摘要:

A coordinated analysis of the Thunderstorm Research International Project storm of July 11, 1978, from 1900 to 2000 UT at the Kennedy Space Center is presented using data from three Doppler radars, a lightning detection and ranging system and a network of 25 electric field mills, and rain gages. This storm produced two cells for which the center of the updraft remained within range of the observational network. Electric field measurements were used to analyze the charge moments transferred by lightning flashes. An attempt was made to analyze as large a percentage as possible of the flashes so that the measurements would be usable to study the charge moment transferred by lightning in the storm. These data were fitted to Weibull distributions which were used to estimate statistical parameters of the lightning for both intracloud and cloud‐to‐ground flashes and to estimate the fraction of the flashes which were below the observation threshold for the two cells studied. The displacement and conduction current densities were calculated throughout the storm from electric field measurements between flashes, and data are presented of values at 5‐min intervals throughout the storm. These values were used to derive the magnitudes and locations of dipole and monopole generators by least squares fitting the measured Maxwell current densities to the displacement‐dominated equations. Constrained fitting was used to examine the uniqueness of the so

ISSN:0148-0227    

DOI:10.1029/JD095iD05p05417

年代:1990

数据来源: WILEY

摘要:

In Part 1 of this paper, the data obtained at the time of the Thunderstorm Research International Project storm at the Kennedy Space Center on July 11, 1978, are discussed and analyzed in a model‐independent manner. Here the parameters of the electrical system that would be consistent with these observations are discussed. Three‐dimensional electrodynamic modeling of the thundercloud electrification allowed estimates to be made of the current moments and electrical power generated continuously throughout the evolution of the two cells of the storm that were studied. The evolution and configuration of the currents were consistent with the separation of an originally neutral ensemble of particles by gravity in the region of 7 km in the region close to the maximum of the updraft velocity. After about 370 s the effect of wind shears would have caused the particles to separate in the convective system of the cells. Rain did not appear to be the dominant charge carrier. The current moments generated were compared with the current moments transferred by intercloud and cloud‐to‐ground lightning. It is shown that for the southern cell, which produced a charge moment of about 8.4 (MC m), lightning utilized about 84% of the charge moment separated, while for the northern cell, which produced about 1.1 (MC m), this figure was approximately 60%. It was shown that the times of initiation and maximum electrical power generated corresponded best with the normalized mass above 7.5 km. It was deduced that the median diameter heavier particles had a fall velocity of about 3 m s−1. The generator currents, flash rates, cloud conductivities, and mean charge per flash were used to estimate the volume associated with the lower region of current d

ISSN:0148-0227    

DOI:10.1029/JD095iD05p05435

年代:1990

数据来源: WILEY

摘要:

The return stroke initial electric field peak normalized to 100 km, the probability of creating a new termination on ground, and the probability of occurrence of a dart‐stepped leader after an interstroke interval of 100 ms or less have been determined as a function of stroke order from simultaneous electric field and TV records of 76 negative cloud‐to‐ground flashes near Tampa, Florida. Fifty percent of the flashes showed multiple terminations on the ground as inferred from TV records (from 29 to 69 percent for separate thunderstorm days). For all flashes, the minimum initial electric field peak normalized to 100 km for first strokes was found to be 2.2 V/m and for subsequent strokes 0.25 V/m. The geometric mean initial electric field peak for 155 second, third and fourth strokes taken together was found to be 1.4 times larger than for 115 strokes of order 5 through 18, with the difference being statistically significant at the 0.005 significance level. The geometric mean initial electric field peak for 63 first strokes in multiple‐stroke flashes was found to be 1.3 times larger than for 13 strokes in single‐stroke flashes. The charge lowered by strokes in New Mexico was observed to exhibit behavior similar to that of the initial electric field peak in Florida described above. Subsequent strokes creating a new termination on ground showed a higher initial electric field peak than strokes following the same channel as the preceding stroke, but lower than for first strokes in a flash. Second strokes showed the maximum probability of both creating a new termination on ground (0.37) and producing a dart‐stepped leader along the channel of the previous stroke after an interstroke interval of 100 ms or less (0.36). None of the 115 strokes of order 5 and higher created a new termination on ground in spite of the fact that previous interstroke intervals for 22 of them were from 101 to 253 ms and did not contain long continuing current. The results presented and other relevant data available in the literature are consistent with the proposed hypothesis that successive strokes provide cumulative conditioning of the lightn

