摘要:

We present data obtained during more than 3 years of nearly continuous measurements of middle atmospheric water vapor. The data are obtained from ground‐based measurements at 22 GHz taken at two sites, one in each hemisphere, using the Naval Research Laboratory water vapor millimeter‐wave spectrometer (WVMS). With the construction of a second instrument, it has been possible to maintain continuous monitoring from both sites since January 1994. The measurements from both instruments show significant seasonal variability. There is a clear annual cycle, with the water vapor above ∼60 km increasing in summer and decreasing in winter. The observed amplitude of the annual oscillation is larger at 45.0°S than at 34.4°N, a result which is qualitatively consistent with the higher latitude of the southern hemisphere site. There is also an indication of a semiannual cycle, particularly at altitudes near 80 km. The annual cycle is consistent with transport due primarily to advection, while the weaker semiannual cycle may be indicative of the effect of gravity waves on diffusive tr

ISSN:0148-0227    

DOI:10.1029/96JD01741

年代:1996

数据来源: WILEY

摘要:

The interaction of a gravity wave with a solar tide is analyzed using ray theory in order to assess whether the temporal oscillation of the tide has any significant effects on the interaction. We consider two types of solution: a “full ray solution,” in which tidal accelerations are included in the gravity wave ray‐tracing equations, and a second “Lindzen solution,” in which they are neglected; the latter is so named because it yields similar results to the parameterization ofLindzen[1981]. Initially, we consider an idealized tide of constant velocity amplitude in a steady isothermal atmosphere, as this allows analytical solutions to be derived. A numerical ray‐tracing code is employed to determine ray solutions within more complex tidal backgrounds. Full ray solutions often differ markedly in amplitude, wavenumber, and trajectory from Lindzen solutions for the same wave, highlighting the importance of tidal accelerations. Tidal accelerations have a stabilizing influence on gravity wave amplitudes by refracting waves to larger intrinsic phase speeds, thus reducing both the occurrence and intensity of tidally modulated gravity wave breaking. Even in the absence of dissipation, time‐varying refraction gives rise to time‐oscillating gravity wave action densities, ∂A/∂t, which in turn lead to wave momentum‐flux densities which are anticorrelated with tidal winds in agreement with mesospheric observations. Although ∂A/∂taverages to zero over one cycle of a constant‐amplitude tide, its phase locking to the tide implies transient flow accelerations which temporarily amplify tidal wind oscillations and might permanently change tidal amplitudes in a nonlinear interactive model. Numerical simulations through more complex tidal backgrounds show instances where permanent ∂A/∂tvalues are induced in a gravity wave through its interaction with the tide. Our results point to weaknesses in a Lindzen parameterization of gravity wave‐tidal interactions. A simple extension of the parameterization is suggested which incorporates some

ISSN:0148-0227    

DOI:10.1029/96JD01660

年代:1996

数据来源: WILEY

摘要:

A steady state two‐dimensional linearized model that extends from the ground into the thermosphere and captures the salient features of migrating diurnal and semidiurnal tidal components is used to investigate the comparative importance of the principal sources of these waves. The results, which have previously gone unreported in the literature, demonstrate the nonnegligible effects of atmospheric absorption of solar radiation at infrared (in the troposphere) and ultraviolet (in the stratosphere) wavelengths on mesospheric and lower thermospheric semidiurnal and diurnal tidal fields, respectively. In addition, latent heat release associated with cloudiness or rainfall in the troposphere is shown to be another plausible source of semidiurnal variability in the upper atmosphere. The important effects of these sources on the dynamics of the mesosphere and lower thermosphere emphasize the need to include realistic parameterizations of migrating tides at the lower boundaries of middle and upper atmospheric general circulation models. The results of this investigation also suggest that updated parameterizations of tropospheric tidal forcing are needed to further current understanding of tidal variability in the upper atmospher

ISSN:0148-0227    

DOI:10.1029/96JD01374

年代:1996

数据来源: WILEY

摘要:

From about 100 nights of ground‐based measurements of fluctuations in airglow brightness and rotational temperature, taken from two sites at 32°S and 37°N, a considerable number of new determinations of complex values of Krassovsky's η are derived, and compared with observations by other investigators and model predictions. Our findings support the model of Hines, Tarasick, and Shepherd (HTS), which leads to practical consequences regarding the usefulness of airglow observations for deriving vertical propagation of atmospheric waves in the mesopause region. It is shown, at least in part, that the vertical wave propagation can be inferred from zenith observations of one airglow emission, alone, and that consistent information can be obtained simultaneously for the two airglow layers. The analysis presented comprises the range of observed periods between 3 and 24 hours. The salient feature is that η values are essentially limited to the fourth quadrant, for both the O2and OH emissions, which means, according to the HTS model, that the majority of the waves observed propagate upward, with vertical wavelengths between 20 and 60 km, and only a few are possibly evanescent. This would not contradict the interpretation that most if not all of the wave signatures may be due to the semidiurnal tide, or tidal trans

