摘要:

An improved return stroke model that is both physically oriented and has a relatively straightforward mathematical basis is proposed. The current at the channel base is specified, and a time‐dependent discharging of the charge stored on the leader channel determines the channel current as a function of height and time. The discharging process is separated into (1) the exponential discharge of the leader head and leader core with a relatively short time constant, less than 1 μs, which we call the “breakdown” time constant, and (2) the exponential discharge of the charge stored around the leader core with a longer time constant, of the order of microseconds. If a typical measured channel‐base current is assumed and if the discharge time constants are properly chosen, electric and magnetic field wave shapes calculated with the model exhibit all the significant characteristics of measured fields. From a comparison of calculated and measured field wave shapes, we find a ratio of the breakdown time constant to the channel‐base current rise time between 1 and 5. Comparison of typical characteristics of field wave shapes from natural and from artificially initiated (triggered) lightning indicates a faster discharging process for triggered lightning. Depending on the breakdown time constant, the return stroke speed determined using the well‐known formula for the transmission‐line model, with inputs being the peak electric field and peak current from the present model, are in the range from about 50 percent to 90 percent of the return stroke speed assumed in the present model. The corresponding transmission‐line model speeds determined from the peak derivatives of the electric field and current are in the range from about 140 percent to 160 percent of the assumed return stroke speed. These results supply some indication of why transmission‐line model speeds determined from the ratio of measured peak current and field derivatives in triggered lightning are greater than the speeds determined from the ratio of measured peak currents and fields. For a given channel‐base current, the initial peak electric field and field derivative derived from the model increases as the height above ground of the strike point increases. The new model can therefore explain the differences in the data obtained from the triggered lightning studies at Kennedy Space Center in 1985 and in 1987 as being due to the different height of the triggering structures in those two years. If natural lightning strikes an elevated object, the increase of the initial electric field and field derivative can result in an additional substantial error in determining the peak current and peak current derivative from the tra

ISSN:0148-0227    

DOI:10.1029/JD095iD09p13621

年代:1990

数据来源: WILEY

摘要:

A study of cosmic rays and thunderstorm frequency has shown a decrease in thunderstorms at the time of high cosmic rays and an increase in thunderstorms 2–4 days later. This was done by superposed epoch analysis of thunderstorms over the eastern two thirds of the United States for 1957–1976. When data for spring and fall months were used, the minimum deepened. When high cosmic rays near full and new moon for these months were key days, the minimum deepened again and was significant at less than the 0.01% level. It is believed that when the Sun, Earth, and Moon are aligned, particulate matter in the lower stratosphere is modulated and acted upon by cosmic rays, bringing about an immediate decrease in thundersto

ISSN:0148-0227    

DOI:10.1029/JD095iD09p13645

年代:1990

数据来源: WILEY

摘要:

The climatology of long‐range atmospheric transport to the Mauna Loa Observatory (MLO) is examined through an analysis of 10‐day isobaric back trajectories for the period 1981–1988, computed using the National Meteorological Center's analyzed gridded winds. The trajectories are classified into distinct transport patterns through the use of cluster analysis, a procedure that avoids the somewhat arbitrary classification by compass sector and instead searches for meaningful groupings within the data themselves. The clustering procedure is performed separately for yearly and monthly sets of 500‐ and 700‐hPa trajectories to examine both year‐to‐year and month‐to‐month variability in flow characteristics. The dominant transport features are quite similar from year to year, and include frequent summertime easterly flow associated with the trade winds and strong, wintertime westerly flow close to the mean axis of the subtropical jet stream. Veering westerly patterns that approach MLO from the north and northeast dominate during winter/summer transition periods. Other noteworthy features include recurring air mass transport from Saharan and Asian desert regions, infrequent interhemispheric flow from as far as 10°S, and occasional flow from high northern latitudes. Ten‐day back trajectories that pass over anthropogenic pollution source regions in western No

