摘要:

Average energy spectral densities are presented for the fast transitions in most of the components that produce large radiation field impulses from cloud‐to‐ground lightning: first and subsequent return strokes; stepped, dart‐stepped, and “chaotic” leaders; and “characteristic” cloud pulses. A disagreement in the previous literature about the spectral energy radiated by return strokes at high frequencies is noted and explained. We show that our spectral amplitudes are not seriously distorted by propagation over less than 35 km of seawater, although as much as 45 km of such propagation does appear to produce significant attenuation above about 10 MHz. First and subsequent return strokes produce identical spectra between 0.2 and 20 MHz. The spectra of stepped and dart‐stepped leader steps are nearly identical and are very similar to that of characteristic pulses. The spectra of leader steps also match return stroke spectra above 2–3 MHz after the former are increased by about 7 dB. The shapes of individual spectra do not depend on their amplitude, so the shapes of the average spectra are probably not distorted by the trigger thresholds used in the data acquisition. Return strokes are the strongest sources of radiation from cloud‐to‐ground lightning in the 0.2‐ to 20‐MHz frequency range, although certain intracloud processes are stronger ra

ISSN:0148-0227    

DOI:10.1029/JD095iD12p20367

年代:1990

数据来源: WILEY

摘要:

We show that the lightning return stroke model by Master, Uman, Lin, and Standler (MULS), a modification of the Lin, Uman, Standler (LUS) model, can be viewed mathematically as having a specified channel‐base current, although in the original formulation it was the channel current sources that were specified. The specification of the MULS channel‐base current is independent of the time dependence of the corona current sources which characterize the MULS model. Further, we show that, when starting from the same channel‐base current, the MULS return stroke model, except for the uniform current component, is mathematically equivalent to the Modified Transmission Line (MTL) model. For both the MULS and the MTL models we present a single analytical expression relating the far fields and the channel‐base current. This formulation provides for a simpler procedure to infer MULS channel‐base currents from remote electric and magnetic field measurements than was previously available and hence to test experimentally the M

ISSN:0148-0227    

DOI:10.1029/JD095iD12p20389

年代:1990

数据来源: WILEY

摘要:

We compare five lightning return stroke current models that exhibit a simple relationship between the current at the return stroke channel‐base and the current along the return stroke channel, namely the Bruce‐Golde (BG) model, the transmission line (TL) model, the Master, Uman, Lin, and Standler (MULS) model, the Traveling Current Source (TCS) model, and the Modified Transmission Line (MTL) model, by assuming a common current wave shape at the channel base and then calculating the channel currents and charges and the resultant electric and magnetic fields. There are basically two characteristics that distinguish the models, namely, (1) the treatment of the return stroke wave front and (2) the spatial and temporal distribution of charge removed from the leader channel. The simpler BG model can be used as an excellent approximation to the TCS model. The latter reduces to the former when the current injected downward by the traveling current source has an infinite speed. The MULS model is equivalent to the MTL model when the MULS uniform current is assumed to be zero. The BG and TCS models produce sharper initial field peaks than do the TL, MTL, and MULS models. The ratio of the peak field derivative to the peak current derivative is near the ratio of the peak field to the peak current for the MULS and MTL models and is equal for the TL model, whereas for the BG and TCS models the ratio of the peak derivatives is about twice the ratio of the peak field to peak current. The TL model is unrealistic for long‐time field calculations due to the fact that no net charge is removed from the channel. The other four models produce overall fields which are reasonable approximations to measured fields from natural lightning even though, for the assumed channel‐base current, the BG and TCS models do not reproduce the observed distant‐field zero crossing and the MTL and MULS models do not reproduce the magnetic “hump” observed after the initial field peak at close range. None of the models can reproduce the fine structure observed in the me

ISSN:0148-0227    

DOI:10.1029/JD095iD12p20395

年代:1990

数据来源: WILEY

摘要:

Highly time resolved streak recordings from nine subsequent strokes in four triggered flashes have been examined for evidence of the occurrence of upward connecting discharges. These photographic recordings were obtained with superior spatial and temporal resolution (0.3 m and 0.5 μs) and were examined with a video image analysis system to help delineate the separate leader and return stroke image tracks. Unfortunately, a definitive determination of the occurrence of connecting discharges in these strokes could not be made. The data did allow various determinations of an upper bound length for any possible connecting discharge in each stroke. Under the simplest analysis approach possible, an “absolute” upper bound set of lengths was measured that ranged from 12 to 27 m with a mean of 19 m; two other more involved analyses yielded arguably better upper bound estimates of 8–18 m and 7–26 m with means of 12 and 13 m, respectively. An additional set of low time‐resolution telephoto recordings of the lowest few meters of channel revealed six strokes in these flashes with one or more upward unconnected channels originating from the lightning rod tip. The maximum length of unconnected channel seen in each of these strokes ranged from 0.2 to 1.6 m with a mean of 0.7 m. This latter set of observations is interpreted as indirect evidence that connecting discharges did occur in these strokes and that the lower bound for their length is

