nasa-gesdisc/es-publications-researchareas huggingface dataset:
Evaluation of Six High-Resolution Satellite and Ground-Based Precipitation Products over Malaysia (2015)

Satellite precipitation products (SPPs) potentially constitute an alternative to sparse rain gauge networks for assessing the spatial distribution of precipitation. However, applications of these products are still limited due to the lack of robust quality assessment. This study compares daily, monthly, seasonal, and annual rainfall amount at 342 rain gauges over Malaysia to estimations using five SPPs (3B42RT, 3B42V7, GPCP-1DD, PERSIANN-CDR, and CMORPH) and a ground-based precipitation product (APHRODITE). The performance of the precipitation products was evaluated from 2003 to 2007 using continuous (RMSE, R2, ME, MAE, and RB) and categorical (ACC, POD, FAR, CSI, and HSS) statistical approaches. Overall, 3B42V7 and APHRODITE performed the best, while the worst performance was shown by GPCP-1DD. 3B42RT, 3B42V7, and PERSIANN-CDR slightly overestimated observed precipitation by 2%, 4.7%, and 2.1%, respectively. By contrast, APHRODITE and CMORPH significantly underestimated precipitations by 19.7% and 13.2%, respectively, whereas GPCP-1DD only slightly underestimated by 2.8%. All six precipitation products performed better in the northeast monsoon than in the southwest monsoon. The better performances occurred in eastern and southern Peninsular Malaysia and in the north of East Malaysia, which receives higher rainfall during the northeast monsoon, whereas poor performances occurred in the western and dryer Peninsular Malaysia. All precipitation products underestimated the no/tiny (<1 mm/day) and extreme (20 mm/day) rainfall events, while they overestimated low (120 mm/day) rainfall events. 3B42RT and 3B42V7 showed the best ability to detect precipitation amounts with the highest HSS value (0.36). Precipitations during flood events such as those which occurred in late 2006 and early 2007 were estimated the best by 3B42RT and 3B42V7, as shown by an R2 value ranging from 0.49 to 0.88 and 0.52 to 0.86, respectively. These results on SPPs uncertainties and their potential controls might allow sensor and algorithm developers to deliver better products for improved rainfall estimation and thus improved water management.

Research areas: Validation
Impact of high resolution land surface initialization in Indian summer monsoon simulation using a regional climate model (2016)

The direct impact of high resolution land surface initialization on the forecast bias in a regional climate model in recent years over Indian summer monsoon region is investigated. Two sets of regional climate model simulations are performed, one with a coarse resolution land surface initial conditions and second one used a high resolution land surface data for initial condition. The results show that all monsoon years respond differently to the high resolution land surface initialization. The drought monsoon year 2009 and extended break periods were more sensitive to the high resolution land surface initialization. These results suggest that the drought monsoon year predictions can be improved with high resolution land surface initialization. Result also shows that there are differences in the response to the land surface initialization within the monsoon season. Case studies of heat wave and a monsoon depression simulation show that, the model biases were also improved with high resolution land surface initialization. These results show the need for a better land surface initialization strategy in high resolution regional models for monsoon forecasting.

Research areas: Atmospheric/Ocean Indicators
Mapping and quantifying Sargassum distribution and coverage in the Central West Atlantic using MODIS observations (2016)

Sargassum washing ashore on the beaches of the Caribbean Islands since 2011 has caused problems for the local environments, tourism, and economies. Although preliminary results of Sargassum distributions in the nearby oceans have been obtained using measurements from the Medium Resolution Imaging Spectrometer (MERIS), MERIS stopped functioning in 2012, and detecting and quantifying Sargassum distributions still face technical challenges due to ambiguous pixels from clouds, cloud shadows, cloud adjacency effect, and large-scale image gradient. In this paper, a novel approach is developed to detect Sargassum presence and to quantify Sargassum coverage using the Moderate Resolution Imaging Spectroradiometer (MODIS) alternative floating algae index (AFAI), which examines the red-edge reflectance of floating vegetation. This approach includes three basic steps: 1) classification of Sargassum-containing pixels through correction of large-scale gradient, masking clouds and cloud shadows, and removal of ambiguous pixels; 2) linear unmixing of Sargassum-containing pixels; and, 3) statistics of Sargassum area coverage in pre-defined grids at monthly, seasonal, and annual intervals. In the absence of direct field measurements to validate the results, limited observations from the Hyperspectral Imager for the Coastal Ocean (HICO) measurements and numerous local reports support the conclusion that the elevated AFAI signals are due to the presence of Sargassum instead of other floating materials, and various sensitivity analyses are used to quantify the uncertainties in the derived Sargassum area coverage. The approach was applied to MODIS observations between 2000 and 2015 over the Central West Atlantic (CWA) region (022N, 6338W) to derive the spatial and temporal distribution patterns as well as the total area coverage of Sargassum. Results indicate that the first widespread Sargassum distribution event occurred in 2011, consistent with previous MERIS findings. Since 2011, only 2013 showed a minimal Sargassum coverage similar to the period of 2000 to 2010; all other years showed significantly more coverage. More alarmingly, the summer months of 2015 showed mean coverage of >2000km2, or about 4 times of the summer 2011 coverage and 20 times of the summer 2000 to 2010 coverage. Analysis of several environmental variables provided some hints on the reasons causing the inter-annual changes after 2010, yet further multi-disciplinary research (including in situ measurements) is required to understand such changes and long-term trends in Sargassum coverage.

