Richard E. Orville

Cloud-to-Ground Lightning in the United States: NLDN Results in the First Decade, 1989-98

Abstract The physical and geographical characteristics of over 216 million cloud-to-ground lightning flashes recorded during the first decade (1989–98) of operation of the National Lightning Detection Network (NLDN) covering the entire continental United States are presented. These characteristics include the total cloud-to-ground flash density, the positive flash density, the percentage of positive flashes, the first stroke negative and positive peak currents, and the multiplicity for negative and positive flashes. All analyses were done with a spatial resolution of 0.2° corresponding to an approximate resolution of 20 km. Flash densities were not corrected for detection efficiency; the measured values are presented. The maximum measured flash density is found to exceed 9 flashes km−2 across Florida in the Tampa–Orlando–Cape Canaveral corridor, near Fort Myers, and between Lake Okeechobee and the Atlantic Ocean. The mean monthly flash count peaks in July at approximately 5.5 million flashes. Positive fla...

Surface and Lightning Sources of Nitrogen Oxides over the United States: Magnitudes, Chemical Evolution, and Outflow

[1] We use observations from two aircraft during the ICARTT campaign over the eastern United States and North Atlantic during summer 2004, interpreted with a global 3-D model of tropospheric chemistry (GEOS-Chem) to test current understanding of regional sources, chemical evolution, and export of NOx. The boundary layer NOx data provide top-down verification of a 50% decrease in power plant and industry NOx emissions over the eastern United States between 1999 and 2004. Observed NOx concentrations at 8–12 km altitude were 0.55 ± 0.36 ppbv, much larger than in previous U.S. aircraft campaigns (ELCHEM, SUCCESS, SONEX) though consistent with data from the NOXAR program aboard commercial aircraft. We show that regional lightning is the dominant source of this upper tropospheric NOx and increases upper tropospheric ozone by 10 ppbv. Simulating ICARTT upper tropospheric NOx observations with GEOS-Chem requires a factor of 4 increase in modeled NOx yield per flash (to 500 mol/ flash). Observed OH concentrations were a factor of 2 lower than can be explained from current photochemical models, for reasons that are unclear. A NOy-CO correlation analysis of the fraction f of North American NOx emissions vented to the free troposphere as NOy (sum of NOx and its oxidation products) shows observed f = 16 ± 10% and modeled f = 14 ± 9%, consistent with previous studies. Export to the lower free troposphere is mostly HNO3 but at higher altitudes is mostly PAN. The model successfully simulates NOy export efficiency and speciation, supporting previous model estimates of a large U.S. anthropogenic contribution to global tropospheric ozone through PAN export.

Enhancement of cloud-to-ground lightning over Houston, Texas

Cloud-to-ground lightning flash data have been analyzed for the twelve-year period 1989–2000, for a geographical area centered on Houston, Texas. Of the 1.6 million cloud-to-ground flashes in this area of study, approximately 752,000 flashes occurred in the summer months of June, July, and August, and 119,000 flashes in the months of December, January, and February. The highest flash densities, greater than 4 flashes km−2 in the summer and 0.7 flashes/km−2 in the winter, are near the urban areas of Houston. We suggest that the elevated flash densities could result from several factors, including, 1) the convergence due to the urban heat island effect, and 2) the increasing levels of air pollution from anthropogenic sources producing numerous small droplets and thereby suppressing mean droplet size. The latter effect would enable more cloud water to reach the mixed phase region where it is involved in the formation of precipitation and the separation of electric charge, leading to an enhancement of lightning.

Global Distribution of Midnight Lightning: September 1977 to August 1978

Abstract A detailed study is reported of the distribution of global midnight lightning from 60°S to 60°N for 365 consecutive days. The period covered is from 1 September 1977 to 31 August 1978. More than 32 000 lightning locations obtained from DMSP photographic data are plotted with an accuracy of 100 km. The locations are in good agreement with features of the general circulation. Land-ocean lightning ratios vary from 2.2 (September) to 4.2 (July) with a mean of 3.2. If these values are corrected for the earths land-ocean area ratio of (2.4)−1, the land-ocean lightning ratios range from 5.3 (September) to 10 (July) with a mean of 7.7.

Lightning Ground Flash Density in the Contiguous United States-1989

Abstract Cloud-to-ground lightning data for the years 1992–95 have been analyzed for geographical distribution of total flashes, positive flashes, and the percentage of flashes that lower positive charge to ground. In the contiguous United States the measured total cloud-to-ground lightning flash counts were 16.3 million (1992), 24.2 million (in both 1993 and 1994), and 22.3 million in 1995. The maximum flash densities occurred in Florida in 1992 (9–11 flashes per square kilometer) and in the Midwest in 1993 (11–13 flashes per square kilometer), coinciding with the storms and floods that dominated the summer of 1993 in the Midwest. In 1994, the area of maximum flash density was again in Florida (11–13 flashes per square kilometer). In 1995, the flash density maxima (9–11 km−2) were in southern Louisiana and near the Kentucky–Illinois border. Positive flash densities had maxima in the Midwest in all four years with values of 0.4 (1992), 1.0 (1993), 0.7 (1994), and 1.8 flashes per square kilometer (1995). T...