ISSN:0148-0227    

DOI:10.1029/JD095iD12p20409

年代:1990

数据来源: WILEY

摘要:

The paper theoretically explains some of the microphysical phenomena observed in Mei‐Yu. In the melting layer the theory shows that most of the changes from the top to the bottom of the melting layer in the particle number density and the particle size distribution slope can be explained by assuming a constant precipitation rate and increasing particle fall speed due to melting. However, the theory implies that some aggregation is going on during melting. In the warm‐rain region, binary interaction of coalescence and breakup are the prominent microphysical processes. In general, coalescence dominates over breakup. But in these cases the two processes will approximately arrive at equilibrium when the precipitation reaches the Earth's surf

ISSN:0148-0227    

DOI:10.1029/JD095iD12p20417

年代:1990

数据来源: WILEY

摘要:

The microphysical processes in both the melting layer and the warm‐rain region of an oceanic mesoscale convective system observed during the Taiwan Area Mesoscale Experiment (TAMEX) are studied using airborne observations. Two flight cases are used, one flown in the transitional region and the other in the stratiform region. In the melting layer the particle number density decreases by an order of magnitude from the top to the bottom of the layer. This is caused by the sharp increase of particle fall speed as snowflakes melt into rain. The ice water content is higher in the transitional region, thus cooling the environment more efficiently during melting. In the warm‐rain region the microphysical process is dominated by coalescence process. Large raindrops are generated and the corresponding median diameter of all raindrops increases by a factor of 2 to 3 from the bottom of the melting layer to the ground level. The breakup process is more efficient in the transitional reg

ISSN:0148-0227    

DOI:10.1029/JD095iD12p20427

年代:1990

数据来源: WILEY

摘要:

This paper presents a parameterization of the joint distribution of size and infrared optical thickness in cumulus cloud. The model involves only two parameters, each of which has a clear physical interpretation, it is shown how knowledge of the size distribution and the mean infrared radiance transmitted by a cloud field can be used to deduce the thickness distribution of the field. One implication of this result is that the conjunction of Advanced Very High Resolution Radiometer data with size statistics, gathered for example in real time by processing high resolution images on board a satellite, can be used to predict the transfer of infrared flux by broken cloud.

ISSN:0148-0227    

DOI:10.1029/JD095iD12p20441

年代:1990

数据来源: WILEY

摘要:

Radiosondes ascending near to mountainous areas in the United Kingdom occasionally encounter very large‐amplitude gravity waves in the stratosphere. These ascents are characterized by large perturbations in the temperature (∼5 K) and vertical velocity (2–10 m s−1) on vertical scales of about 2 km. Some of the profiles suggest a large vertical flux of horizontal momentum (order 1 Nm−2). Examples of these waves are illustrated, and the situations in which they are observed are described. Simple linear wave theory has been used to diagnose many of the important wave p

ISSN:0148-0227    

DOI:10.1029/JD095iD12p20451

年代:1990

数据来源: WILEY

摘要:

We describe a data analysis technique for determining gravity wave intrinsic parameters including wave propagation direction. The technique involves measuring the altitude variations of the wave‐induced density perturbations of the atmospheric Na layer. This technique can be used with airborne lidars, multiple ground‐based lidars, and steerable lidars. In this paper the technique is applied to airborne Na lidar data obtained during a roundtrip flight from Denver, Colorado, to the Pacific Coast in November 1986. During the flight, strong wave perturbations were observed in the Na layer near the Pacific Coast over a horizontal distance of nearly 700 km. The intrinsic horizontal wavelength of this wave was estimated to be about 85 km, and the vertical wavelength was 4.1 km. The intrinsic period was about 102 min, and the propagation direction was almost due so

ISSN:0148-0227    

DOI:10.1029/JD095iD12p20457

年代:1990

数据来源: WILEY

摘要:

This study concerns analysis of climatic variability in general circulation models, first to determine how faithfully the models reproduce measures of the present variability, and then to examine how variability changes in CO2perturbed cases. The control run output of three different versions of the NCAR Community Climate Model is examined for selected regions of the United States including the Great Plains, the Southeast, the Great Lakes region, and the upper West Coast. Mainly temperature and precipitation are analyzed, but some preliminary analysis of relative humidity is also included. Inter annual variability is analyzed in one version (Chervin) for only the present day climate. Results indicate that this version underestimates the interannual variability of temperature, but generally successfully reproduces the relative variability of precipitation. Two versions of the CCM tend to overestimate daily temperature variability while the third (Dickinson) accurately reproduces or underestimates it. Reasons for these differences are discussed. Daily precipitation variability is overestimated by two model versions (Chervin and Dickinson) but fairly well reproduced by the Washington version. The CO2perturbed run of one version (Washington) produces mixed results for changes in daily temperature variability and tends to produce increased daily variability for precipitation.

ISSN:0148-0227    

DOI:10.1029/JD095iD12p20469

年代:1990

数据来源: WILEY