Research areas: Ecosystems
Seasonal, interannual, and long-term variabilities in biomass burning activity over South Asia (2016)

The seasonal, interannual, and long-term variations in biomass burning activity and related emissions are not well studied over South Asia. In this regard, active fire location retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS), the retrievals of aerosol optical depth (AOD) from MODIS Terra, and tropospheric column NO2 from Ozone Monitoring Instrument (OMI) are used to understand the effects of biomass burning on the tropospheric pollution loadings over South Asia during 20032013. Biomass burning emission estimates from Global Fire Emission Database (GFED) and Global Fire Assimilation System (GFAS) are also used to quantify uncertainties and regional discrepancies in the emissions of carbon monoxide (CO), nitrogen oxide (NOx), and black carbon (BC) due to biomass burning in South Asia. In the Asian continent, the frequency of fire activity is highest over Southeast Asia, followed by South Asia and East Asia. The biomass burning activity in South Asia shows a distinct seasonal cycle that peaks during February-May with some differences among four (north, central, northeast, and south) regions in India. The annual biomass burning activity in north, central, and south regions shows an increasing tendency, particularly after 2008, while a decrease is seen in northeast region during 20032013. The increase in fire counts over the north and central regions contributes 24 % of the net enhancement in fire counts over South Asia. MODIS AOD and OMI tropospheric column NO2 retrievals are classified into high and low fire activity periods and show that biomass burning leads to significant enhancement in tropospheric pollution loading over both the cropland and forest regions. The enhancement is much higher (110176 %) over the forest region compared to the cropland (3462 %) region. Further efforts are required to understand the implications of biomass burning on the regional air quality and climate of South Asia.

Research areas: Environmental Impacts, Air Quality
The solar dimming/brightening effect over the Mediterranean Basin in the period 19792012 (2016)

Numerous studies have shown that the solar radiation reaching the Earth's surface is subjected to multi-decadal variations with significant spatial and temporal heterogeneities in both magnitude and sign. Although several studies have examined the solar radiation trends over Europe, North America and Asia, the Mediterranean Basin has not been studied extensively. This work investigates the evolution and trends in the surface net short-wave radiation (NSWR, surface solar radiation - reflected) over the Mediterranean Basin during the period 19792012 using monthly re-analysis datasets from the Modern Era Retrospective-Analysis for Research and Applications (MERRA) and aims to shed light on the specific role of clouds on the NSWR trends. The solar dimming/brightening phenomenon is temporally and spatially analyzed over the Mediterranean Basin. The spatially-averaged NSWR over the whole Mediterranean Basin was found to increase in MERRA by +0.36Wm2 per decade, with higher rates over the western Mediterranean (+0.82Wm2 per decade), and especially during spring (March-April-May; +1.3Wm2 per decade). However, statistically significant trends in NSWR either for all-sky or clean-sky conditions are observed only in May. The increasing trends in NSWR are mostly associated with decreasing ones in cloud optical depth (COD), especially for the low (<700hPa) clouds. The decreasing COD trends (less opaque clouds and/or decrease in absolute cloudiness) are more pronounced during spring, thus controlling the increasing tendency in NSWR. The NSWR trends for cloudless (clear) skies are influenced by changes in the water-vapor content or even variations in surface albedo to a lesser degree, whereas aerosols are temporally constant in MERRA. The slight negative trend (not statistically significant) in NSWR under clear skies for nearly all months and seasons implies a slight increasing trend in water vapor under a warming and more humid climatic scenario over the Mediterranean.

