摘要:

A lightning flash, cloud or cloud‐to‐ground, can be modeled by a time‐varying dipole, whose orientation may differ significantly from vertical, especially for cloud flashes. A direction finder measuring the vertical electric field and the horizontal components of the magnetic field over an infinitely conducting ground can estimate the location of the dipole with an accuracy that depends on the dipole orientation. A vertical dipole gives the best accuracy and a horizontal one the worst. With more than one direction finder, at different sites, the uncertainty for a nonvertical dipole can be removed. The method is based on the use of a pseudodirection angle, which for a vertical dipole coincides with the actual direction, and various time derivatives of the received s

ISSN:0148-0227    

DOI:10.1029/96JD00067

年代:1996

数据来源: WILEY

摘要:

In an extension of earlier work [Helsdon et al., 1992;Mazur, 1989], we have developed a one‐dimensional, electrostatic representation of the electric field change and charge redistribution in a cloud subsequent to a lightning flash. In this parameterization, these effects are determined by the properties of the vertical electric field. We assume lightning is initiated when the electric field magnitude reaches a prescribed breakdown value at some point. The lightning channel is represented by a prolate spheroidal conductor of a specified radius, whose length is determined by a crude energy criterion. Charge is induced on the channel surface and this charge modifies the surrounding electric field. We have applied this parameterization to the vertical electric field resulting from an idealized charge sounding to explore the features of the charge profile that determine whether lightning is likely to be intracloud or cloud to ground. We discuss the results of this analysis in terms of lightning patterns observed climatologically and the evolution of lightning patterns in a thunderstorm during its life cycl

ISSN:0148-0227    

DOI:10.1029/96JD00941

年代:1996

数据来源: WILEY

摘要:

Measurements of electric charges of raindrops, electric field, and Maxwell current were made at Abidjan (5°N, 4°W) from October 1988 to June 1990. The examination of the data obtained allows us to differentiate between storms and showers. The former are characterized by high global precipitation charge density (about 8 μC m−2) and by raindrops, 90% of which carry a positive charge of average value 86 pC, while the average negative charge is −22 pC only. The majority of positive raindrops are generally associated with the active phase of the storm and lead to strong precipitation currents of the order of −4.3 nA m−2, on average. These values, compared with those of the displacement current (JD), calculated from electric field records, and with those of the experimental Maxwell current (JM), show that the approximation ofJMby the displacement current alone is not valid. In those cases, the precipitation current (JP) must be included. The showers are characterized by weaker global precipitation charge density (about 1.6 μC m−2) and weaker charges (about 25 pC), equally distributed between positive and negative ones. The precipitation currents associated with them are consequently weak, and in those cases the Maxwell current may be approximated by the displacement

ISSN:0148-0227    

DOI:10.1029/95JD03657

年代:1996

数据来源: WILEY

摘要:

The radiative effects of tropical clouds at the tropopause and the ocean surface have been estimated by usingin situmeasurements from the Central Equatorial Pacific Experiment (CEPEX). The effect of clouds is distinguished from the radiative effects of the surrounding atmosphere by calculating the shortwave and longwave cloud forcing. These terms give the reduction in insolation and the increase in absorption of terrestrial thermal emission associated with clouds. At the tropopause the shortwave and longwave cloud forcing are nearly equal and opposite, even on daily timescales. Therefore the net effect of an ensemble of convective clouds is small compared to other radiative terms in the surface‐tropospheric heat budget. This confirms the statistical cancellation of cloud forcing observed in Earth radiation budget measurements from satellites. At the surface the net effect of clouds is to reduce the radiant energy absorbed by the ocean. Under deep convective clouds the diurnally averaged reduction exceeds 150 W m−2. The divergence of flux in the cloudy atmosphere can be estimated from the difference in cloud forcing at the surface and tropopause. The CEPEX observations show that the atmospheric cloud forcing is nearly equal and opposite to the surface forcing. Based upon the frequency of convection, the atmospheric forcing approaches 100 W m−2when the surface temperature is 303 K. The cloud forcing is closely related to the frequency of convective cloud systems. This relation is used in conjunction with cloud population statistics derived from satellite to calculate the change in surface cloud forcing with sea surface temperature. The net radiative cooling of the surface by clouds increases at a rate of 20 W m−2K−1during the CEPEX observi

ISSN:0148-0227    

DOI:10.1029/95JD02534

年代:1996

数据来源: WILEY

摘要:

The possibility of simulating tropical cyclones (TCs) using the National Center for Atmospheric Research community climate model (CCM2) is explored. Daily outputs from two long‐term simulation runs using the standard T42 resolution CCM2 are examined to identify simulated tropical cyclones (STCs) using a search scheme that selects qualified STCs resembling observed TCs. The two simulation cases are a 20‐year run driven by climatological sea surface temperatures (SSTs) and a 10‐year run, corresponding to the decade from 1979 to 1988, with the same model configuration except for the use of observed SSTs. A composite technique is adopted to reveal the horizontal and vertical structures of well‐developed STCs, and a comparison with those of observed TCs is presented. Then, the climatologies of STCs from the two simulation cases are discussed in terms of their genesis, movements, and seasonal and interannual variations through the comparisons with observed TC statistics. Despite obvious shortcomings of the standard CCM2, such as a coarse horizontal resolution, the structure and climatology of STCs identified in both climate runs are in reasonably good agreement with those of observed TCs. The annual STC frequency shows a better agreement with the observed SST run than the climatological SST run, while many other aspects of STCs in the two climate runs are com

ISSN:0148-0227    

DOI:10.1029/95JD03774

年代:1996

数据来源: WILEY

摘要:

Composite relationships among outgoing longwave radiation, sea level pressure, surface winds, and upper tropospheric circulation are examined for northern winter during subseasonal episodes of eastward progression of convection from the Indian Ocean to the western Pacific. This evolution often culminates with westerly wind burst events and strong air‐sea interaction associated with regional‐scale convective blowups in the western equatorial Pacific. We first document some of these interactions in the composites for two timescales, the submonthly (6–30 days) and that of the Madden‐Julian Oscillation (MJO) timescale (30–70 days). We then analyze the December 1992 period during the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) to illustrate how these composite relationships are manifested in a case study. Convection in the Indian Ocean for the composites is shown to be associated with a northern hemisphere wave train at 200 mbar, arcing through the midlatitudes, that can contribute to convective blowups farther east on the submonthly (6–30 days) timescale in the Intertropical Convergence Zone (ITCZ) in the eastern Pacific. The eastern Asian trough that is part of this wave train is associated with pressure surges from the northern hemisphere and subsequent convection over Southeast Asia. As the MJO convective envelope moves east to Australasia, midlatitude wave trains in either hemisphere include upper level troughs east of Asia and Australia and pressure surges from either hemisphere that contribute to pressure rises over the Indonesian region and a subsequent shift of the convective envelope to the western Pacific. The vertical wind structure for the December 1992 case study is consistent with the composite surface and upper level winds and also shows strong vertical wind shear in the boundary layer, a sharply defined westerly maximum near 700 mbar and an intensification of the upper level easterlies near 100 mbar. Very deep westerlies (to 200 mbar) are confined to shorter timescales. The case study illustrates the various time and space scale interactions noted in the composites. Reciprocal interactions between the tropics and the midlatitudes on the submonthly and MJO timescales in both the composites and the case study involve pressure surges and wave interaction that influence subsequent convection as the convective envelope migrates eastward from the tropical Indian to Pacific

ISSN:0148-0227    

DOI:10.1029/96JD01014

年代:1996

数据来源: WILEY

摘要:

The seasonal diurnal zonal and meridional winds observed from 2 to 18 km in the tropical Pacific using five 50‐MHz wind profilers are presented. The hourly data sets ranged in length from 686 days to 2934 days of observations. The observed diurnal winds are compared with tidal model winds forced by solar absorption of water vapor that are primarily the (1,1,1) tidal mode. Similarities between observed and modeled diurnal winds off the equator (12°S) suggest these off‐equator observations are forced by solar absorption of water vapor and are global oscillations. Differences between observed and modeled winds near the equator (2°N and 1°S) suggest these observations are forced by local or mesoscale processes. The vertical structure of meridional wind near the equator supports the diurnal forcing by the radiation of cloud tops and/or cirrus clouds above 12 km or the latent heat release of precipitating clouds extending throughout the troposphere. While near‐equator observations appear to be locally forced, the global distribution of these “local” forcing mechanisms may generate global oscillations that cannot be resolved with these wind profiler data from the Pacific basin. The observed winds below 4 km appear to be affected by boundary layer processes and have different characteristics than wind

ISSN:0148-0227    

DOI:10.1029/96JD01013

年代:1996

数据来源: WILEY

摘要:

Observations of tropospheric winds have been made nearly continuously since early 1986 in the altitude range 2–18 km using a 50 MHz VHF wind profiler at Christmas Island (2°N, 157°W). Tropospheric zonal winds observed at Christmas Island show a pronounced annual variation in the middle to upper troposphere with westerly winds typically observed during northern winter. The annual variation is strongest during La Niña and weakest during El Niño. The amplitude of the cycle has a broad peak in the upper troposphere. Empirical orthogonal function (EOF) analysis of the vertical structure of the annual variation shows that the principal mode explains nearly 90% of the variance in the 90‐ to 365‐day frequency band. We attribute the annual variation in zonal winds to the annual variation in deep tropical heating associated with deep convection. The annual variation in tropical convection gives rise to annual variations in the surface pressure and pressure difference between Darwin and Tahiti. The pressure difference is greatest during northern hemisphere winter months. Other measures of the annual variation in tropical convection are provided by the tropical tropopause height and by highly reflective clouds (HRC). The tropical tropopause is highest during the northern hemisphere winter months when convection is most active over the maritime continent/western Pacific warm pool region and lowest in northern summer when convection is most active over the land masses of the northern hemisphere. The HRC show an annual migration in the centers of active convection. The annual latitudinal migration of the active centers of convection over the western Pacific with respect to the equatorial waveguide appears to be an important factor in determining the annual variation evident in the remote response of the zonal winds observed over Christm

ISSN:0148-0227    

DOI:10.1029/96JD00029

年代:1996

数据来源: WILEY

摘要:

The annual cycle of the net mass transport across the extratropical tropopause is examined. Contributions from both the global‐scale meridional circulation and the mass variation of the lowermost stratosphere are included. For the northern hemisphere the mass of the lowermost stratosphere has a distinct annual cycle, whereas for the southern hemisphere, the corresponding variation is weak. The net mass transport across the tropopause in the northern hemisphere has a maximum in late spring and a distinct minimum in autumn. This variation and its magnitude compare well with older estimates based on representative90Sr mixing ratios. For the southern hemisphere the seasonal cycle of the net mass transport is weaker and follows roughly the annual variation of the net mass flux across a nearby isentropic surfac

ISSN:0148-0227    

DOI:10.1029/96JD00821

年代:1996

数据来源: WILEY

摘要:

A global three‐dimensional atmospheric model, the NCAR CCM2 general circulation model, has been adapted to study the hourly to yearly variability of CO2in the atmosphere. Features of this CCM2‐based model include high spatial resolution (2.8° × 2.8° latitude/longitude), 18 vertical levels, a 15‐min time step, and an explicit, nonlocal atmospheric boundary layer parameterization. The surface source/sink relationships used include exchange with the ocean, the terrestrial biosphere, biomass burning, and fossil fuel release of CO2. The timing and magnitude of the model seasonal cycle are compared to observational data for 28 sites. The seasonal cycle of atmospheric CO2is generally well predicted by the model for most of the northern hemisphere, but estimates of the amplitude of the seasonal cycle in the southern hemisphere are overpredicted. To address this aspect more rigorously, we have used the monthly surface oceanpCO2maps created by the Max‐Planck‐Hamburg ocean general circulation model to asses the ocean seasonality on the atmospheric surface CO2seasonality. The globally averaged interhemispheric gradient in atmospheric CO2concentrations, as computed with the chosen source/sink distributions, is a factor of two too high compared to data, and selected longitudinal bands may be up to 50% higher than the zonal mean. The high temporal resolution of this model allows the infrequent yet real extrema in atmospheric CO2concentrations to be captured. The vertical attenuation of the seasonal cycle of atmospheric CO2is well simulated by the boundary layer/free troposphere interaction in the model in the northern hemisphere. Conversely, an increasing amplitude of the seasonal cycle aloft is found in the midlatitude southern hemisphere indicating interhemispheric transport effects from north to south. We use two different models of the terrestrial biosphere to examine the influence on the computed seasonal cycle and find appreciable differences, especially in continental sites. A global three‐dimensional chemical transport model is used to assess the production of CO2from the oxidation of CO throughout the volume of the atmosphere. We discuss these CO + OH → CO2+ H results within the context of inverse model approaches to ascertaining the global and regional source/sink patterns of CO2. Deficiencies in the model output as compared to observational data are discussed within the context of guiding

ISSN:0148-0227    

DOI:10.1029/95JD03680

年代:1996

数据来源: WILEY