Global frequency and distribution of lightning as observed from space by the Optical Transient Detector
TL;DR: In this article, the authors used the OTD measurements to construct lightning climatology maps that demonstrate the geographical and seasonal distribution of lightning activity for the globe, and found that lightning occurs mainly over land areas, with an average land/ocean ratio of 10:1.
Abstract: of uncertainty for the OTD global totals represents primarily the uncertainty (and variability) in the flash detection efficiency of the instrument The OTD measurements have been used to construct lightning climatology maps that demonstrate the geographical and seasonal distribution of lightning activity for the globe An analysis of this annual lightning distribution confirms that lightning occurs mainly over land areas, with an average land/ocean ratio of 10:1 The Congo basin, which stands out year-round, shows a peak mean annual flash density of 80 fl km 2 yr 1 in Rwanda, and includes an area of over 3 million km 2 exhibiting flash densities greater than 30 fl km 2 yr 1 (the flash density of central Florida) Lightning is predominant in the northern Atlantic and western Pacific Ocean basins year-round where instability is produced from cold air passing over warm ocean water Lightning is less frequent in the eastern tropical Pacific and Indian Ocean basins where the air mass is warmer A dominant Northern Hemisphere summer peak occurs in the annual cycle, and evidence is found for a tropically driven semiannual cycle INDEX TERMS: 3304 Meteorology and Atmospheric Dynamics: Atmospheric electricity; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3324 Meteorology and Atmospheric Dynamics: Lightning; 3394 Meteorology and Atmospheric Dynamics: Instruments and techniques;
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TL;DR: In this article, the authors show the results of a systematic search through seven full years of the TRMM database to find indicators of uncommonly intense storms, such as strong (> 40 dBZ) radar echoes extending to great heights, high lightning flash rates, and very low brightness temperatures at 37 and 85 GHz.
Abstract: The instruments on the Tropical Rainfall Measuring Mission (TRMM) satellite have been observing storms as well as rainfall since December 1997. This paper shows the results of a systematic search through seven full years of the TRMM database to find indicators of uncommonly intense storms. These include strong (> 40 dBZ) radar echoes extending to great heights, high lightning flash rates, and very low brightness temperatures at 37 and 85 GHz. These are used as proxy variables, indicating powerful convective updrafts. The main physical principles supporting this assertion involve the effects of such updrafts in producing and lofting large ice particles high into the storm, where TRMM's radar easily detects them near storm top. TRMM's passive microwave radiometer detects the large integrated ice water path as very low brightness temperatures, while high lightning flash rates are a physically related but instrumentally independent indicator. The geographical locations of these very intense convective storms ...
789 citations
TL;DR: A multivariate quantification of environmental drivers for the observed, current distribution of vegetation fires using statistical models of the relationship between fire activity and resources to burn, climate conditions, human influence, and lightning flash rates at a coarse spatiotemporal resolution is presented.
Abstract: Climate change is expected to alter the geographic distribution of wildfire, a complex abiotic process that responds to a variety of spatial and environmental gradients. How future climate change may alter global wildfire activity, however, is still largely unknown. As a first step to quantifying potential change in global wildfire, we present a multivariate quantification of environmental drivers for the observed, current distribution of vegetation fires using statistical models of the relationship between fire activity and resources to burn, climate conditions, human influence, and lightning flash rates at a coarse spatiotemporal resolution (100 km, over one decade). We then demonstrate how these statistical models can be used to project future changes in global fire patterns, highlighting regional hotspots of change in fire probabilities under future climate conditions as simulated by a global climate model. Based on current conditions, our results illustrate how the availability of resources to burn and climate conditions conducive to combustion jointly determine why some parts of the world are fire-prone and others are fire-free. In contrast to any expectation that global warming should necessarily result in more fire, we find that regional increases in fire probabilities may be counter-balanced by decreases at other locations, due to the interplay of temperature and precipitation variables. Despite this net balance, our models predict substantial invasion and retreat of fire across large portions of the globe. These changes could have important effects on terrestrial ecosystems since alteration in fire activity may occur quite rapidly, generating ever more complex environmental challenges for species dispersing and adjusting to new climate conditions. Our findings highlight the potential for widespread impacts of climate change on wildfire, suggesting severely altered fire regimes and the need for more explicit inclusion of fire in research on global vegetation-climate change dynamics and conservation planning.
778 citations
TL;DR: The Gulf Stream is the upper limb of the Atlantic meridional overturning circulation, which has varied in strength in the past and is predicted to weaken in response to human-induced global warming in the future.