ISSN:0148-0227    

DOI:10.1029/JD095iD05p05447

年代:1990

数据来源: WILEY

摘要:

Statistics on the occurrence of long continuing current as a function of the number of strokes per flash, the stroke order, and the duration of the interstroke interval preceding the continuing current have been derived from electric field and other records of 346 flashes from five separate studies in Florida and two separate studies in New Mexico. Electric field records of 76 flashes were analyzed for the duration of the leaders preceding strokes initiating long continuing current and for the initial electric field peak of both strokes initiating long continuing current and strokes preceding those initiating long continuing current. Only 6 percent of single‐stroke flashes contained long continuing current, while 49 percent of multiple‐stroke flashes did; 1.4 percent of first strokes (in single and multiple‐stroke flashes) were followed by long continuing current while this percentage varied between 3 and 15 percent for subsequent strokes of order 2 through 26. The percentage of five‐stroke flashes terminated by long continuing current is about three times lower than the percentage of two‐, three‐, four‐, six‐, seven‐, or eight‐stroke flashes. Further, the total number of five‐stroke flashes is about two to three times lower than the number of two‐, three‐, four‐, and six‐stroke flashes. The continuing current occurrence statistics for Florida and for New Mexico were, in general, very similar. Leaders that were followed by long continuing current and those that were not showed no appreciable difference in duration. The geometric mean of the initial electric field peak for strokes preceding strokes initiating long continuing current was 1.5 times larger than for regular strokes; and a similar tendency was found for the charge lowered by strokes. The data presented are interpreted physically to indicate (1) that the lightning channel to ground is cumulatively conditioned by sequential strokes, (2) that the positive charge introduced into the upper portions of the channel to ground by large return strokes can influence some salient flash characteristics, and (3) that flashes draw their charge from potentially interconnected regions each of which can support, on average, 4 strokes. The probability of finding a long continuing current after a relatively short interstroke interval, in the range from 8 to 32 ms, was about 0.35, about 3 times higher than for all intervals. The probability of finding a stroke initiating a long continuing current following a stroke with a relatively large initial electric field peak, in the range from 8 to 15 V/m (normalized to 100 km), was about 0.32, four times larger than for all strokes. When a stroke with a relatively large initial electric field peak of 5 V/m or higher was followed by a relatively short interstroke interval of 8 to 32 ms, the next stroke initiated a long continuing current in more than half the cases. The statistics on long continuing current occurrence as a function of return stroke initial electric field peak and interstroke interval duration may be applicable for detecting the occurrence of long continuing current using presently‐operating long

ISSN:0148-0227    

DOI:10.1029/JD095iD05p05455

年代:1990

数据来源: WILEY

摘要:

An analysis of electromagnetic waveform records and video images of a multistroke cloud‐to‐ground (CG) strike to the NASA F‐106B instrumented airplane is presented. The CG flash started as a lightning strike triggered by the airplane and later produced multiple return strokes to the ground (the ground strike network registered six return strokes). Although there are some uncertainties in data interpretation resulting from lack of independent measurements with other than the airborne instruments, recoil streamers and eight sequences of dart leaders followed by return strokes (two more than were indicated by the ground network) were identified in the airborne data. At least three of the subsequent return strokes were attached to the airplane. The analysis provides evidence that formation of recoil streamers and dart leaders is accompanied by a surge in continuous current. This feature is similar to that observed in the bidirectional leader development during lightning initiation on the air

ISSN:0148-0227    

DOI:10.1029/JD095iD05p05471

年代:1990

数据来源: WILEY

摘要:

Following the eruption of the Mount St. Helens volcano on May 18, 1980, atmospheric waves were recorded by a network of microbarographs located over 7000 km from the source. Analysis of these data requires the use of complex processing techniques based on a high‐resolution method to extract the signals produced by the St. Helens source from spurious waves or noise in each record. This facilitates interpretation of the wave trains in terms of propagation modes. It is thus shown that Lamb modeL0is present in the low‐frequency part of all the signals, whereas acoustic modes (more probablyA′2) are needed to explain all the properties of the high‐frequency part, which is clearly observed for a westward and a southward prop

ISSN:0148-0227    

DOI:10.1029/JD095iD05p05485

年代:1990

数据来源: WILEY

摘要:

The surface wind field inland of the intense coastal katabatic wind regime at Terra Nova Bay, Antarctica, has been studied both observationally and numerically. Airborne surveys of wind‐induced features on the snow surface have been used to construct the time‐averaged winter surface airflow pattern. The surface motion field has also been simulated by a mesoscale primitive equation model using terrain slopes with a horizontal resolution of 32 km. Both methods of analysis demonstrate that the intense katabatic airstream at Terra Nova Bay is forced by converging air currents in the continental interior. The broadscale confluence zone becomes organized into two regions within about 180 km of the coast. The primary route for katabatic mass transport into the Terra Nova Bay area is Reeves Glacier valley, but an important secondary source is provided by airflow down David Glacier. The former is generated by the Coriolis‐induced concentration of mass transport on the left side (looking downwind) of the broadscale confluence zone as well as by the near‐coastal mountain deflection of airflow into the valley head. The confluence zone feeding into David Glacier valley stretches over 100 km into the interior and is forced by the broadscale terrain configuration of the ice sheet. Source areas for the two airstreams diner with Reeves Glacier valley being fed by cold air formed well into the interior, whereas the airflow down David Glacier is sustained by radiative cooling over lower parts of the ice sheet which are much closer to the Ross Sea. Evaluation of the observational and modeling results reveals both the strengths and weaknesses of each approach. Airborne surveys of sastrugi orientations are a highly successful method for establishing the detailed pattern of surface airflow. However, a systematic examination of sastrugi dimensions suggests that such work could also be carried out using SPOT‐type satellite observations, which is a more cost‐effective approach than aircraft surveying. The double‐jet structure of katabatic airflow into the Terra Nova Bay area is well represented by the primitive equation modeling, but much finer grids are needed to resolve the near‐coastal d

ISSN:0148-0227    

DOI:10.1029/JD095iD05p05495

年代:1990

数据来源: WILEY

摘要:

The angular momentum of the Earth‐atmosphere system is conserved to good approximation because external torques are small. Variations in the angular momentum of the atmosphere must therefore be accompanied by and directly connected with variations in the angular momentum of the Earth and hence with variations in the length of day (lod). The atmospheric angular momentum budget simulated in the Canadian Climate Centre general circulation model is investigated in order to assess the fidelity of the model to the atmosphere but, more importantly, to investigate the nature of the simulated interaction between the atmosphere and the underlying surface and the implications for variations in the length of day. These interaction terms are difficult to obtain based on observations. The general circulation model successfully reproduces many of the observed aspects of the angular momentum budget, including the partition of the angular momentum between hemispheres, the dominance of the seasonal cycle and, as well as may be determined, the climatological distribution of the mountain torque and stress torque interaction terms. As to the simulated interaction between atmosphere and underlying surface on shorter time scales, it is found that on time scales of days the mountain and stress torque terms contribute to the rate of change of angular momentum (and by implication the lod) in the ratio of 60–40%, while at weekly and longer time scales the stress torque dominates by increasing amounts. The notable contemporaneity of changes in the length of day and of atmospheric angular momentum suggests that much of the interaction occurs directly with the solid Earth and that the oceans, despite their large area, play a lesser role. This is verified in the simulation where the sum of the stress and mountain torque over land contributes of the order of 85–90% to the variation in angular momentum for time scales of days to m

ISSN:0148-0227    

DOI:10.1029/JD095iD05p05511

年代:1990

数据来源: WILEY

摘要:

The paper presents and discusses a large set of in situ measurements of fine‐scale structure in the mesosphere and lower thermosphere at high latitudes. The experimental technique used is a rocket‐borne electrostatic positive ion probe with linear amplification and high precision and time resolution. This type of probe has proved to be very efficient. The accuracy and limitations of the experiment are discussed. The measured ion density fluctuations are used as passive tracers for motions in the nonionized air. Quantitative estimates of the characteristics of turbulence have been derived. The method of analysis is carefully discussed and related to previous work, and a revision of earlier methods is proposed. The results show a characteristic minimum in turbulent intensity at or below the mesopause reg

ISSN:0148-0227    

DOI:10.1029/JD095iD05p05533

年代:1990

数据来源: WILEY

摘要:

An ion‐chemical model for the terrestrial D region is used to investigate the change in degree of hydration for proton hydrates when the summer polar neutral atmosphere is perturbed by a gravity wave. The drastically changing ion composition which results has consequences for the energy‐wave number spectrum of the scalar tracer pla

ISSN:0148-0227    

DOI:10.1029/JD095iD05p05549

年代:1990

数据来源: WILEY