摘要:

To accurately simulate the present Earth climate requires an atmospheric longwave radiation model whose computed fluxes agree to within ±1% of the only presently available standard, line by line (LBL) calculations. This standard derives from the agreement to within ±1% of various LBL calculations (Luther et al., 1988). To model potential future climate change requires an atmospheric longwave radiation model that can easily incorporate many trace gases. To meet these requirements, a longwave band model of 100 cm−1resolution is presented. Random band model transmissions are modified to account for overabsorption compared to LBL calculations, with empirical coefficients chosen to give good agreement with LBL spectral fluxes. The longwave band model (LWBM) integrated fluxes agree to within ±1% of LBL fluxes, while heating rates agree to within ±5%. The LWBM is used in the National Center for Atmospheric Research Community Climate Model Version 1 to compute longwave radiation. It is demonstrated that by not including minor bands of CO2and O3, other trace gases (CH4, N2O, CFCl3, CF2Cl2), or nonblack surface emissivity effects results in a significant bias in tropical clear‐sky outgoing longwave radiation of 8–10 W m−2over oceans and up to 15 W m−2over

ISSN:0148-0227    

DOI:10.1029/92JD00806

年代:1992

数据来源: WILEY

摘要:

Reference radiative transfer solutions in the near‐infrared spectrum, which account for the spectral absorption characteristics of the water vapor molecule and the absorbing‐scattering features of water drops, are employed to investigate and develop broadband treatments of solar water vapor absorption and cloud radiative effects. The conceptually simple and widely used Lacis‐Hansen parameterization for solar water vapor absorption is modified so as to yield excellent agreement in the clear sky heating rates. The problem of single cloud decks over a nonreflecting surface is used to highlight the factors involved in the development of broadband overcast sky parameterizations. Three factors warrant considerable attention: (1) the manner in which the spectrally dependent drop single‐scattering values are used to obtain the broadband cloud radiative properties, (2) the effect of the spectral attenuation by the vapor above the cloud on the determination of the broadband drop reflection and transmission, and (3) the broadband treatment of the spectrally dependent absorption due to drops and vapor inside the cloud. The solar flux convergence in clouds is very sensitive to all these considerations. Ignoring effect 2 tends to overestimate the cloud heating, particularly for low clouds, while a poor treatment of effect 3 leads to an underestimate. A new parameterization that accounts for the aforementioned considerations is accurate to within ∼30% over a wide range of overcast sky conditions, including solar zenith angles and cloud characteristics (altitudes, drop models, optical depths, and geometrical thicknesses), with the largest inaccuracies occurring for geometrically thick, extended cloud systems containing large amounts of vapor. Broadband methods that treat improperly one or more of the above considerations can yield substantially higher errors (>35%) for some overcast sky conditions while having better agreements over limited portions of the parameter range. For example, a technique that considers effect 3 but ignores effect 2 yields a partial compensation of errors of opposite sign, such that the resulting inaccuracy for geometrically thick clouds can be less than 20%. In contrast to the marked sensitivity of the cloud heating rates, the maximum relative errors in the reflected flux at the top of the overcast atmosphere and the transmitted flux at the surface do not vary appreciably under the various broadband treatments; with the new parameterization, the relative errors are less than 15%. In applying the broadband concept to overcast atmospheres with multiple cloud decks, there are cases when the errors can be larger than stated above. Hence a general use of broadband methods in weather prediction and climate models (e.g., general circulation models) should be accompanied by a realization of the potential inaccuracies that can occur for specific overcast

ISSN:0148-0227    

DOI:10.1029/92JD00932

年代:1992

数据来源: WILEY

摘要:

Collocated observations from the Advanced Very High Resolution Radiometer (AVHRR), High‐Resolution Infrared Sounder/2 (HIRS/2), and Earth Radiation Budget Experiment (ERBE) instruments onboard the NOAA 9 satellite are combined to describe the broadband and spectral radiative properties of the Earth‐atmosphere system. Broadband radiative properties are determined from the ERBE observations, while spectral properties are determined from the HIRS/2 and AVHRR observations. The presence of clouds, their areal coverage, and cloud top pressure are determined from a combination of the HIRS/2 and the AVHRR observations. The CO2slicing method is applied to the HIRS/2 to determine the presence of upper level clouds and their effective emissivity (product of cloud area amount and emissivity). The AVHRR data collocated within the HIRS/2 field of view are utilized to determine the uniformity of the scene and retrieve sea surface temperature. Changes in the top of the atmosphere longwave and shortwave radiative energy budgets, and the spectral distribution of longwave radiation are presented as a function of cloud amount and cloud top pressure. The radiative characteristics of clear sky conditions over oceans are presented as a function of sea surface temperature and atmospheric water vapor structure. The importance of spectral information in assessing changes in the top of the atmosphere radiative energy budget is demonstra

ISSN:0148-0227    

DOI:10.1029/92JD00847

年代:1992

数据来源: WILEY

摘要:

The empirical formula proposed by Willett et al. (1989) for the estimation of lightning return‐stroke peak current,I, from measured peak electric field,E, at a rangeD, is analyzed and discussed. The formula of Willett et al. (1989), obtained from the regression ofEonI, is not the least squares fit and hence is not the best expression for predicting the peak current from the measured peak electric field. Based on the same data, the least squares fit and, hence, the best expression is obtained from the regression ofIonE, given byI= 1.5 − 0.037DE, whereIis in kA and taken as negative,Eis positive and in V/m, andDis in km. The Willett et al. (1989) formula results in an error with respect to the best peak current estimating expression that varies from −15% to +2.6% over the range of peak field values of 1.9 to 11 V/m (normalized to 100 km) used to derive the two relations. When the data of Willett et al. (1989) are separated into a high returnstroke speed group (1.5 × 108to 1.9 × 108m/s) and a low return‐stroke speed group (1.2 × 108to 1.4 × 108m/s), theI‐Eregression lines differ for the two groups, with the difference in the regression line slopes being statistically significant at the 0.01 significance level. If the difference between intercepts of these two regression lines, found to be statistically insignificant, is neglected, the observed difference in slopes suggests that the group with the higher measured return‐stroke speed is associated with a lower peak electric field for the same peak current. Finally, the practical applications of the Willett et al. (1989) formula presently found in the literature are reviewed, and the several cases of improper use, mostly related to misinterpretation of Willett et al. 's (1989) sign convention

ISSN:0148-0227    

DOI:10.1029/92JD00720

年代:1992

数据来源: WILEY

摘要:

A thorough examination of the results of a time‐dependent computer model of a dipole thunderstorm revealed that there are numerous similarities between the time‐averaged electrical properties and the steady state properties of an active thunderstorm. Thus, the electrical behavior of the atmosphere in the vicinity of a thunderstorm be can be determined with a formulation similar to what was first described by Holzer and Saxon in 1952. From the Maxwell continuity equation of electric current, a simple analytical equation was derived that expresses a thunderstorm's average current contribution to the global electric circuit in terms of the generator current within the thundercloud, the intracloud lightning current, the cloud‐to‐ground lightning current, the altitudes of the charge centers, and the conductivity profile of the atmosphere. This equation was found to be nearly as accurate as the more computationally expensive numerical model, even when it is applied to a thunderstorm with a reduced conductivity thundercloud, a time‐varying generator current, a varying flash rate, and a changing ligh

ISSN:0148-0227    

DOI:10.1029/92JD00857

年代:1992

数据来源: WILEY

摘要:

Peak currents and polarities for the first return strokes of approximately 5000 cloud‐to‐ground lightning flashes have been studied. The flashes were associated with eight mesoscale convective systems (MCSs) that occurred in tropical northern Australia in 1989 during the second field phase of the Down Under Doppler and Electricity Experiment (Dundee). Statistical analysis of the lightning data produced an average peak current (independent of polarity) of 39 kA. When considered with the average cloud‐top heights of the tropical MCSs examined herein, the 39‐kA peak current average appears to be in general agreement with Orville's (1990) hypothesis of latitudinal variation in peak current. In six of the Dundee MCSs consisting of both convective and stratiform precipitation components, we found that positive peak current maxima tended to occur in the trailing stratiform region while positive peak current minima were generally situated in convective precipitation. We also observed this pattern in a middle‐latitude MCS that occurred on June 10–11, 1985. The magnitude of the positive peak current maxima in the tropical MCSs and the middle‐latitude MCS increased coincidentally in time with the growth of the stratiform regions, and reached peak values when the stratiform regions were most intense. Given the apparent dependence of positive peak current magnitude on the life‐cycle of the stratiform region and the spatial separation between positive peak current maxima and the convective line, we offer further support for a noninductive charging mechanism being responsible for the electrical charging and subsequent lightning observed in the trailing stratiform

ISSN:0148-0227    

DOI:10.1029/92JD00798

年代:1992

数据来源: WILEY

摘要:

A dual, scanning Fabry‐Perot spectrometer at Mawson, Antarctica (67.6°S, 62.9°E), was used to obtain simultaneous direct solar and daytime sky spectra of the sodium D2 Fraunhofer absorption line at λ589 nm. The intensity of the Ring component of skylight was determined from these spectra. Our observations indicated that the absolute intensity of this component varied diurnally, maximizing at local noon. Observations over ice and snow gave greater absolute Ring intensities than those over open water. Fractional Ring intensities were typically 6% of the adjacent day sky continuum. During full daylight, no significant variation in fractional Ring intensity was observed with changing solar zenith angle; sharp increases were observed during twilight. These observations are consistent with inelastic molecular scattering in the atmosphere being the cause of the Ring ef

ISSN:0148-0227    

DOI:10.1029/92JD00768

年代:1992

数据来源: WILEY

摘要:

A technique is presented to estimate ice cloud particle characteristic sizes and concentrations as well as the integrated ice water path from simultaneous ground‐based radar and infrared radiometer measurements. The approach is based on the theoretical consideration of infrared thermal radiative transfer within a cloud and can be applied to clouds that are semitransparent in the infrared “window” and horizontally extensive. The suggested technique is applied to radar and infrared radiometer data collected during the Cloud Lidar and Radar Exploratory Test (CLARET‐I) experiment. Retrieved values of ice cloud microphysical parameters are in general agreement with results obtained by other

ISSN:0148-0227    

DOI:10.1029/92JD00968

年代:1992

数据来源: WILEY

摘要:

During the period April 1989 through December 1990, O3concentrations in the marine boundary layer at Barbados, West Indies, show a pronounced seasonal cycle. Daily averaged values in the winter and spring often fall in the range of 25–35 ppbv for periods of several days, and they seldom fall below 20 ppbv. In contrast, during the summer, values typically fall in the range of 10–20 ppbv. During the winter‐spring period, there is a very strong negative correlation between O3and a number of aerosol species, including NO3−. These anticorrelations appear to be driven by changing transport patterns over the North Atlantic as opposed to chemical reactions involving O3and nitrogen species in the atmosphere. Analyses of isentropic trajectories clearly show that high O3and low NO3−are associated with transport from higher latitudes and high altitudes. Conversely, high NO3−and relatively low O3are associated with transport from Africa. Our study suggests that North America and the middle troposphere (and stratosphere) are not strong sources for NO3−over the tropical North Atlantic. The strong correlation of NO3−with210Pb and the weaker correlation with Saharan dust indicates that NO3−is derived principally from continental surface sources, probably in Europe and North Africa, but not from the Saharan soil material itself. During several extended periods, NO3−and210Pb were strongly correlated and their concentrations were high relative to nss SO4=; these factors, coupled with trajectories originating in Africa, suggest that African biomass burning was a significant source at these times. In contrast, biomass burning appears to be a minor source for O3as measured at Barbados, perhaps accounting for an enhancement of about 5 ppbv at most d

ISSN:0148-0227    

DOI:10.1029/92JD00894

年代:1992

数据来源: WILEY

摘要:

An intercomparison has been conducted among five instruments which measure gas phase ammonia at low concentration in the atmosphere: (1) a photofragmentation/laser‐induced fluorescence (PF/LIF) instrument; (2) a molybdenum oxide annular denuder sampling/chemiluminescence detection (MOADS) technique; (3) a tungsten oxide denuder sampling/chemiluminescence detection (DARE) system; (4) a citric acid coated denuder sampling/ion chromatographic analysis (CAD/IC) method; and (5) an oxalic acid coated filter pack sampling/colorimetric analysis (FP/COL) method. Mixtures of ammonia in air at flow rates of 1800 (STP) L/min and concentrations from 0.1 to 14 parts per billion by volume (ppbv) with the addition of possible interferants (CH3NH2, CH3CN, NO, NO2, O3, and H2O) were provided for simultaneous tests. In addition, the five instruments made simultaneous ambient air measurements both from a common manifold and from their separate inlets located at a common height above the ground. Several conclusions were reached: (1) No artifacts or interferences were conclusively established for any of the techniques, although CH3NH2may interfere in the MOADS system. (2) Measurements from the PF/LIF and the CAD/IC methods agreed well with the prepared mixtures over the full range of ammonia concentrations. The high specificity and time resolution (l min) of the PF/LIF results allowed data from this technique to be used as a basis set for comparisons. (3) For fog‐free conditions, ambient measurements from all of the instruments generally agreed to within a factor of 2 for ammonia levels above 0.5 ppbv. The CAD/IC and PF/LIF instruments agreed to within 15% on average for all ambient data. (4) The slope from linear regression analysis of separate inlet ambient air measurements indicated that the DARE data agreed with those from the PF/LIF system to within 7%. The linear regression line intercept was 216 parts per trillion by volume (pptv), which may indicate a positive interference in the DARE data, but the DARE data were closer to the PF/LIF data (50–100 pptv higher) at the lowest ambient ammonia levels. (5) The FP/COL method measured about 66% of the ammonia as determined by the PF/LIF technique and measured even lower fractional levels in the prepared samples. These low values indicate a loss of ammonia, possibly on the Teflon prefilter, under the conditions of this study (cold temperatures and generally low relative humidity). (6) The MOADS ambient air data were about 64% of the ammonia that was observed by the PF/LIF method for levels greater than about 1 ppbv, and below this level it overestimated ammonia. Problems with the MOADS calibrations and inlets may have been responsible. (7) Ambient air data from the one period of fog formation indicated that ambient ammonia was primarily partitioned into the condensed phase, leaving the interstitial air greatly depleted. Volatilization of absorbed ammonia from water droplets entrained in the sampled air appeared to influence the MOADS system. This did not appear to affect the FP/COL, CAD/IC, or PF/LIF results as a result of fast sample flows and/or operation at ambient tempera

ISSN:0148-0227    

DOI:10.1029/92JD00721

年代:1992

数据来源: WILEY