ISSN:0148-0227    

DOI:10.1029/96JD01723

年代:1996

数据来源: WILEY

摘要:

The annual cycles in height and temperature of the tropopause over the tropical western Pacific island of Truk (7.5°N, 151.8°E) are derived for the period 1980–1988 from daily radiosonde profiles. Earlier work has shown that tropopause properties are fairly uniform over large areas of the tropics, so that conclusions drawn from the Truk observations can be taken as representative of the western Pacific warm pool region, and can be applied with caution to wider regions of the tropics. The relative roles of tropical convection and extratropical wave driving in determining the tropopause properties are discussed, and it is suggested that the chief role of wave driving is to lower the temperature in the uppermost part of the troposphere during the northern winter months, causing a decrease in local stability, and allowing penetrative overshooting of convective turrets to take place more readily than during the rest of the year. As the winter progresses, the overshooting convective turrets create widespread cirrus anvils, incorporating stratospheric air with a high potential temperature, and establishing a new higher tropopause. Anomalously high tropopause potential temperatures during the northern spring months are tentatively attributed to the combined wave‐driving action of the two hemispheres, as wave activity increases with the approach of winter in the southern hemisphere, but still remains at a high level during northern spring. The day‐to‐day variability in tropopause height and temperature is discussed, and is shown to have implications for troposphere‐stratosphere exchange. The use of monthly mean tropopause temperatures to calculate the saturation vapor pressure of water entering the stratosphere may give misleading results, since injection is likely to take place primarily in association with the transient rises in tropopause height and decreases in temperature that occur more or less randomly from day to day, while the monthly means also include the drops in height and increases in temperature that do not represent injections into the s

ISSN:0148-0227    

DOI:10.1029/96JD01622

年代:1996

数据来源: WILEY

摘要:

The Earth Radiation Budget Experiment (ERBE) used angular direction models (ADMs) to convert satellite scanning radiometer measurements to radiative fluxes at the top of the atmosphere. The ADMs were assumed to depend only on the physical characteristics of the scene being observed, and they were taken to be independent of the instrument's spatial resolution. As is shown here, however, the angular distribution of the radiation fields derived from ERBE observations depends on the field of view size. Because the spatial resolution of the field of view depends on the satellite view zenith angle, the dependence of the angular distribution on field of view size suggests that the radiative fluxes derived from ERBE have biases which depend on satellite view zenith angle. ERBE scanner observations from the Earth Radiation Budget Satellite (ERBS) are averaged for nadir pixels to construct observations that have a constant field of view size for each of the ERBE satellite view zenith angle bins. The angular distribution of the radiation field for the constant size field of view observations exhibits systematic differences of the order of 2–5% from that obtained with the unaltered, full resolution scanner observations. While the differences are small, they are frequently significant at the 90% confidence level, given the variation in the observations and the sample size (3 months). Reflected radiances constructed from the constant size fields of view are generally more anisotropic than those constructed from the full resolution scanner observations. The population of scene type obtained for the constant size fields of view shows that the ERBE scene identification algorithm has to be modified in order to develop angular direction models which are consistent with the observed anisotropy and also independent of field of view siz

ISSN:0148-0227    

DOI:10.1029/96JD01156

年代:1996

数据来源: WILEY

摘要:

Simple threshold scene identification methods are developed to reduce the effects of errors in scene identification on the anisotropy of reflected and emitted radiances inferred from Earth Radiation Budget Experiment (ERBE) scanner observations. The ERBE maximum likelihood estimate (MLE) scene identification is assumed to be accurate for nadir fields of view. Various combinations of neighboring ERBE scanner fields of view at nadir are used to determine the population of cloud scene types as a function of field of view size. Longwave and shortwave thresholds are then determined for each of the ERBE solar zenith, satellite view zenith, and relative azimuth angular bins so that the population of cloud scene types at a particular satellite view zenith angle is consistent with the field of view size at the particular satellite view zenith angle. Differences between the anisotropy of reflected sunlight and emitted longwave radiation obtained using the new scene identification method and that obtained using the ERBE MLE method show that the ERBE radiative fluxes have satellite view zenith angle dependent biases. Thresholds are also developed for cloud scene identification with fields of view that are constructed to have a constant size with satellite view zenith angle. The angular dependence of reflected sunlight and emitted longwave radiation for scenes identified with these thresholds show little dependence on field of view size. This lack of dependence is a necessary condition for using scanning radiometer data to obtain radiative fluxes.

