摘要:

The Faculté Polytechnique of Mons has participated in the last three programs of artificially triggered lightning in Saint‐Privat d'Allier, France (1986, 1990, and 1991). Measurements of the vertical part of the electric field E and electric field derivative (dE/dt) only 50 m from the discharge were made in 1990 and 1991. Our data, combined with those of the Centre National d'Etudes des Télécommunications, the Centre d'Etudes Nucléaires de Grenoble, and the Ecole Centrale de Lyon, lead us to a statistical study of the electrical parameters obtained from artificially triggered discharges in Saint‐Privat d'Allier. We define the statistical distributions of these electrical parameter populations and establish some relations between them. We compare artificially triggered lightning with natural lightning and conclude that the triggered lightning strokes are very similar to the subsequent strokes of natural lightning. We also compare the Saint‐Privat d'Allier triggered lightning with the Kennedy Space Center (Florida) triggered lightning. The Kennedy Space Center is at sea level and has a high ground conductivity, while Saint‐Privat d'Allier is a mountainous country with strong resistivity. Main results are, for the current, 50% of triggered strokes are higher than 12.1 kA in Florida (statistics of 305 strokes) and 9.8 kA in Saint‐Privat d'Allier (54 strokes); for dl/dt, 50% of triggered strokes have a dl/dt higher than 91.4 kA/μs in Florida (134 strokes) and 36.8 kA/μs in Saint‐Privat d'Allier (47 strokes); 5% of the strokes have a dI/dt higher than 297.7 kA/μs in Florida. The best relation (best correlation coefficient) between dI/dt (kA/μs) and I (kA) is dI/dt = 2.6 I1.34in Florida (134 strokes) and dI/dt = 2.0 I1.28in Saint‐Privat

ISSN:0148-0227    

DOI:10.1029/94JD00912

年代:1994

数据来源: WILEY

摘要:

Ionization rates for different times on two fair‐weather days are computed using Hoppel's (1985) theoretically deduced aerosol‐size‐dependent attachment coefficients and the aerosol‐size spectra derived from our measurements of mobility spectra of atmospheric ions made at a height of 1 m above ground at Pune (18°32′N, 73°51′E, 559 m above mean sea level), India. The ionization rates are minimum in the afternoon and increase to their maximum values of 116.52 and 103.93 ion pairs cm−3s−1in the early morning. Increase in ionization rate is found to accompany with the increase in the total aerosol concentration. Our results suggest that this increase in ionization rate may be due to addition of some radioactive aerosol particles, primarily in the size range of

ISSN:0148-0227    

DOI:10.1029/94JD01335

年代:1994

数据来源: WILEY

摘要:

The relationship between deep equatorial convection over the western Pacific and atmospheric circulation during November to February, 1986–1992 is studied using cross correlations between outgoing longwave radiation (OLR) and National Meteorological Center global analyses. We focus on intraseasonal convective events on the 6‐ to 30‐day timescale over two regions: just east of Borneo and over the Tropical Ocean and Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE) domain to the northeast of New Guinea. Although comparatively little work has been done on this timescale, we show that the bulk of the intraseasonal variability in both convection and low‐level circulation over the tropical western Pacific occurs in this period range. As a comparison with these higher‐frequency events, the evolution of COARE convection on the 30‐ to 70‐day timescale is also considered. The results of the historical data analysis are compared to a case study of deep convection associated with strong surface westerly winds during November 1989. This analysis is meant to serve as a benchmark for future case studies from the Tropical Ocean‐Global Atmosphere COARE experiment and other periods. In general, 6‐ to 30‐day equatorial convection in the regions studied occurs in conjunction with low‐level equatorial westerly wind anomalies, paired anomalous cyclonic circulations straddling the equator, and a strengthening of the sea level pressure gradient along the equator. The increase in the pressure gradient appears most often tied to the simultaneous equatorward movement of high‐pressure systems originating over southeastern Asia and Australia. A surge of trades over the central Pacific also accompanies the development of convection over both study regions. Following the convective peak, the low‐level westerlies slowly decay, as the northern cyclone moves northwestward away from the equator in a track characteristic of a tropical depression. It is also observed that low‐level westerly anomalies occur frequently without deep convection. The vertical structure during convective events along the equator on the 30‐ to 70‐day timescale and over Indonesia in the 6‐ to 30‐day band is baroclinic, with easterly anomalies at upper levels overlying low‐level westerlies. At 200 mbar a strengthened meridional outflow from the OLR signal into a wave train on the equatorward side of the Asian jet is observed to develop following the convective peak, suggesting forcing of this circulation by the convection. Over the COARE domain there is evidence that 6‐ to 30‐day convection often occurs in conjunction with deep westerly flow. It is suggested that these types of events are frequently characterized by vertical propagation of westerlies from the surface to the upper troposphere, as is shown for a case study from November 1989. This westerly burst episode is shown to have many features in common with the “

ISSN:0148-0227    

DOI:10.1029/94JD01486

年代:1994

数据来源: WILEY

摘要:

Measurements of a scalar flux from an extended surface are frequently made with the eddy correlation technique consisting of a vertical velocity sensor and a sensor for the scalar of interest. In many cases the two sensors have to be mounted with a significant horizontal separation to avoid flow interference. Consequently, the technique will underestimate the scalar flux. This paper addresses the issue of flux underestimation due to this separation. A model is developed in the framework of Monin‐Obukhov similarity for the spatial covariance of vertical velocity and air temperature in the unstable surface layer. It allows the underestimation of sensible heat flux to be assessed using information on separation orientation relative to wind direction, atmospheric stability, measurement height, and separation distance. The coefficient in the model is evaluated with observations made over a potato field and a clover field. The principles established here should also be applicable to fluxes of scalars other than sensible hea

ISSN:0148-0227    

DOI:10.1029/94JD00942

年代:1994

数据来源: WILEY

摘要:

The response of a general circulation model to a change in its treatment of cloud solar forcing is investigated. Radiation field data from the forecast model of the Navy Operational Global Atmospheric Prediction System for five Julys (1979–1983) are presented in an investigation of the effect of a change from grid cell averaged clouds to maximally overlapping clouds in the model's solar radiation scheme. The model results are compared with Nimbus 7 Earth Radiation Budget top of the atmosphere (TOA) solar and longwave irradiances and with derived surface solar irradiance data. Although the maximal overlap scheme performs considerably better than the grid cell averaging scheme (reducing maximum deficiencies in TOA and surface solar irradiance by over 100 W m−2), significant errors remain. The simulated correlation between TOA net solar and longwave irradiance improves at low latitudes in the northern hemisphere, with little change at higher latitudes. This improved correlation is consistent with the greater consistency between the treatments of solar and longwave cloud radiative forcing brought to the model by the new solar radiation scheme. The change in the radiation treatment is shown to have the greatest direct effect on solar radiation over convective regions, a consequence of the scarcity of optically thick clouds produced by the model's cloud parameterization in other regions. The model responds with an increase in convective activity over land and an increase in the flux of moisture from sea to land. Planetary cooling over the oceans increases because of a decrease in cloud cover. From mid to high latitudes in the northern hemisphere, there are scattered regions of increased cloud water content associated with increased tropospheric temperatures. Over land the model response in terms of TOA downwelling solar irradiance tends to counter the increase in solar irradiance caused by the model change in all latitudinal zones in the northern hemisphere. This response is caused primarily by changes in the cloud fields, which thus act as a negative feedback following the change in cloud solar forcing. The significance of this response is examined with respect to the perturbation in solar irradiance represented by the model change. An estimate of this perturbation is obtained by taking the difference in solar irradiance diagnosed by the two cloud solar forcing treatments for simulations employing the grid cell averaging scheme. The response is significantly greater in magnitude in the tropics than at midlatitudes, both in an absolute sense and as a percentage of this perturbation. Because TOA longwave irradiance exhibits a positive response in the tropics, and a negative response at midlatitudes, however, the percentage response in net TOA downwelling irradiance is actually greater in magnitude at midlatitudes. In a number of regions the cloud feedback is very large, showing the importance for cloud field prediction of improvements in the treatment of cloud solar forcing. Such cloud feedback also explains the small improvement seen here in the prediction of TOA solar irradiance in certain regions/Increases in surface sensible heating and longwave cooling are generally considerably less than increases in surface latent heating, though a notable exception occurs in arid central Asia. A large ground temperature increase in that region is strongly correlated at low levels with the atmospheric temperature increase observed at midlatitudes in the northern hemisph

ISSN:0148-0227    

DOI:10.1029/94JD00359

年代:1994

数据来源: WILEY

摘要:

Air‐water fluxes of oxygen and carbon dioxide have been calculated for a model which incorporates a turbulent air layer and takes into account the effects of the fluxes of sensible and latent heat on the temperature of the liquid surface. The calculated fluxes are compared with the experimental results of Liss et al (1981), Smith and Jones (1985), and Smith et al. (1991). The results of this comparison clearly demonstrate both the importance of coupling, in the sense of irreversible thermodynamics, of heat and matter fluxes at the gas‐water interface and the important role of the surface temperature of the liquid in controlling the magnitude and sometimes even the direction of the gas flux. The large carbon dioxide fluxes found by Smith and coworkers can be accounted for by assuming eddy diffusion, rather than molecular diffusion, on the seaside of the interface. This is consistent with an earlier suggestion that their measurements were affected by proximity to a surf zone. The present calculations might serve as the basis of a practical method of determining air‐sea fluxes of CO2and other trace

ISSN:0148-0227    

DOI:10.1029/94JD00816

年代:1994

数据来源: WILEY

摘要:

This paper introduces a simple method for deriving climatological values of the longwave flux emitted from the clear sky atmosphere to the ice‐free ocean surface. Simulations of the clear sky longwave fluxes to space and to the surface are employed in this study to assist in the development of this flux retrieval which requires monthly averaged column‐integrated water vaporwand the clear sky top‐of‐atmosphere (TOA) outgoing longwave flux (both available from satellite measurements). It is shown using both theory and data from simulations how the ratio of the surface to TOA flux is a simple function ofwand a validation of the simple relationship is presented based on a limited set of surface flux measurements. The rms difference between the retrieved surface fluxes and the simulated surface fluxes is approximately 6 W m−2. The clear sky column cooling rate of the atmosphere is derived from the Earth Radiation Budget Experiment (ERBE) values of the clear sky TOA flux and the surface flux retrieved using Special Scanning Microwave Imager (SSM/I) measurements ofwtogether with ERBE clear sky fluxes. The relationship between this column cooling rate,w, and the sea surface temperature (SST) is explored and it is shown how the cooling rate systematically increases as bothwand SST increase. The uncertainty implied in these estimates of cooling are approximately ±0.2 K d−1. The effects of clouds on this longwave cooling are also explored in a limited way by placing bounds on the possible impact of clouds on the column cooling rate based on certain assumptions about the effect of clouds on the longwave flux to the surface. While a more global assessment of the cloud effect must await use of new satellite data that will allow us to estimate the contributions by clouds to these surface fluxes, it is shown in this paper how the longwave effects of clouds in a moist atmosphere where the column water vapor exceeds approximately 30 kg m−2may be estimated from presently available satellite data with an uncertainty estimated to be approximately 0.2 K d−1. Based on an approach described in this paper, we show how clouds in these relatively moist regions decrease the column cooling by almost 50% of the clear sky values and the existence of significant longitudinal gradients in column radiative heating across the equatorial and subtropic

ISSN:0148-0227    

DOI:10.1029/94JD01151

年代:1994

数据来源: WILEY

摘要:

The determination of the concentration of OH in the Earth's troposphere is of fundamental importance to an understanding of the chemistry of the lower atmosphere. Although many experiments to measure OH concentration have been performed in recent years, very few operate at sensitivities necesssary to measure the extremely low amount of OH in the clean troposphere (0.1–0.2 parts per trillion by volume at summertime local noon). This paper describes an informal intercomparison campaign held at Fritz Peak, Colorado, in summer 1991 to intercompare the OH concentrations determined from a spectroscopic instrument and an in situ chemical conversion instrument, both with sensitivities at or below 5×105molecules cm−3. Ancillary measurements including those of O3, CO, NO, NO2, NOy, H2O, SO2, aerosols, solar flux, and meteorological parameters were also performed to test photochemical theories of OH formation. These measurements also provided a means for comparing air masses at the long path and in situ sites. The intercomparison was very successful with measured values of OH concentration in agreement within one standard error much of the time. OH concentrations were typically low, rarely above 4×106cm−3, with only slow growth during the morning hours, indicating the possible presence of scavenger species. Model results suggest higher than measured OH concentrations or the presence of scavenger

ISSN:0148-0227    

DOI:10.1029/94JD00740

年代:1994

数据来源: WILEY

摘要:

Meteorological conditions, extremely conducive to fire development and spread in the spring of 1987, resulted in forest fires burning over extremely large areas in the boreal forest zone in northeastern China and the southeastern region of Siberia. The great China fire, one of the largest and most destructive forest fires in recent history, occurred during this period in the Heilongjiang Province of China. Satellite imagery is used to examine the development and areal distribution of 1987 forest fires in this region. Overall trace gas emissions to the atmosphere from these fires are determined using a satellite‐derived estimate of area burned in combination with fuel consumption figures and carbon emission ratios for boreal forest fire

ISSN:0148-0227    

DOI:10.1029/94JD01024

年代:1994

数据来源: WILEY

摘要:

Monitoring stratospheric aerosols containing H2SO4using the brightness temperature (BT) difference between 11 and 8.3 μm (BT8–BT11) spectral channels is demonstrated using theoretical calculations and satellite observations. Assuming an aqueous solution of 50% and 75% sulfuric acid, radiative transfer calculations indicate that over oceans an increase in the optical depth of the stratospheric aerosol results in an increase in BT8–BT11. Theoretical simulations suggest that the technique is sensitive to visible optical depths greater than approximately 0.15. The simulations also demonstrate a lack of sensitivity to the particle size distribution. Changes in pre‐ and post‐Pinatubo observations by the HIRS2 on board the NOAA 10 are consistent with observed optical depth measurements and confirm the sensitivity of these channels to the presence of the aerosol. The technique is also applied to cold tropical convective clouds and desert regions where the signal, though evident, is less conclusive. Time series analysis is applied to the NOAA 10 and NOAA 12 combined BT8–BT11observations to detect the periodicity of the spread of the volcanic aerosol. Over a region of the southern Pacific a 18‐ to 26‐day period is present. Model simulations were conducted to demonstrate a trispectral technique with observations near 8, 11, and 12 μm. The trispectral approach has high potential in that the spectral signature of cirrus, water vapor, and H2SO4aerosols are different. Observations from NOAA 10 and NOAA 11 are combined to demonstrate the capabilities of these infrared wavelengths of detecting the aerosol. The signal is clearly evident when a region of the South Atlantic is compared for pre‐ and post‐

ISSN:0148-0227    

DOI:10.1029/94JD01331

年代:1994

数据来源: WILEY