The North American Lightning Detection Network (NALDN)—First Results: 1998–2000

Abstract Cloud-to-ground lightning data have been analyzed for the years 1998–2000 for North America (Canada plus the contiguous United States) for all ground flashes, positive flashes, the percentage of positive lightning, peak currents for negative and positive lightning, and for negative and positive multiplicity. The authors examined a total of 88.7 million flashes divided among the three years: 31.1 million (1998), 29.5 million (1999), and 28.2 million (2000). Annual flash densities are derived from 245–424 km2 regions and are uncorrected for flash detection efficiency. The highest flash densities in Canada are along the U.S.–Canadian border (1–3 flashes km−2), and in the United States along the Gulf of Mexico coast and Florida (exceeding 9 flashes km−2). Maximum annual positive flash densities in Canada generally range primarily from 0.1 to 0.3 flashes km−2, and in the United States to over 0.7 flashes km−2 (areas in the Midwest, the Gulf Coast, and Florida). Areas of greater than 20% positive light...

Cloud-to-Ground Lightning Observations from TOGA COARE: Selected Results and Lightning Location Algorithms

Abstract Recently, observations of electrified oceanic convection and associated cloud-to-ground (CG) lightning were obtained over the tropical western Pacific Ocean during TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment). During COARE, observations of convection were made using a variety of instrument platforms including ship and airborne Doppler radars, an advanced lightning direction finder (ALDF) network, and a shipborne inverted electric field mill. This study focuses on data collected by the COARE ALDF network, fusion of those data with observations, and the methods used to calculate accurate CG return stroke locations. Analysis of CG lightning data and Doppler radar data indicates that lightning-producing oceanic convection is characterized by deep, vertically developed convective cells with radar reflectivities exceeding 30-dBZ above the height of the −10°C level. In several cases a peak in CG frequency occurred coincident with the descent of precipitation...

Cloud-to-ground lightning flash characteristics in the contiguous United States : 1989-1991

Wideband magnetic direction finders have been used to obtain a cloud-to-ground lightning flash count for the contiguous United States, an area of 7.7×106 km2, for the period 1989 through 1991. Over 46 million flashes to ground were recorded and are divided among the three years, 13.4 million in 1989, 15.9 million in 1990, and 16.9 million in 1991. Maximum flash densities occur in Florida and increase each year, from 9 flashes km−2 (1989) to 13 flashes km−2 (1991). The database contains 1.7 million positive flashes divided among the three years, 0.4 million in 1989, 0.6 million in 1990, and 0.7 million in 1991. In 1990 and 1991 the positive flash density maximum occurred in Florida but elsewhere in 1989. Secondary maximum positive flash densities occur throughout the Midwest. The annual mean percentage of positive flashes in the total lightning count is 3.7% for the period 1989–1991. The interannual variation is small, ranging from 3.1% (1989) to 4.0% (1991). However, in any given year the geographical variation of the percentage of positive flashes is large. The percentage of positive flashes is 2% (1989) at the latitude of Florida but near 25% and higher at the latitudes of the upper Midwest, Maine, and Oregon.

Global Lightning Flash Frequency

Abstract Lightning flashes recorded in photographs by two satellites in the Defense Meteorological Satellite Program (DMSP) are used to calculate flash frequencies for land and ocean regions in 10° latitude belts between 60°S and 60°N for dusk and midnight local times. Data are analyzed for the first week of each month between March 1974 and February 1975. We find that the annual land-ocean ratio of global lightning at dusk ranges from 8 to approximately 20, depending on whether lightning near the coast is judged to be from thunderstorms produced predominantly by ocean effects or land effects. A similar analysis for data obtained near midnight yields ratios which range from 4 to 8. The global land-ocean lightning ratio is significantly higher during the northern summer than during the southern summer. The dusk lightning flash frequency as a function of latitude peaks at 10–20°N during the northern summer and 0–10°N during the southern summer. The midnight flash frequency peaks at 0–10°N throughout the yea...

Global lightning: Total, cloud and ground flash estimates

A worldwide survey of the ratio of cloud flashes to ground flashes in thunderstorms using CGR3 (Cloud-Ground Ratio #3) instruments at sites covering a latitude range from 59.9°N to 27.3°S between 1986 and 1991 inclusive has provided information on the latitudinal variation of total flash density and the ratio of cloud flashes to total flashes. Two surveys of global lightning occurrence using satellites, the Defense Meteorological Satellite Program (DMSP) survey using optical sensors to detect lightning at near local midnight and the Ionospheric Sounding Satellite-b (ISS-b) survey using radio-frequency lightning detection equipment, were used in conjunction with the CGR3 observations to obtain the latitudinal variation of total flash density over each major land mass and each major ocean. These three surveys and other sources provided data on the annual and diurnal variation in lightning occurrence at various latitudes. We have combined the above information in a computational model of global lightning occurrence and have separated total flashes into cloud flash and ground flash components. Using this model, we estimate the global annual number of total flashes to be 2.05×109 yr−1, corresponding to a mean rate of 65 s−1. This estimate is probably low because of inadequate detection of weak cloud flashes. Depending on the assumption made concerning the latitudinal variation of the ratio of cloud flashes to total flashes, the mean rates of occurrence are between about 51 s−1 and 55 s−1 for cloud flashes and between about 10 s−1 and 14 s−1 for ground flashes. The uncertainty in these estimates is a factor of 2 in either direction. The calculated global diurnal variation has a peak rate of occurrence of about 1.6 times the mean rate at about 1400 universal time (UT) in the northern hemisphere spring and has its lowest rate of about 0.5 times the mean rate at about 0300 UT in the northern hemisphere summer. About 54% of all lightning occurs in the northern hemisphere. The mean global land total flash density is about 8.3 km−2 yr−1. This is about 3.4 times the mean global total flash density over oceans.