摘要:

Forty simultaneous, submicrosecond time‐resolved measurements of triggered lightning returnstroke current (I), current derivative (dI/dt), and electric field derivative (dE/dt) were made in Florida and France in 1985 and 1986. Peak currents ranged from about 5 to 50 kA, peakdI/dtamplitudes from 60 to 260 kA/μs in 1985 and from 20 to 140 kA/μs in 1986. The mean peakdI/dtvalues, 111 kA/μs (1985) and 68 kA/μs (1986), are 2–3 times higher than data from instrumented towers, and peakIanddI/dtappear to be positively correlated. ThedE/dtanddI/dtwaveform pairs have similar shapes, and the peak amplitudes are linearly proportional. Return‐stroke velocity, computed from the ratio of peakdE/dtanddI/dtsignal amplitudes using an expression derived from the radiation field term of the transmission line model (TLM), averaged 2.9×108m/s and 3.0×108m/s in 1985 and 1986, respectively, which is about 2 times higher than most optical measurements. The TLM velocity may be erroneous because (1) thedE/dtmeasurement was made only 50 m from the lightning channel, where fields other than the radiation field component, that is near fields, may contribute to the totaldE/dtand (2) fine structure on the measuredEfields was not consistent with a single upwardly propagating return‐stroke current wave assumed by the TLM (two waves are mor

ISSN:0148-0227    

DOI:10.1029/JD095iD01p00811

年代:1990

数据来源: WILEY

摘要:

The focus of this study is the control run performance of four general circulation models (GCMs): the Oregon State University (OSU) two‐layer atmospheric GCM (AGCM), the OSU coupled ocean‐atmosphere model (CGCM), the Goddard Institute for Space Studies (GISS) nine‐layer AGCM, and the European Centre for Medium‐Range Weather Forecasts (ECMWF) T21 model. The analysis variable is monthly mean sea level pressure (MSLP), and model validation is performed for a limited domain (North America/Atlantic/Europe). The first part of the investigation deals with the magnitude and gross spatial structure of model errors in means and interannual variability (for January and July only). These errors are examined with the aid of maps of time‐mean MSLP, difference fields, and local variance ratios. The significance of the local (grid point by grid point) differences in means and variances is then determined by performing univariatet‐ andF‐tests. This information on the spatial structure of large‐scale systematic errors is important for understanding the results of significance tests performed on the overall fields. In the second part of the investigation, the statistics recommended by Wigley and Santer (this issue) for use in model validation are applied to test the overall significance of observed/simulated differences in means, variances, and spatial patterns over the entire annual cycle. Significance levels are determined with the pool permutation procedure (PPP) introduced by Preisendorfer and Barnett (1983). Results indicate that all four models have highly significant errors in the mean field and spatial pattern over the entire annual cycle. Errors in the temporal variance are generally less significant, and significance levels for variance tests can depend critically on the choice of averaging period for observed validation data. The actual test statistic values show that there are considerable differences in model performance. The ECMWF T21 model simulates the spatial pattern and time‐mean MSLP field with greater fidelity than the other model

ISSN:0148-0227    

DOI:10.1029/JD095iD01p00829

年代:1990

数据来源: WILEY

摘要:

The comparison of spatial fields of meteorological variables is an essential component of model validation studies and is central in assessing the significance of any change between a perturbed and control run of a general circulation model. Comparisons may be made of statistics which define the time‐mean state, the temporal variability about this state, and/or spatial variability. Comparisons may also be made of the two time‐mean spatial patterns, or of the temporal evolutions of spatial patterns. We consider here a suite of univariate and multivariate statistics which may be used to make these comparisons. Some of these statistics have been used previously, while others are either new or have not previously been used in the present context. The use of these statistics, their differences and similarities, and their relative performances are illustrated by considering mean sea level pressure changes between the decades 1951–1960 and 1971–1980 over an area covering North America, the North Atlantic Ocean, and Europe. Significance levels are assessed using the pool‐permutation procedure of Preisendorfer and Barnett (1983) (henceforth P+B). This overcomes problems arising from nonideal behavior of the data (particularly spatial autocorrelation), unknown sampling distributions, and multiplicity in the case of univariate statistics. A subset of statistics is identified as most useful. For tests of differences in means these are the grid point by grid pointt‐test, a test comparing the overall means, and P+B's SITES statistic. For tests of differences in temporal variability they are the grid point by grid pointF‐test, and SPRET1 (the ratio of the spatial means of the time variances). SPRET1 is a modification of P+B's SPRED statistic designed to identify the direction of any variance difference. As a test of spatial variability differences, we identify SPREX1 (the ratio of the time means of the spatial variances), and for comparing spatial patterns the best statistic is the (spatial) correlation coefficient between the time‐mean fields. For comparing the temporal evolution of spatial patterns, we recommend using the time‐mean anomaly field correlation which is a more easily interpreted equivalent to P+B'

ISSN:0148-0227    

DOI:10.1029/JD095iD01p00851

年代:1990

数据来源: WILEY

摘要:

A narrow band model is presented to simulate the transfer of infrared radiation through a cloudless nocturnal boundary layer. The absorption and emission by the rotational and vibrational lines of water vapor, carbon dioxide and ozone in the spectral region from 0–2800 cm−1is modeled with the Goody band model, while the absorption of the water vapor continuum in the atmospheric window region is evaluated from the empirical formulas of Roberts et al. (1976). To approximate the integration of the absorption coefficient over the inhomogeneous path, the Curtis‐Godson method is used, and to approximate the integration over all zenith angles a diffusion factor of 5/3 independent of path length is adopted. It is shown that in the troposphere the results of the narrow band model diverge from cooling rate calculations performed by line by line models by about 0.2K d−1(∼20%). Moreover, from a comparison with 44 observations of downward flux at the surface, it is shown that the simulated flux underestimates the observed flux by about 6 W

