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Gerald V. Frost, Uma S. Bhatt, Matthew J. Macander, Amy S. Hendricks, and M. Torre Jorgenson

Abstract

Alaska’s Yukon-Kuskokwim Delta (YKD) is among the Arctic’s warmest, most biologically productive regions, but regional decline of the Normalized Difference Vegetation Index (NDVI) has been a striking feature of spaceborne Advanced High Resolution Radiometer (AVHRR) observations since 1982. This contrast with “greening” prevalent elsewhere in the Low Arctic raises questions concerning climatic and biophysical drivers of tundra productivity along maritime-continental gradients. We compared NDVI time-series from AVHRR, the Moderate Resolution Imaging Spectroradiometer (MODIS), and Landsat for 2000–2019, and identified trend drivers with reference to sea-ice and climate datasets, ecosystem and disturbance mapping, field measurements of vegetation, and knowledge exchange with YKD elders. All time-series showed increasing maximum NDVI; however, while MODIS and Landsat trends were very similar, AVHRR-observed trends were weaker and had dissimilar spatial patterns. The AVHRR and MODIS records for time-integrated NDVI were dramatically different; AVHRR indicated weak declines, whereas MODIS indicated strong increases throughout the YKD. Disagreement largely arose from observations during shoulder seasons, when there is partial snow cover and very high cloud frequency. Nonetheless, both records shared strong correlations with spring sea-ice extent and summer warmth. Multiple lines of evidence indicate that despite frequent disturbances and high interannual variability in spring sea-ice and summer warmth, tundra productivity is increasing on the YKD. Although climatic drivers of tundra productivity were similar to more continental parts of the Arctic, our intercomparison highlights sources of uncertainty in maritime areas like the YKD that currently, or soon will challenge historical concepts of “what is Arctic.”

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Michael Peterson and Geoffrey Stano

Abstract

Lighting megaflashes extending over >100 km distances have been observed by the Geostationary Lightning Mappers (GLMs) on NOAA’s 16-series Geostationary Operational Environmental Satellites (GOES). The hazards posed by megaflashes are unclear, however, due to limitations in the GLM data. We address these by reprocessing GOES-16 GLM measurements from 1/1/2018 to 1/15/2020 and integrating them with Earth Networks Global Lightning Network (ENGLN) observations. 194,880 GLM megaflashes are verified as natural lightning by ENGLN. Of these, 127,479 flashes occurred following the October 2018 GLM software update that standardized GLM timing. Reprocessed GLM/ENGLN lightning maps from these post-update cases provide a comprehensive view of how individual megaflashes evolve.

This megaflash dataset is used to generate statistics that describe their hazards. The average megaflash produces 5-7 CG strokes that are spread across 40-50% of the flash extent. As flash extent increases beyond 100 km, megaflashes become concentrated in key hotspot regions in North and South America while the number of CG and IC events per flash and the overall peak current increase. CGs in the larger megaflashes occur over 80% of the flash extent measured by GLM, while the majority contain regions where the megaflash is the only lightning activity in the preceding hour. These statistics demonstrate that there is no safe location below an electrified cloud that is producing megaflashes and current lightning safety guidance is not always sufficient to mitigate megaflash hazards.

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Ansar Khan, Samiran Khorat, Rupali Khatun, Quang-Van Doan, U. S. Nair, and Dev Niyogi

Abstract

India responded to the COVID-19 pandemic through a three-phase nationwide lockdown: 25 March - 14 April, 15 April - 3 May and 4 - 17 May, 2020. We utilized this unique opportunity to assess the impact of restrictions on the air quality of Indian cities. We conducted comprehensive statistical assessments for the Air Quality Index (AQI) and criteria pollutant concentrations for 91 cities during the lockdown phases to the preceding seven days (pre-lockdown phase 18-24March,2020) and corresponding values from the same days of the year in 2019. Both comparisons show statistically significant country-wide mean decrease in AQI (33%), PM2.5 (36%), PM10 (40%), NO2 (58%), O3 (5%), SO2 (25%), NH3(28%), and CO(60%). These reductions represent a background or the lower bound of air quality burden of industrial and transportation sectors. The northern region was most impacted by the first two phases of the lockdown, while the southern region was most affectedin the last phase. The northeastern region was least affected, followed by the eastern region which also showed an increase in O3during the lockdown. Analysis of satellite retrieved Aerosol Optical Depth (AOD) shows that effects of restrictions on particulate pollution to be variable- locally confined in some areas or having a broader impact in other regions. Anomalous behavior over the eastern region suggestsa differing role of regional societal response or meteorology. The study results have policy implications as they provide the observational background values for the industrial and transportation sector’s contribution to urban pollution.

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Mark R. Jury and América R. Gaviria Pabón

Abstract

Satellite and reanalysis products are used to study the atmospheric environment, aerosols, and trace gases in smoke plumes over South America in the period 2000–18. Climatic conditions and fire density maps provide context to link biomass burning across the southern Amazon region (5°–15°S, 50°–70°W) to thick near-surface plumes of trace gases and fine aerosols. Intraseasonal weather patterns that underpin greater fire emissions in the dry season (July–October) are exacerbated by high pressure over a cool eastern Pacific Ocean, for example in September 2007. Smoke-plume dispersion simulated with HYSPLIT reveals a slowing of westward transport between sources in eastern Brazil and the Andes Mountains. During cases of thick smoke plumes over southern Amazon, an upper ridge and sinking motions confine trace gases and fine aerosols below 4 km. Long-term warming, which tends to coincide with the zone of biomass burning, is +0.03°C yr−1 in the air and +0.1°C yr−1 at the land surface. Our study suggests that weather conditions promoting fire emissions also tend to limit dispersion.