Research areas: Sun-Earth Interactions
Coupling remote sensing bio-optical and three-dimensional hydrodynamic modeling to study the phytoplankton dynamics in a tropical hydroelectric reservoir (2015)

The goal of this study was to determine the extent to which mixing and stratification processes influence the horizontal surface phytoplankton distribution in a tropical hydroelectric reservoir in Brazil. We used a synergistic approach that combines remote sensing bio-optical empirical and three-dimensional hydrodynamic modeling to represent phytoplankton dynamics. Seasonal differences were evaluated by simulating the bio-optical and the three-dimensional hydrodynamic models for two periods: the austral summer and winter of 2009. The three-dimensional hydrodynamic model simulations showed that the water column was completely mixed during winter; the water column remained stratified during summer. We also noticed a permanent thermocline during the summer between 15 and 25m in the reservoir. In both seasons, the surface current was wind driven and preferentially directed eastward. The bio-optical model showed that the horizontal surface phytoplankton distribution, as indicated by chlorophyll-a concentration, was heterogeneous during winter and homogenous during summer. Moreover, higher concentrations were observed by the empirical bio-optical model during winter (the highest mean areal concentration was 24gL1) than in summer (2.1gL1). This difference was explained by short timescale events, such as cold front passages, which contributed to the proliferation of phytoplankton in winter by enhancing vertical mixing in the water column. Lake number analysis showed that upwelling events were the most common mechanisms driving the mixing process during winter (LN<1 82% of the time). Combining remote sensing and three-dimensional hydrodynamic modeling makes it possible to more fully analyze the dynamics of the horizontal surface phytoplankton distribution under different meteorological conditions. The two modeling techniques can be used in a complementary manner and serve as an essential tool for the environmental monitoring of aquatic systems.

Research areas: Ecosystems, Atmospheric/Ocean Indicators
Correlating Remote Sensing Data with the Abundance of Pupae of the Dengue Virus Mosquito Vector, Aedes aegypti, in Central Mexico (2014)

Using a geographic transect in Central Mexico, with an elevation/climate gradient, but uniformity in socio-economic conditions among study sites, this study evaluates the applicability of three widely-used remote sensing (RS) products to link weather conditions with the local abundance of the dengue virus mosquito vector,  Aedes aegypti (Ae. aegypti). Field-derived entomological measures included estimates for the percentage of premises with the presence of Ae. aegypti pupae and the abundance of Ae. aegypti pupae per premises. Data on mosquito abundance from field surveys were matched with RS data and analyzed for correlation. Daily daytime and nighttime land surface temperature (LST) values were obtained from Moderate Resolution Imaging Spectroradiometer (MODIS)/Aqua cloud-free images within the four weeks preceding the field survey. Tropical Rainfall Measuring Mission (TRMM)-estimated rainfall accumulation was calculated for the four weeks preceding the field survey. Elevation was estimated through a digital elevation model (DEM). Strong correlations were found between mosquito abundance and RS-derived night LST, elevation and rainfall along the elevation/climate gradient. These findings show that RS data can be used to predict Ae. aegypti abundance, but further studies are needed to define the climatic and socio-economic conditions under which the correlations observed herein can be assumed to apply.

Research areas: Public Health
Uncertainty of different atmospheric ozone retrievals and its effect on temporal trends and radiative transfer simulations in the Iberian Peninsula (2014)

AbstractUncertainty in total ozone column (TOC) values is quantified for eight different databases through a direct comparison with ground-based data at three Spanish locations, the maximum uncertainty being about 10.5 Dobson unit. A long-term TOC series is constructed using the mentioned databases from 1950 to 2011 for nine locations in the Iberian Peninsula. The monthly TOC climatology in the nine locations is presented. An exhaustive analysis is performed of TOC series trends and their statistical significance in the periods 19502011, 19501984, and 19852011. A significant ozone reduction between 1950 and 2011 appears in the Iberian Peninsula with a strong reduction from the late 1970s to the mid-1990s and with more or less constant or slightly increasing levels in the last 17 year. These trends are recalculated taking into account of uncertainty in the TOC values of the series, with a decrease in the number of statistically significant trends emerging. The statistically significant trends in annual and in the averaged Iberian Peninsula series are usually still significant even considering the uncertainty. Finally, a study is carried out of the uncertainty caused by TOC uncertainty in total shortwave and erythemal ultraviolet irradiances simulated under cloudless skies using a radiative transfer model.