Abstract: The Gulf Stream is a warm Atlantic current that transports heat northward, keeping Western Europe significantly warmer than North America in winter. It is known to influence short-term weather phenomena such as surface winds and cyclone formation, but its effects on longer-term climate and at higher levels in the atmosphere are poorly understood. Now a combination of weather analyses, satellite data and an atmospheric general circulation model reveals that the Gulf Stream's influence is felt well above the near-surface portion of the atmosphere. The current anchors a tall wall of atmospheric upward motion that penetrates into the upper troposphere and supports deep raining clouds. This provides a pathway by which the Gulf Stream can affect local climate, and possibly climate in remote regions via an effect on planetary wave propagation. The cover graphic represents surface current speeds in blue-white colours (white is the fastest) and upward wind velocities in yellow-red colours (red for stronger winds), along with land-surface topography in eastern North America. The Gulf Stream's influence on the atmosphere is examined using a combination of operational weather analyses and satellite observations. The results indicate that the Gulf Stream anchors a rain band in which upward motion of air penetrates deep into the upper troposphere, well above the near-surface portion of the atmosphere. These mechanisms provide a pathway by which the Gulf Stream can affect local climate, and possibly also climate in remote regions. The Gulf Stream transports large amounts of heat from the tropics to middle and high latitudes, and thereby affects weather phenomena such as cyclogenesis1,2 and low cloud formation3. But its climatic influence, on monthly and longer timescales, remains poorly understood. In particular, it is unclear how the warm current affects the free atmosphere above the marine atmospheric boundary layer. Here we consider the Gulf Stream’s influence on the troposphere, using a combination of operational weather analyses, satellite observations and an atmospheric general circulation model4. Our results reveal that the Gulf Stream affects the entire troposphere. In the marine boundary layer, atmospheric pressure adjustments to sharp sea surface temperature gradients lead to surface wind convergence, which anchors a narrow band of precipitation along the Gulf Stream. In this rain band, upward motion and cloud formation extend into the upper troposphere, as corroborated by the frequent occurrence of very low cloud-top temperatures. These mechanisms provide a pathway by which the Gulf Stream can affect the atmosphere locally, and possibly also in remote regions by forcing planetary waves5,6. The identification of this pathway may have implications for our understanding of the processes involved in climate change, because the Gulf Stream is the upper limb of the Atlantic meridional overturning circulation, which has varied in strength in the past7 and is predicted to weaken in response to human-induced global warming in the future8.
639 citations
University of Hawaii1, United States Naval Research Laboratory2, National Oceanic and Atmospheric Administration3, Oregon State University4, University of California, Los Angeles5, Scripps Institution of Oceanography6, University of Rhode Island7, Woods Hole Oceanographic Institution8, Hokkaido University9
TL;DR: In this paper, the physical processes that lie behind the interaction of sharp SST gradients and the overlying marine atmospheric boundary layer and deeper atmosphere, using high-resolution satellite data, field data and numerical models, are examined.
600 citations
TL;DR: The history leading to modern LLSs that sense lightning radiation fields at multiple remote sensors, focusing on the interactions between enabling technology, scientific discovery, technical development, and uses of the data are described.
Abstract: Lightning in all corners of the world is monitored by one or more land- or space-based lightning locating systems (LLSs). The applications that have driven these developments are numerous and varied. This paper describes the history leading to modern LLSs that sense lightning radiation fields at multiple remote sensors, focusing on the interactions between enabling technology, scientific discovery, technical development, and uses of the data. An overview of all widely used detection and location methods is provided, including a general discussion of their relative strengths and weaknesses for various applications. The U.S. National Lightning Detection Network (NLDN) is presented as a case study, since this LLS has been providing real-time lightning information since the early 1980s, and has provided continental-scale (U.S.) information to research and operational users since 1989. This network has also undergone a series of improvements during its >20-year life in response to evolving detection technologies and expanding requirements for applications. Recent analyses of modeled and actual performance of the current NLDN are also summarized. The paper concludes with a view of the short- and long-term requirements for improved lightning measurements that are needed to address some open scientific questions and fill the needs of emerging applications.
586 citations
References
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TL;DR: The Schumann resonance, a global electromagnetic phenomenon, is shown to be a sensitive measure of temperature fluctuations in the tropical atmosphere, which increases nonlinearly with temperature in the interaction between deep convection and ice microphysics.
Abstract: The Schumann resonance, a global electromagnetic phenomenon, is shown to be a sensitive measure of temperature fluctuations in the tropical atmosphere. The link between Schumann resonance and temperature is lightning flash rate, which increases nonlinearly with temperature in the interaction between deep convection and ice microphysics.
453 citations
TL;DR: In this paper, four years of observations from the NASA Optical Transient Detector (OTD) and Global Atmospherics National Lightning Detection Network (NLDN) are combined to determine the geographic distribution of the intracloud/cloud to ground lightning ratio over the continental United States.