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Paolina Bongioannini Cerlini, Lorenzo Silvestri, Silvia Meniconi, and Bruno Brunone

Abstract

This paper concerns the simulation of the water table elevation in shallow unconfined aquifers where infiltration is assumed as the main mechanism of recharge. The main aim is to provide a reliable tool for groundwater management that satisfies water supply managers. Such a tool is a candidate as a physically based alternative to the use of empirical methods or general circulation models. It is based on the use of two widely available sets of data: the water table elevation measurements and soil moisture time series. In fact, the former are usually provided by government agencies on public websites whereas the latter are included in the atmospheric global datasets (reanalysis). It is notable that data from reanalysis are accessible to any citizen and organization around the world on an open-access basis (e.g., Copernicus). In the proposed method, the measured water table elevations are correlated quantitatively with the water fluxes toward the aquifer evaluated using the soil moisture data from ERA5 reanalysis (provided by ECMWF) within a Richards equation–based approach. The analysis is executed using data from the Umbria region (Italy) on both a daily and monthly scale. In fact, these are the time intervals of interest for a proper management of groundwater resources. The proposed relationships include both a logarithmic and linear term and point out the possible different regimes of the shallow aquifers with regard to the recharge due to infiltration. These different mechanisms reflect in the different role played by the water fluxes toward the aquifer in terms of water table elevation changes according to the considered time scale.

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Jason Naylor and Aaron D. Kennedy

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This study analyzes the frequency of strong, isolated convective cells in the vicinity of Louisville, Kentucky. Data from the Severe Weather Data Inventory are used to compare the frequency of convective activity over Louisville with the observed frequency at nearby rural locations from 2003 to 2019. The results show that Louisville experiences significantly more isolated convective activity than do the rural locations. The difference in convective activity between Louisville and the rural locations is strongest during summer, with peak differences occurring between May and August. Relative to the rural locations, Louisville experiences more isolated convective activity in the afternoon and early evening but less activity after midnight and into the early morning. Isolated convective events over Louisville are most likely during quiescent synoptic conditions, whereas rural events are more likely during active synoptic patterns. To determine whether these differences can be attributed primarily to urban effects, two additional cities are shown for comparison—Nashville, Tennessee, and Cincinnati, Ohio. Both Nashville and Cincinnati experience more isolated convective activity than all five of their nearby rural comparison areas, but the results for both are statistically significant at four of the five rural locations. In addition, the analysis of Cincinnati includes a sixth comparison site that overlaps the urbanized area of Columbus, Ohio. For that location, differences in convective activity are not statistically significant.

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A. S. Alhumaima and S. M. Abdullaev

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The primary aim of this work is to study the response of the normalized difference vegetation index (NDVI) of landscapes in the lower Tigris basin to current global and regional climate variability presented, respectively, by the global circulation indices and monthly temperatures and precipitation extracted from five observational/reanalysis datasets. The second task is to find the dataset that best reflects the regional vegetation and climate conditions. Comparison of the Köppen–Trewartha bioclimatic landscapes with the positions of botanical districts, land-cover types, and streamflow estimates led to the conclusion that only two datasets correctly describe regional climatic zones. Therefore, searching for the NDVI response to regional climate variability requires the use of normalized analogs of temperatures and precipitations, as well as the Spearman rank correlation. We found that March/April NDVI, as proxies of the maximum biological productivity of the regional landscapes, are strongly correlated with October–March precipitation derived from three datasets and January–March temperatures derived from one dataset. We discovered the significant impact of autumn–winter El Niño–Southern Oscillation and winter Indian Oceanic dipole states on regional weather (e.g., all five recent severe droughts occurred during strong La Niña events). However, the strength of this impact on the vegetation was clearly linked to the zonal landscape type. By selecting pairs of the temperature/precipitation time series that best correlated with NDVI at a given landscape, we have built a synthetic climate dataset. The landscape approach presented in this work can be used to validate the viability of any dataset when assessing the impacts of climate change and variability on weather-dependent components of Earth’s surface.

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Mary K. Butwin, Sibylle von Löwis, Melissa A. Pfeffer, Pavla Dagsson-Waldhauserova, Johann Thorsson, and Throstur Thorsteinsson

Abstract

The 2010 eruption of Eyjafjallajökull produced volcanic ash that was mostly deposited to the south and east of the volcano, with the thickest deposits closest to the eruption vents. For months following the eruption there were numerous reports of resuspended volcanic ash made by weather observers on the ground. A saltation sensor (SENSIT) and an optical particle counter (OPC) located on the southern side of Eyjafjallajökull measured posteruptive particulate matter (PM) saltation and suspension events, some of which were also observable by satellite imagery. During the autumn/winter following the eruption, visible satellite images and the SENSIT show that PM measured by the OPC was only detected when winds had a northerly component, making the source on the slopes of Eyjafjallajökull. During the largest observed events, particles >10 μm were suspended but measured in extremely low concentrations (<1 particle per centimeter cubed). The saltation measurements, however, show high concentrations of particles >100 μm in size during these events. During the largest events, winds were at least 5 m s−1 with a relative humidity < 70%. Ground conditions in Iceland change quickly from unfavorable to favorable for the suspension of particles. It is hypothesized that this is due to the porosity of the surface material allowing water to filter through quickly as well as the fast drying time of surface material. The high moisture content of the atmosphere and the ground do not appear to be a deterrent for large PM events to occur in Iceland.

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Rezaul Mahmood, Joseph Santanello, and Xiaoyang Zhang
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Rezaul Mahmood
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