vegetation abundance and land surface temperature did not return to prefire levels, for any burn severity class, after five years. Lastly, this work provides evidence that fire weather conditions conducive to large wildfires will become more extreme and extend later into the fall season in most areas of California by 2100. The methods of investigation used here may be applied to other regions of the world in an effort to inform the mitigation and suppression of large wildfires.

    Cover page: Pyrogeography of California: A Study of Wildfire-Climate Dynamics
    • Thesis
    • Peer Reviewed
    UC Santa Barbara Electronic Theses and Dissertations (2024)

    Recent climatic shifts and deforestation in the Brazilian Amazon have likely ushered the region into a new ecological and climatic regime with profound implications for biodiversity, ecosystem services, and local communities. This dissertation employs a comprehensive approach to address Brazil's National Adaptation Plan goals by integrating climate modeling and evaluation, field interviews, and risk mapping methodologies across three interrelated studies.The first paper evaluates the performance of thirteen Coupled Model Intercomparison Project phase 6 (CMIP6) models in simulating precipitation within the Amazon River Basin. The study assesses model efficacy in capturing precipitation variability using spatial pattern mapping, Taylor diagram analysis, and empirical orthogonal function analysis. It provides insights crucial for agricultural and hydrological planning. The second study delves into the adaptation strategies of small-scale farmers in Rondônia, highlighting the transformation in agricultural practices due to changing climate conditions. Through qualitative interviews, it explores how increased temperatures and altered precipitation patterns drive shifts from traditional crops to cattle ranching and milk production. It emphasizes the need for policy interventions that support sustainable farming and climate resilience. Lastly, the third paper investigates the climatic impacts of deforestation in unprotected areas of the Amazon. Advanced climate-land surface modeling quantifies temperature, precipitation, and evapotranspiration changes, mapping out the risks to agricultural productivity. This risk mapping integrates climatic, agricultural, demographic, and socioeconomic data to identify heightened-risk municipalities, advocating for more robust conservation measures. These studies contribute a deeper understanding of the Brazilian Amazon's relationships between climate change and human-environmental systems. The findings underline the importance of integrating scientific research with policymaking to mitigate climate change's impacts and support local communities' sustainability.

      Cover page: Climate Dynamics and Agricultural Adaptability in the Brazilian Amazon
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2016)
      This study investigates the impact of burned areas on the surface energy balance and monthly precipitation in northern Africa as simulated by a state-of-the-art regional model. Mean burned area fraction derived from MODIS date of burning product was implemented in a set of 1-year long WRF-NMM/SSiB2 model simulations. Vegetation cover fraction and LAI were degraded daily based on mean burned area fraction and on the survival rate for each vegetation land cover type. Additionally, ground darkening associated with wildfire-induced ash and charcoal deposition was imposed through lower ground albedo for a period after burning. In general, wildfire-induced vegetation and ground condition deterioration increased mean surface albedo by exposing the brighter bare ground, which in turn caused a decrease in monthly surface net radiation. On average, the wildfire-season albedo increase was approximately 6.3 % over the Sahel. The associated decrease in surface available energy caused a drop in surface sensible heat flux to the atmosphere during the dry months of winter and early spring, which gradually transitioned to a more substantial decrease in surface evapotranspiration in April and May that lessened throughout the rainy season. Overall, post-fire land condition deterioration resulted in a decrease in precipitation over sub-Saharan Africa, associated with the weakening of the West African monsoon progression through the region. A decrease in atmospheric moisture flux convergence was observed in the burned area simulations, which played a dominant role in reducing precipitation in the area, especially in the months preceding the monsoon onset. The areas with the largest precipitation impact were those covered by savannas and rainforests, where annual precipitation decreased by 3.8 and 3.3 %, respectively. The resulting precipitation decrease and vegetation deterioration caused a drop in gross primary productivity in the region, which was strongest in late winter and early spring. This study suggests the cooling and drying of atmosphere induced by burned areas caused the strengthening of subsidence during pre-onset and weakening of upward atmospheric motion during onset and mature stages of the monsoon leading to a waning of convective instability and precipitation. Monthly mid-tropospheric vertical wind showed a strengthening of downward motion in winter and spring seasons, and weakening of upward movement during the rainy months. Furthermore, precipitation energy analysis revealed that most of precipitation decrease originated from convective events, which supports the hypothesis of reduced convective instability due to wildfires.
