摘要:

A photochemical model of nitric oxide from 40 to 250 km has been used to study the distribution of NO in the mesosphere and lower thermosphere. The model includes a parameterization of NO photolysis that can account for the opacity of a varying thermosphericNOlayer. This model is used to test the suggestion that increases in thermospheric NO which increase this opacity ”shield” NO at lower altitudes by decreasing the photolytic destruction rate. The model was ”tuned” to reproduce two recent observations of thermospheric NO, the high latitude rocket experiment ofEparvier and Barth[1992] and the midlatitude ground‐based microwave experiment ofClancy et al.[1992]. In the first case, the model indicated that the large thermospheric NO layer could have caused significant shielding and up to a factor of 2 effect on the mesospheric NO is seen. On the other hand, for the midlatitude observation the NO optical pathlength was too small to be significant. A time dependent version of the model was used to study the response of mesospheric NO to auroral energy input during winter. Under conditions which combine rapid transport and NO self‐shielding, the model suggests that NO density increases could propagate into the upper s

ISSN:0148-0227    

DOI:10.1029/94JD01206

年代:1994

数据来源: WILEY

摘要:

A new principle of studying the mobility spectrum versus agening of atmospheric ions has been developed. By exposing generated ions to an airstream and a counteracting adjustable electric field, the level when the force of the electric field will be greater than the mechanical (represented by the airstream) can be stated. By measuring the ion current versus the electric field at a fixed air stream, the mobility spectrum can be deduced. By selecting different values for air speed, electric field and traveling distance for the ions, the mobility spectrum versus agening of the ions can be deduced. Examples of recordings together with analysis are given.

ISSN:0148-0227    

DOI:10.1029/94JD02174

年代:1994

数据来源: WILEY

摘要:

A new lightning return stroke model is proposed in which the lightning channel, previously charged by the leader, is exponentially discharged with the discharge time constant being a general function of height. The new model is both more general and more physically reasonable than the Diendorfer‐Uman model (Diendorfer and Uman, 1990) and its modification by Thottappillil et al. (1991). In the new model a single height‐variable discharge time constant is used instead of the two height‐invariant discharge time constants used for natural lightning in the Diendorfer‐Uman model. As an illustration of the use of the new model, the measured electric fields for three triggered lightning return strokes about 5‐km distant are compared with the fields calculated from the model, using simultaneously measured channel base current and return stroke speed as inputs to

ISSN:0148-0227    

DOI:10.1029/94JD02085

年代:1994

数据来源: WILEY

摘要:

An aircraft locally distorts the ambient thundercloud electric field. In order to determine the field in the absence of the aircraft, an aircraft calibration is required. In this work a matrix inversion method is introduced for calibrating an aircraft equipped with four or more electric field sensors and a high‐voltage corona point that is capable of charging the aircraft. An analytic, closed form solution for the estimate of a (3×3) aircraft calibration matrix is derived, and an absolute calibration experiment is used to improve the relative magnitudes of the elements of this matrix. To demonstrate the calibration procedure, we analyze actual calibration data derived from a Lear jet 28/29 that was equipped with five shutter‐type field mill sensors (each with sensitivities of better than 1 V/m) located on the top, bottom, port, starboard, and aft positions. As a test of the calibration method, we analyze computer‐simulated calibration data (derived from known aircraft and ambient fields) and explicitly determine the errors involved in deriving the variety of calibration matrices. We extend our formalism to arrive at an analytic solution for the ambient field, and again carry all errors expl

ISSN:0148-0227    

DOI:10.1029/94JD01682

年代:1994

数据来源: WILEY

摘要:

A system has been developed to measure wideband electric field derivatives (dE/dt) at five ground stations in a 15 km × 15 km network at Kennedy Space Center. Individual station responses are normalized using digital filters. Pulse‐timing resolution is improved to much less than the 50‐ns sample interval by interpolation using packing in the frequency domain. A time tag for each pulse is defined as the mean of the times of the rising‐edge half peak, peak, and falling‐edge half peak. The standard deviation in these times defines the timing error and is shown to be a function of noise and bandwidth rather than digitization rate. Each of the four unknowns for a pulse source location (x,y,z) and time of occurrence (t) is found from the five time‐tag measurements using different weightings for all five combinations of the four‐station hyperbolic equations. Weighting factors and errors in x,y,z and t are estimated using error propagatio

ISSN:0148-0227    

DOI:10.1029/94JD02150

年代:1994

数据来源: WILEY

摘要:

Observational evidence is presented for the nucleation of condensation nuclei (CN) in the clear air adjacent to an isolated, marine, cumulus cloud. Two separate regions of particle nucleation are identified: one located above the cloud top, and the second located downwind of the cloud near the level of the anvil outflow. The regions of high CN concentrations were located in extremely clean marine air, with unactivated aerosol surface area (excluding the nucleation mode)<2 μm2cm−3, air temperature −31°C, and higher relative humidities than the undisturbed environment. Vertical profile measurements downwind of the cloud showed that CN concentrations at the level of the anvil outflow (4.9 km) were 8 times greater than at any other level between the surface and 5.3 km. A conceptual model is formulated in which aerosol particles, sulfur dioxide (SO2), sulfuric acid vapor (H2SO4), dimethyl sulfide (DMS), and ozone (O3) from the boundary layer are entrained into the cumulus cloud. Total aerosol number concentrations and unactivated aerosol surface area decrease with height in the cloud due to Brownian diffusion and diffusiophoresis of cloud interstitial aerosol to hydrometeors, coalescence scavenging by cloud droplets, collisional scavenging by ice particles, and subsequent removal by precipitation. The air that is detrained from the cloud raises the relative humidity and vents the clean air, SO2, H2SO4, DMS, and O3to the near‐cloud environment. Hydroxyl radicals then oxidize the SO2and DMS to H2SO4. Under the conditions of high relative humidity, low total aerosol surface area, low temperatures, and high SO2concentrations near cloud top, significant concentrations of new particles can be produced by homogeneous‐bimolecular nucleation of sulfuric acid solution droplets from H2SO4and H2O vapor molecules. The concentration of CN as a function of time is calculated for the case described in this paper using a bimodal integral nucleation model. The model results show that significant numbers of CN could have been produced within a few hours by the homogeneous‐bimolecular nucleation of sulfuric acid solution droplets under the observed conditions provided the concentration of SO2near cloud top was enhanced by vertica