ISSN:0148-0227    

DOI:10.1029/JD095iD01p00867

年代:1990

数据来源: WILEY

摘要:

The residual mean circulation (RMC) formulation of zonally averaged transport in the middle atmosphere produces a circulation which depends on the distributions of net diabatic heating and temperature. We have derived such circulations from two temperature data sets, National Meteorological Center (NMC) and Limb Infrared Monitor of the Stratosphere (LIMS), using the same radiative transfer code (Rosenfield et al., 1987). We have used these circulations to transport N2O in a photochemical model. The circulations and the resulting N2O distributions are notably different during the northern hemisphere winter, with that based on the NMC temperatures producing too much upward transport in the tropical stratosphere as judged by comparison with the stratospheric and mesoscale sounder data. The experiment demonstrates that model calculations, in general, and perturbation assessments, in particular, are likely to be quite sensitive to the choice of input temperature data (where this is not computed self‐consistently). It also reveals what appears to be a seasonally dependent bias in NMC zonally averaged temperatures with respect to those obtained from the LIMS instrument during 1978/197

ISSN:0148-0227    

DOI:10.1029/JD095iD01p00873

年代:1990

数据来源: WILEY

摘要:

Ground‐based microwave techniques have supplied the only long‐term measurements of water vapor in the mesosphere. We review the entire current data base, which consists of measurements obtained in three separate experiments over an 8‐year period. The data from all three experiments indicate that the water vapor seasonal variation at mid‐latitudes in the upper mesosphere is dominated by an annual component with low mixing ratios in winter and high mixing ratios in summer. This suggests that the vertical distribution of water vapor in the upper mesosphere (up to 80 km) is controlled by advective rather than diffusive processes. This is consistent with the low mesosphericKzzvalues (≈ 105cm2s−1) deduced from the vertical gradient of the microwave water vapor measurements by Strobel et al. (1987). However, it is difficult to reconcile the predominantly annual water vapor variation with the semiannual variation in ozone at 78 km observed by the Solar Mesosphere Explorer. We perform a series of one‐dimensional photochemical/vertical transport model calculations which verify that (within the context of the hydrogen/oxygen chemistry considered in the model), the seasonal variation of water vapor cannot be the mechanism for the semiannual ozone variation. This variation is either a manifestation of some heretofore unknown ozone photochemical mechanism, or it could be driven by a seasonal variation in the vertical transport of atomic oxygen from the thermosphere. One possible vertical transport scenario for producing the semiannual ozone variation (while retaining the annual water vapor variation)

ISSN:0148-0227    

DOI:10.1029/JD095iD01p00883

年代:1990

数据来源: WILEY

摘要:

Solar Mesosphere Explorer (SME) ozone data were analyzed for short‐term periodic oscillations in the altitude region from 50 to 90 km. A 5‐day oscillation was found to be a dominant feature at high southern latitudes near the December solstice in the altitude range from 50 to 80 km. The wave feature appears to travel westward with a speed consistent with that of a wave number one planetary‐scale oscillation circling the globe in 5 days. The amplitude also appears to be consistent with the temperature oscillations predicted by Geisler and Dickinson (1976). This oscillation was not found to be a dominant feature in SME ozone during northern hemisphere s

ISSN:0148-0227    

DOI:10.1029/JD095iD01p00895

年代:1990

数据来源: WILEY

摘要:

Dissociation of ozone in the Chappuis bands has been used as an O atom source to study isotope effects occurring in the O(3P)+O2(3∑g) recombination reaction. The ozone produced was found to be enriched in both of its heavy isotopes. The pressure dependence (5–1000 torr) and temperature dependence (127–360 K) of this isotope effect have been investigated. The enrichment is approximately constant from 5 torr to 100 torr and decreases at higher pressures. It increases with temperature, with50O3showing a slightly faster rate of increase than49O3. The results of this experiment have clearly isolated the source of the isotope effect to the gas phase O(3P) + O2(3∑g) recombination reaction. For comparison, we also present isotope ratios of ozone formed in an electric discharge. None of the results, however, have shown the large enhancement of 40% or more in mass 50 observed in some stratospheric measu

ISSN:0148-0227    

DOI:10.1029/JD095iD01p00901

年代:1990

数据来源: WILEY

摘要:

Wave number spectra from Nimbus 7 temperature‐humidity infrared radiometer 6.7‐μm water vapor data are analyzed using series 4800 km long, in regions free of high clouds and frontal zones. In these regions, the brightness temperatures approximate temperatures on a water vapor isosteric (constant density) surface, rather than averages over a broad vertical layer. Power above the noise can be extracted down to wavelengths of about 60 km. Fitting the power spectrum versus horizontal wave numberkto ak−npower law for wavelengths from 60 to a few hundred kilometers gives slopes ofn= 2.7 to 3.0, depending on the exact wave numbers that are fitted. Thunderstorms and convective cloud systems may constitute an energy source for the reverse energy cascade which produces a −5/3 spectral slope. Our results suggest that when these features are not present, the enstrophy‐cascading process that gives a −3 slope may govern the motion at scales smaller than it has heretofore b

ISSN:0148-0227    

DOI:10.1029/JD095iD01p00909

年代:1990

数据来源: WILEY