        Cover page: Impact of burned areas on the northern African seasonal climate from the perspective of regional modeling
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2019)
          Cover page: On the effects of wildfires on precipitation in Southern Africa
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2014)
          Regional climate models (RCMs) have been developed and extensively applied for dynamically downscaling coarse resolution information from different sources, such as general circulation models (GCMs) and reanalyses, for different purposes including past climate simulations and future climate projection. Thus far, the nature, the methods, and a number of crucial issues concerning the use of dynamic downscaling are still not well understood. The most important issue is whether, and if so, under what conditions dynamic downscaling is really capable of adding more information at different scales compared to the GCM or reanalysis that imposes lateral boundary conditions (LBCs) to the RCMs. There are controversies regarding the downscaling ability. In this review paper we present several factors that have consistently demonstrated strong impact on dynamic downscaling ability in intraseasonal and seasonal simulations/predictions and future projection. Those factors include setting of the RCM experiment (e.g. imposed LBC quality, domain size and position, LBC coupling, and horizontal resolution); as well as physical processes, mainly convective schemes and vegetation and soil processes that include initializations, vegetation specifications, and planetary boundary layer and surface coupling. These studies indicate that RCMs have downscaling ability in some aspects but only under certain conditions. Any significant weaknesses in one of these aspects would cause an RCM to lose its dynamic downscaling ability. This paper also briefly presents challenges faced in current RCM dynamic downscaling and future prospective, which cover the application of coupled ocean-atmosphere RCMs, ensemble applications, and future projections. © 2014 Elsevier B.V.
            Cover page: A review on regional dynamical downscaling in intraseasonal to seasonal simulation/prediction and major factors that affect downscaling ability
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2019)
            Recent studies have shown that spring land surface temperature (LST) and subsurface temperature (SUBT) over the high elevation areas in the western US (WUS) have significant impacts on the downstream summer droughts/floods in North America. In this paper, both the National Centers for Environmental Prediction—Global Forecast System (NCEP-GFS) general circulation model (GCM) and the weather research and forecasting (WRF) regional climate model (RCM) are employed, where RCM scenarios utilized initial and lateral boundary conditions derived from the corresponding NCEP-GFS scenarios. Here we use a late spring flood in the US Southern Great Plains (SGP) case to examine whether simulation of the LST/SUBT downstream effects is sensitive to the domain size choice, change in dynamical cores within the same model, as well as to the representation of surface processes parameterizations. Although all RCM experiments with different settings simulate reasonably geographical patterns of observed LST and precipitation anomalies, we found that the choice of the domain size is crucial for proper downscaling the LST/SUBT downstream effects to accurately produce the observed precipitation/LST anomalies over the SGP/WUS, respectively, along with the associated large-scale features. The southern boundary location has been identified to be crucial in producing the SGP Low Level Jet strength, which in turn brings more moisture from the Gulf of Mexico to the SGP and thereby resulting in a better simulation of the precipitation anomaly in that area. The sensitivity of the simulation of the LST/SUBT downstream effect to dynamical cores is assessed by inter-comparing the Non-hydrostatic Mesoscale Model (NMM) and the Advanced Research WRF dynamic cores. We find NMM was better at generating the large-scale eastward wave train, a crucial process associated with the LST/SUBT downstream effect. Meanwhile, this study also shows that the LST/SUBT downstream effects were not significantly dependent on the surface process parameterizations, although the Simplified Simple Biosphere model version 3 (SSiB3) highlighted a better performance over SSiB2.