ISSN:0148-0227    

DOI:10.1029/94JD01926

年代:1994

数据来源: WILEY

摘要:

We measured dimethylsulfide (DMS) in air (DMSa) and surface seawater (DMSw) on board the R/VMeteorduring February–March 1991 on the tropical South Atlantic. Samples for the determination of DMSawere taken through a fluorinated ethylene/propylene Teflon inlet ≈33 m above sea level, preconcentrated by adsorption onto gold wool in quartz tubes, and analyzed by gas chromotography with flame photometric detection. The DMSainstrument is fully automated, providing improved precision, and processes up to four samples per hour. Over most of the cruise track, which followed the 19°S parallel between Brazil and Africa, DMSw, was significantly correlated to climatologically averaged chlorophyll concentrations obtained from coastal zone color scanner data, suggesting that remote sensing may be useful for estimating seawater DMS levels at least in some ocean regions. The cruise track proceeded from waters of low roductivity (off the coast of Brazil and in the subtropical gyre) to higher productivity (the Benguela Current and the upwelling region off Namibia and Angola); meteorological conditions were steady with consistent easterly winds. DMS values for air and water were low (≈ 50 ppt and 1–2 nmol L−1, respectively) in the areas of low productivity and increased simultaneously (≈ 100–300 ppt and 3–15 nmol L−1) as productivity increased. DMS sea‐to‐air fluxes (average 7.3 μmol m−2d−1) were calculated based on different parameterizations; for the study region the differences between the results obtained from the different models were minor. DMSawas strongly correlated to its emission flux from the sea surface as estimated from DMSwand meteorological parameters. This suggests that the air/sea transfer parameterizations used are suitable for providing estimates

ISSN:0148-0227    

DOI:10.1029/94JD01837

年代:1994

数据来源: WILEY

摘要:

The distribution and variations of atmospheric CO2from 1981 to 1992 were determined by measuring CO2mixing ratios in samples collected weekly at a cooperative global air sampling network. The results constitute the most geographically extensive, carefully calibrated, internally consistent CO2data set available. Analysis of the data reveals that the global CO2growth rate has declined from a peak of ∼2.5 ppm yr−1in 1987–1988 to ∼0.6 ppm yr−1in 1992. In 1992 we find no increase in atmospheric CO2from 30° to 90°N. Variations in fossil fuel CO2emissions cannot explain this result. The north pole‐south pole CO2difference increased from ∼3 ppm during 1981–1987 to ∼4 ppm during 1988–1991. In 1992 the difference was again ∼3 ppm. A two‐dimensional model analysis of the data indicates that the low CO2growth rate in 1992 is mainly due to an increase in the northern hemisphere CO2sink from 3.9 Gt C yr−1in 1991 to 5.0 Gt C yr−1in 1992. The increase in the north pole‐south pole CO2difference appears to result from an increase in the southern hemisphere CO2s

ISSN:0148-0227    

DOI:10.1029/94JD01951

年代:1994

数据来源: WILEY

摘要:

The Amazon Basin tropical rain forest is a key region to study processes that are changing the composition of the global atmosphere, including the large amount of fine mode aerosol particles emitted during biomass burning that might influence the global atmosphere. Three background monitoring stations, Alta Floresta, Cuiabá, and Serra do Navio, are operating continuously measuring aerosol composition. Fine (dp<2.0 μm) and coarse (2.0

ISSN:0148-0227    

DOI:10.1029/94JD01023

年代:1994

数据来源: WILEY

摘要:

Sensible heat flux measurements were carried out to examine the characteristics related to frequency and duration of extreme flux events above a desert surface. First, the flux bursts were identified by a modified hyperbolic hole “quadrant analysis” in which the threshold criterion depends on the mean and standard deviation of the flux time statistics. Second, the frequency distribution of the extreme flux burst length was examined. It was found that the frequency distribution of the burst length is well approximated by two‐parameter power laws which account for two distinct types of bursting phenomena: one is organized, coherent, and has a relatively long duration, while the other is short lived, less organized, and has a relatively short duration. Linear regression analysis was used to determine the two power exponents of the burst length probability density function. Both exponents compare favorably with other reported measurements on concentration bursting in dispersing plumes from an elevated source. The cutoff burst length from one power law to the next also matches other studies. A strong correlation between bursting frequency and flux integral time scales was observed under a variety of atmospheric stability and surface wetness conditions. This strong correlation suggested a relationship between atmospheric stability and the mean time interval between bursts. It was found that the variation in the interval between bursts for the dynamic and dynamic convective sublayers is strongly related to the variation in stability. This was not the case for near‐convective and stable atmospheric con

ISSN:0148-0227    

DOI:10.1029/94JD01679

年代:1994

数据来源: WILEY