ISSN:0148-0227    

DOI:10.1029/JD095iD09p13651

年代:1990

数据来源: WILEY

摘要:

As part of the Global Tropospheric Experiment/Amazon Boundary Layer Experiment 2A, micrometeorological measurements were made using a 45 m scaffolding tower located in the Ducke Reserve Forest site (2°57′S, 59°57′W) 26 km northeast of Manaus, Amazonas, Brazil. We report the daily variation within and above the forest of several meteorological parameters such as temperature, humidity, winds, radiation exchange, and energy partition into latent and sensible heat fluxes. Estimates of eddy diffusivity coefficients for momentum, heat, and water vapor were made using the observed profiles. These results provide information on the physical processes involved in the exchange of momentum, heat, and water vapor between the forest and the air layer above. To account for the anomalies in eddy diffusivities, a brief discussion is presented based on turbulent transport proc

ISSN:0148-0227    

DOI:10.1029/JD095iD09p13669

年代:1990

数据来源: WILEY

摘要:

Single‐star scintillation spatiotemporal cross‐correlation functions enable estimates of altitude, CN2and horizontal wind velocity of each atmospheric turbulent layer between 2 and 20 km to be made. Furthermore, for the first time with such a technique, the standard deviations σvof the horizontal wind fluctuations are also estimated. The experimental results are compared with those obtained by other methods and good agreement is found. The σvestimations are interpreted in terms of the horizontal wind velocity spectrum and the measurements extend into the buoyancy range. These results encourage the belief that the use of a faster correlator will enable us to experimentally sample the buoyancy range for length scales ranging from 100 m to several tens of thousands of m

ISSN:0148-0227    

DOI:10.1029/JD095iD09p13683

年代:1990

数据来源: WILEY

摘要:

A two‐dimensional (latitude‐height) tropospheric tracer model has been used to simulate the global distributions of CO2,85Kr,222Rn, and SO2. A nondiffusive parameterization has been developed describing subgrid scale vertical transport, including transport in convective clouds and frontal zones of midlatitude cyclones. The parameters governing this transport have been estimated using the seasonal variation of CO2. For long‐lived tracers such as CO2and85Kr the parameterization yields results that are similar to those obtained with the subgrid scale vertical transport described as an eddy diffusion process only. For short‐lived tracers, with lifetimes of a few days to weeks, the nondiffusive parameterization results in vertical concentration profiles that are much steeper and probably more realistic than those obtained with a diffusive parameter

ISSN:0148-0227    

DOI:10.1029/JD095iD09p13691

年代:1990

数据来源: WILEY

摘要:

From May 13, through August 21, 1986, and again from April 3, through May 28, 1987, a dust devil census was conducted at the Permanent High Explosive Test Site on White Sands Missile Range, New Mexico. In a 260‐km2observation area, a total of 3134 dust devils were counted on 97 days (out of 112 observing days). Within the observation area, the dust devils were concentrated in a relatively small area and showed strong correlation with roadways and other cleared areas. However, from the summer of 1986 to the spring of 1987, the center of activity shifted from a relatively undisturbed region to an area whose surface had been recently modified. This shift is attributed to changes in soil moisture and surface thermal properties. It is suggested that the co‐location of the center of activity in 1987 with the modified surface is an example of inadvertent weather modification on a very local scale. The dust devils had a daily distribution very similar to that found by Sinclair (1969) in comparable terrain in the vicinity of Tucson, Arizona. The time of the first observed dust devil was very regular on days with dry surface conditions at sunrise and clear skies through the morning. The time of the first dust devil was delayed and the degree of activity reduced if precipitation had occurred on a previous day in the observation area. The production of dust devils often ended abruptly with the onset of an afternoon thundershower in the observation a

ISSN:0148-0227    

DOI:10.1029/JD095iD09p13707

年代:1990

数据来源: WILEY

摘要:

We present kinetic energy horizontal and vertical wave number spectra of wind and density perturbations at mid‐latitudes for the altitude range from approximately 80 to 100 km. The spectra are inferred from Na density profiles collected with an airborne lidar during two flights conducted in November 1986. The two flights included one round trip from Denver, Colorado, to Springfield, Illinois, and another round trip from Denver to the Pacific Coast. The horizontal wave number spectra exhibited an approximatelykh−1.2dependence at horizontal scales from 70 to 700 km, while the vertical wave number spectra exhibited an approximatelykz−2.7dependence at vertical scales from 2 to 10 km. These spectra are compared with spectra obtained from ground‐based lidar, radar, shuttle re‐entry, and Global Atmospheric Sampling Program observations. The rms horizontal wind velocity inferred from the airborne Na lidar data increased with time and with longitude from the Pacific Coast to the Great Plains. The inferred mean rms horizontal wind velocity was 32 m s−1, and the mean horizontal velocity variance was about 1100 m2s−2. This horizontal velocity variance is comparable to horizontal velocity variances inferred from radar observations, but larger than horizontal velocity variances inferred from shuttle re‐entry observations. During the round trip flight from Denver, Colorado, to Springfield, Illinois, two quasi‐monochromatic waves were observed. The longer wavelength wave had an intrinsic zonal wavelength of 772 km, intrinsic zonal phase velocity of 35 m s−1westward, and intrinsic period of 6.1 hours. This wave appears to be propagating almost due west. The shorter wavelength wave had an intrinsic zonal wavelength of 263 km, zonal phase velocity of 43 m s−1westward, an

ISSN:0148-0227    

DOI:10.1029/JD095iD09p13723

年代:1990

数据来源: WILEY

摘要:

In November 1986, joint radar and Na lidar observations were conducted in Colorado to study mesopause dynamics. The radar observations were conducted at Platteville, Colorado with a Stratosphere Troposphere (ST) radar in a meteor echo detection mode. The Na lidar observations included ground‐based observations at Broomfield and Denver, Colorado, and airbome observations during three flights over the Great Plains, Rocky Mountains, and Pacific Coast. Both radar and lidar identified a 6‐hour wave that was remarkably coherent during the November campaign. The vertical wavelength of the 6‐hour wave estimated from the radar data was 8 to 9 km, the amplitude of the zonal wind component was 10 m S−1, and the amplitude of the meridional wind component was 4 m s−1. The vertical wavelength estimated from the ground‐based and airborne lidar data was 7.3 km, and the amplitude of the scalar horizontal wind ranged from 13 to 16 m s−1. The zonal wavelength of this wave was approximately 772 km, and the intrinsic zonal phase velocity was 35 m s−1westward. The 6‐hour wave was dominant in the height range of 80–90 km, which corresponds to the bottomside of the Na layer. An additional 2‐hour wave was observed with the Na lidar during both the ground‐based and airborne observations. The vertical wavelength of this wave was about 12 km, and the zonal wavelength was 260 km. The intrinsic zonal phase velocity was 43 m s−1westward. This 2‐hour wave was also present only on the

ISSN:0148-0227    

DOI:10.1029/JD095iD09p13737

年代:1990

数据来源: WILEY

摘要:

The transport of trace species by waves is affected by their production and/or destruction by photochemical processes. For this reason the transport or “diffusion” matrices used in two‐dimensional models should be different for each constituent. This paper describes a method for altering the meridional and vertical dynamical transport coefficients for use with photochemically active species. The transport represents the net meridional eddy flux by planetary waves and the net vertical eddy flux by gravity waves. Output from integrations using the control (species independent) and chemical eddy transport coefficients are compared. The change in the meridional (planetary wave) transport results in modest, but nevertheless significant, changes in the distribution of trace species. In many cases, for example odd oxygen and nitric acid, the model changes bring the distributions closer to those observed. In contrast, the incorporation of the chemical eddy effect into the gravity wave transport has almost no effect on any of the model f

ISSN:0148-0227    

DOI:10.1029/JD095iD09p13749

年代:1990

数据来源: WILEY