              Cover page: Dynamical downscaling the impact of spring Western US land surface temperature on the 2015 flood extremes at the Southern Great Plains: effect of domain choice, dynamic cores and land surface parameterization
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2019)
              This paper evaluates multi-decadal simulations of the UCLA version of Climate Forecast System version 2, in which the default Noah land surface model has been replaced with the Simplified Simple Biosphere Model version-2. To examine the influence of the atmosphere–ocean (AO) interaction on the variability, two different simulations were conducted: one with interactive ocean component, and the other constrained by the prescribed sea surface temperature. We evaluate the mean seasonal climatology of precipitation and temperature, along with the model’s ability to reproduce atmospheric variability at different scales over the globe, including extratropical modes of atmospheric variability, and long-term trends of global and hemispheric temperature and regional precipitation. Here, we particularly selected two monsoon regions, East Asia and West Africa, where the simulation of multi-decadal variations which has heretofore been a challenging task, to examine decadal variation of monsoon precipitation. In general, temperature anomaly trends were better captured than those of precipitation in both simulations. Results suggest that the AO interaction, represented as latent heat flux, contributes to improve reproducibility of global-wide climatology, extratropical modes of atmospheric variability, and variability in the multi-decadal climate simulation, as well as for inter-decadal variability of the East Asian summer monsoon.
                Cover page: Evaluation of multi-decadal UCLA-CFSv2 simulation and impact of interactive atmospheric-ocean feedback on global and regional variability
                • Article
                • Peer Reviewed
                UCLA Previously Published Works (2014)
                Land use and land cover (LULC) over Africa have changed substantially over the last 60 years and this change has been proposed to affect monsoon circulation and precipitation. This study examines the uncertainties of model simulated response in the African monsoon system and Sahel precipitation due to LULC change using a set of regional model simulations with different combinations of land surface and cumulus parameterization schemes. Although the magnitude of the response covers a broad range of values, most of the simulations show a decline in Sahel precipitation due to the expansion of pasture and croplands at the expense of trees and shrubs and an increase in surface air temperature. The relationship between the model responses to LULC change and the climatologists of the control simulations is also examined. Simulations that are climatologically too dry or too wet compared to observations and reanalyses have weak response to land use change because they are in moisture or energy limited regimes respectively. The ones that lie in between have stronger response to the LULC changes, showing a more significant role in land–atmosphere interactions. Much of the change in precipitation is related to changes in circulation, particularly to the response of the intensity and latitudinal position of the African Easterly Jet, which varies with the changes in meridional surface temperature gradients. The study highlights the need for measurements of the surface fluxes across the meridional cross-section of the Sahel to evaluate models and thereby allowing human impacts such as land use change on the monsoon to be projected more realistically.
                  Cover page: Assessment of uncertainties in the response of the African monsoon precipitation to land use change simulated by a regional model
                  • 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)
                    Recurrent drought and associated heatwave episodes are important features of the US climate. Many studies have examined the connection between ocean surface temperature changes and conterminous US droughts. However, remote effects of large-scale land surface temperature variability, over shorter but still considerable distances, on US regional droughts have been largely ignored. The present study combines two types of evidence to address these effects: climate observations and model simulations. Our analysis of observational data shows that springtime land temperature in northwest US is significantly correlated with summer rainfall and surface temperature changes in the US Southern Plains and its adjacent areas. Our model simulations of the 2011 Southern Plains drought using a general circulation model and a regional climate model confirm the observed relationship between land temperature anomaly and drought, and suggest that the long-distance effect of land temperature changes in the northwest US on Southern Plains droughts is probably as large as the more familiar effects of ocean surface temperatures and atmospheric internal variability. We conclude that the cool 2011 springtime climate conditions in the northwest US increased the probability of summer drought and abnormal heat in the Southern Plains. The present study suggests a strong potential for more skillful intra-seasonal predictions of US Southern Plains droughts when such facts as ones presented here are considered.
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                    Cover page: Spring land temperature anomalies in northwestern US and the summer drought over Southern Plains and adjacent areas
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