Abstract: Four years of observations from the NASA Optical Transient Detector (OTD) and Global Atmospherics National Lightning Detection Network (NLDN) are combined to determine the geographic distribution of the intracloud/cloud to ground lightning ratio over the continental United States. The mean ratio over this region is 2.64-2.94, with a standard deviation of 1.1-1.3 and anomalies as low as 1.0 or less over the Rocky and Appalachian mountains and as high as 8-10 in the central-upper midwest. There is some indication that the ratio covaries with ground elevation, although the relationship is nonunique. Little evidence is found to support a latitudinal covariance, despite significant variation in the climatological mean tropopause pressure over the latitudes considered. The dynamic range of local variability is comparable to the range of values cited by previous studies for latitudinal variation from the deep tropics to midlatitudes. Local high anomalies of this ratio in the midwest are coincident with anomalies in the climatological percentage of positive CG occurrence, as well as in the occurrence of large positive CGs characteristic of MCS convective and trailing stratiform regions. This suggests that storm type, morphology and level of organization may dominate over environmental cofactors in the local determination of this ratio.
329 citations
TL;DR: In this paper, the effect of convective available potential energy (CAPE) and its nonlinear influence on the growth and accumulation of ice particles aloft, which are believed to promote charge separation by differential particle motions, was examined for both break period and monsoonal regimes in the vicinity of Darwin, Australia.
Abstract: Radar and electrical measurements for deep tropical convection are examined for both “break period” and “monsoonal” regimes in the vicinity of Darwin, Australia. Break period convection consists primarily of deep continental convection, whereas oceanic-based convection dominates during monsoonal periods, associated with the monsoon trough over Darwin. Order-of-magnitude enhancements in lightning flash rates for the “break period” regime are associated with 10–20-dB enhancements in radar reflectivity in the mixed-phase region of the convection compared with the monsoonal regime. The latter differences are attributed to the effect of convective available potential energy (CAPE) and its nonlinear influence on the growth and accumulation of ice particles aloft, which are believed to promote charge separation by differential particle motions. CAPE, in turn, is largely determined by the boundary-layer wet-bulb temperature. Modest differences (1°–3°C) in wet-bulb potential temperature between land and s...
322 citations
TL;DR: In this paper, the authors combined both TRMM precipitation radar (PR) and Lightning Imaging Sensor (LIS) data to examine "wet-season" vertical structures of tropical precipitation across a broad spectrum of locations in the global Tropics.
Abstract: Observation of the vertical profile of precipitation over the global Tropics is a key objective of the Tropical Rainfall Measuring Mission (TRMM) because this information is central to obtaining vertical profiles of latent heating. This study combines both TRMM precipitation radar (PR) and Lightning Imaging Sensor (LIS) data to examine “wet-season” vertical structures of tropical precipitation across a broad spectrum of locations in the global Tropics. TRMM-PR reflectivity data (2A25 algorithm) were utilized to produce seasonal mean three-dimensional relative frequency histograms and precipitation ice water contents over grid boxes of approximately 5°–10° in latitude and longitude. The reflectivity histograms and ice water contents were then combined with LIS lightning flash densities and 2A25 mean rainfall rates to examine regional relationships between precipitation vertical structure, precipitation processes, and lightning production. Analysis of the reflectivity vertical structure histograms ...
278 citations
TL;DR: In this article, the effective LIS thresholds, expressed as radiances emitted normal to cloud top, are 4.0 ± 0.7 and 7.6 ± 3.3 μJ sr−1 m−2 for night and local noon.
Abstract: Laboratory calibration and observed background radiance data are used to determine the effective sensitivities of the Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS), as functions of local hour and pixel location within the instrument arrays. The effective LIS thresholds, expressed as radiances emitted normal to cloud top, are 4.0 ± 0.7 and 7.6 ± 3.3 μJ sr−1 m−2 for night and local noon; the OTD thresholds are 11.7 ± 2.2 and 16.8 ± 4.6 μJ sr−1 m−2. LIS and OTD minimum signal-to-noise ratios occur from 0800 to 1600 local time, and attain values of 10 ± 2 and 20 ± 3, respectively. False alarm rate due to instrument noise yields ∼5 false triggers per month for LIS, and is negligible for OTD. Flash detection efficiency, based on prior optical pulse sensor measurements, is predicted to be 93 ± 4% and 73 ± 11% for LIS night and noon; 56 ± 7% and 44 ± 9% for OTD night and noon, corresponding to a 12%–20% diurnal variability and LIS:OTD ratio of 1.7. Use of the weighted daily mean det...
272 citations