Research areas: Validation
Satellite-derived estimates of ultrafine particle concentrations over eastern North America (2013)

High concentrations of ultrafine particles (UFP, i.e., particles with diameter < 100 nm) impact both human health and Earth's climate. Recent innovations in remote sensing technologies and data retrievals offer the potential for predicting UFP concentrations based on data from satellite-borne instrumentation. Herein we present a physically based statistical algorithm to estimate UFP concentrations across eastern North America using remotely sensed aerosol optical depth, Angstrom exponent, ultraviolet solar radiation flux, and ammonia and sulfur dioxide concentrations. The proposed algorithm is built and independently evaluated using an array of in situ observations. The algorithm is able to capture up to 60% of the variability in daily measured UFP number concentrations at a regionally representative reference site and is thus applied to generate seasonal UFP concentration estimates across eastern North America. The resulting UFP concentrations are cross-evaluated with simulations from a global aerosol microphysics model. There is a negative bias in the model output relative to the satellite-driven proxy, which is largest (up to 76%) in summer and may be due to overestimation of UFP from the satellite-based algorithm derived herein, due to the higher availability of remote sensing data in clear-sky conditions or uncertainty in the model simulation of new particle formation. Nevertheless, the model and algorithm indicate similar spatial and seasonal variability (spatial correlation coefficients of 0.10 to 0.56), indicating the value of the satellite-based UFP proxy in global and regional model evaluation exercises and in efforts to identify regions where future in situ data collection should be prioritized.

Research areas: Validation
Enhanced SO2 concentrations observed over northern India: role of long-range transport (2013)

The combustion of fossil fuels (coal and petroleum products) constitutes a source of continuous release of anthropogenic SO2 into the atmosphere. Furthermore, natural sources such as volcanoes can inject large amounts of SO2 directly into the troposphere and sometimes even into the stratosphere. These event-based volcanic eruptions provide solitary opportunities to study the transport and transformation of atmospheric constituents. In this study, we present an episode of high SO2 concentration over northern India as a result of long-range transport from Africa using multiple satellite observations. Monthly averaged column SO2 values over the Indo-Gangetic Plain (IGP) were observed in the range of 0.60.9 Dobson units (DU) during November 2008 using observations from the Ozone Monitoring Instrument (OMI). These concentrations were conspicuously higher than the background concentrations (<0.3 DU) observed during 20052010 over this region. The columnar SO2 loadings were highest on 6 November over most of the IGP region and even exceeded 6 DU, a factor of 1020 higher than background levels in some places. These enhanced SO2 levels were not reciprocated in satellite-derived NO2 or CO columns, indicating transport from a non-anthropogenic SO2 source. As most of the local aerosols over the IGP region occur below 3 km, a well-separated layer at 45 km was observed from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. Wind fields and back-trajectory analysis revealed a strong flow originating from the Dalaffilla volcanic eruption in Ethiopia during 46 November 2008. Although volcanic SO2 plumes have been extensively studied over many parts of Asia, Europe, and the USA, analysis of such events for the IGP region is being reported for the first time in this study.

Research areas: Volcanic Eruptions, Air Quality
Micrometeorological conditions and surface mass and energy fluxes on Lewis Glacier, Mt Kenya, in relation to other tropical glaciers (2013)

Abstract. The Lewis Glacier on Mt Kenya is one of the best-studied tropical glaciers, but full understanding of the interaction of the glacier mass balance and its climatic drivers has been hampered by a lack of long-term meteorological data. Here we present 2.5 yr of meteorological data collected from the glacier surface from October 2009 to February 2012. The location of measurements is in the upper portion of Lewis Glacier, but this location experiences negative annual mass balance, and the conditions are comparable to those experienced in the lower ablation zones of South American glaciers in the inner tropics. In the context of other glaciated mountains of equatorial East Africa, the summit zone of Mt Kenya shows strong diurnal cycles of convective cloud development as opposed to the Rwenzoris, where cloud cover persists throughout the diurnal cycle, and Kilimanjaro, where clear skies prevail. Surface energy fluxes were calculated for the meteorological station site using a physical mass- and energy-balance model driven by measured meteorological data and additional input parameters that were determined by Monte Carlo optimization. Sublimation rate was lower than those reported on other tropical glaciers, and melt rate was high throughout the year, with the glacier surface reaching the melting point on an almost daily basis. Surface mass balance is influenced by both solid precipitation and air temperature, with radiation providing the greatest net source of energy to the surface. Cloud cover typically reduces the net radiation balance compared to clear-sky conditions, and thus the frequent formation of convective clouds over the summit of Mt Kenya and the associated higher rate of snow accumulation are important in limiting the rate of mass loss from the glacier surface. The analyses shown here form the basis for future glacier-wide mass and energy balance modeling to determine the climate proxy offered by the glaciers of Mt Kenya.

Research areas: Cryospheric Indicators