ISSN:0148-0227    

DOI:10.1029/96JD01157

年代:1996

数据来源: WILEY

摘要:

Collocated measurements from the Advanced Very High Resolution Radiometer (AVHRR) and the Earth Radiation Budget Experiment (ERBE) scanner are used to examine the radiative forcing of atmospheric aerosols generated from biomass burning for 13 images in South America. Using the AVHRR, Local Area Coverage (LAC) data, a new technique based on a combination of spectral and textural measures is developed for detecting these aerosols. Then, the instantaneous shortwave, longwave, and net radiative forcing values are computed from the ERBE instantaneous scanner data. Results for the selected samples from 13 images show that the mean instantaneous net radiative forcing for areas with heavy aerosol loading is about −36 W/m2and that for the optically thin aerosols are about −16 W/m2. These results, although preliminary, provide the first estimates of radiative forcing of atmospheric aerosols from biomass burning using satellite d

ISSN:0148-0227    

DOI:10.1029/96JD02161

年代:1996

数据来源: WILEY

摘要:

To improve the estimation of primary production at a regional scale, an assessment of the utility of fraction of absorbed photosynthetically active radiation (fAPAR) estimated from spectral vegetation indices (VI) for the case of the Sahelian vegetation was studied. Simulations using a three‐dimensional radiative transfer model were conducted for two types of structurally distinct vegetation canopies: millet crop (regularly distributed clumps of vegetation over bare soil) and savanna (mixture of a grass layer and a sparse woody layer). A realistic range of values is extracted for each vegetation input variable (leaf area index, ground cover, height and spatial distribution) from published literature. Bidirectional reflectance factors were calculated in the NOAA‐advanced very high resolution radiometer (AVHRR) spectral bands for a geometric configuration representative of the NOAA satellite series. Two vegetation indices were tested: normalized difference vegetation index (NDVI) and modified soil adjusted vegetation index (MSAVI). The simulations indicate that the fAPAR‐VI relationship is sensitive to the geometry of measurement and soil optical properties, especially in the case of a millet crop. Simple linear models that include this variability and are directly applicable to atmospherically corrected AVHRR data are proposed, and the error of estimation of fAPAR is evaluated. MSAVI gives better results, especially when the brightness of soil is known. For natural vegetation the relationship between fAPAR and MSAVI is then reduced to a coefficient that is a constant (∼2.2). For millet canopies this coefficient is a function of Sun and view angles. Finally, the utility of our approach is illustrated with two examples of fAPAR‐VI relationships applied to a NOAA ti

ISSN:0148-0227    

DOI:10.1029/96JD01621

年代:1996

数据来源: WILEY

摘要:

A new precipitation retrieval scheme for use in the tropics is developed using a combination of visible (VIS)/IR and microwave satellite observations. The new scheme combines the advantages of both the ample coverage of VIS/IR sampling and the physical directness between precipitation and microwave radiances. A VIS/IR algorithm is “trained” using Special Sensor Microwave/Imager (SSM/I) derived values of precipitation. Cloud top temperatures and visible optical depths provided by the International Satellite Cloud Climatology Project (ISCCP) level DX analyses are “binned” by every 10 K and every 10 unit optical depth, respectively. Lookup tables for the probability of rain and mean rainfall rate are constructed for each cloud top temperature/optical depth cell whenever the optical depth is available (daytime). For the nighttime, the tables are based only on cloud top temperature. The instantaneous rainfall rates are obtained by multiplying the mean rainfall rate by the probability of rain, both characterized by the cloud top temperature and/or visible optical depth associated with the satellite pixel. The satellite retrieval always retains results from optimal parameters, which means SSM/I results are used whenever SSM/I samplings are available; otherwise, VIS/IR results are used during daytime, while IR‐only results are used during nighttime. The final precipitation product can have resolution as high as 3 hours in time and about 0.3° in space. However, averaging in space and/or time improves the statistics of the derived rainfall rates, since the statistics of the algorithm are more fully utilized. The satellite‐derived rainfall product compares well with ship rain gauge data in terms of rain/no rain determination, and agrees well with daily rainfall accumulation derived from a large‐scale moisture budget. The satellite retrieved rainfall rates are biased high relative to radar observations. In the absence of satisfactory data with which to validate the satellite‐derived rainfall rates, it can be concluded that there is useful information in the ISCCP data set that relates to precipitation and that the inclusion of visible optical depth information improves the daytime retrieval of precipitation relative to

ISSN:0148-0227    

DOI:10.1029/96JD01771

年代:1996

数据来源: WILEY