摘要:

Knowledge of air mass is vital for the interpretation of twilight measurements of trace gases, as well as the conversion of measured slant column amounts to vertical abundances for comparison with model predictions. Radiative transfer computations were used to determine NO2air mass values for clear skies at 450 and 650 nm using a discrete ordinate (two different formulations), Monte Carlo, and an integral equation method. All four methods yielded agreement to within 6% at a solar zenith angle of 90° when the absorber was located in the stratosphere. For a tropospheric absorber, differences as large as 21% occurred at 90°. Since only the Monte Carlo method treats the scattered radiation in spherical geometry, it is more accurate for computing tropospheric air masses where multiple scattering is significant. The other three models use a conceptual approximation by treating the scattered radiation in plane parallel geometry. However, for absorbers in the stratosphere, major saving of computing time without any loss of accuracy is obtained using the discrete ordinate or integral equation method as compared to the Monte Carlo metho

ISSN:0148-0227    

DOI:10.1029/96JD00054

年代:1996

数据来源: WILEY

摘要:

A 90–150 day signal is identified in the global precipitable water field generated by the Global Data Assimilation Systems (GDAS) of the Goddard Laboratory for Atmospheres (GLA), National Meteorological Center, and European Centre for Medium‐Range Weather Forecasts. The finding of this intraseasonal signal in global precipitable water is significant for two reasons: (1) it suggests that there is 90–150 day intraseasonal variability in the atmospheric branch of the global hydrological cycle and (2) it provides a useful parameter to test the sensitivity of the GDAS‐generated hydrological data. This newly identified intraseasonal signal in the global precipitable water was verified with Special Sensor Microwave/Imager precipitable water data over oceans and station‐mixing ratio data over the continental United States. Based upon some simple statistical analyses and global and regional composite charts, it was found that the 90–150 day low‐frequency oscillations contained in different GDAS data sets are more coherent with each other in regions with good data coverage but are poorly correlated over the data‐sparse areas. Furthermore, the GLA GDAS provides the most realistic representation of this intraseasonal global precipitab

ISSN:0148-0227    

DOI:10.1029/95JD02369

年代:1996

数据来源: WILEY

摘要:

This study presents an analytical expression, derived from radiative theory, for relating water vapor radiances to layer‐average relative humidities. This “radiance‐to‐humidity transformation” provides a simple, yet reliable, means of interpreting satellite observations of the upwelling radiation in the 6.3‐μm water vapor absorption band in terms of a more familiar water vapor quantity. Despite its simplicity, when compared to detailed radiative transfer calculations of the upper (6.7 μm) tropospheric water vapor radiance, the transformation is demonstrated to be accurate to within ∼1 K. Similar levels of accuracy are found when the transformation is compared to detailed calculations of the middle (7.3 μm) and lower (8.3 μm) tropospheric water vapor radiance, provided that the emission from the underlying surface is taken into account. On the basis of these results, the radiance‐to‐humidity transformation is used to interpret TIROS operational vertical sounder observed water vapor radiances in terms of the relative humidity averaged over deep layers of the upper, middle, and lower troposphere. We then present near‐global maps of the geographic distribution and climatological variations of upper, middle, and lower‐tropospheric humidity for the period 1981–1991. These maps clearly depict the role of the large‐scale circulation in regulating the location and temporal variatio

ISSN:0148-0227    

DOI:10.1029/96JD00280

年代:1996

数据来源: WILEY

摘要:

Four atmospheric transmittance models, LOWTRAN 7, MODTRAN 3, FASCOD3P, and the Thomason model, are investigated to quantify the relationship between water vapor transmittance as function of water vapor amount,Tw(U), for an instrument specific band pass in the 0.94‐μm region. In a second step an empiricalTw(U) function is established using long term measurements with our high‐precision Sun photometer (SPM) in Bern, Switzerland along with 1300 simultaneous and collocated water vapor retrievals performed with a dual‐channel microwave radiometer (MWR). In order to avoid a possible bias in the empiricalTw(U) function, the MWR data set is prescreened by comparing retrievals coincident with radiosonde ascents. Over a 2½‐year period of common observations, radiosondes and SPM agreed to within 0.19 cm (13%) of columnar water vapor (CWV) using the empiricalTw(U) relationship. Completely independent comparisons with an additional MWR and two Fourier transform spectrometers yielded agreement within 13% and 9%, respectively. Comparing empirical and modeled results, we found that with respect to the experimental data, LOWTRAN 7, MODTRAN 3, and FASCOD3P reported higher water vapor transmittances over almost the entire range of realistic absorber amounts. By relating these differences to differences in retrieved CWV for the case of two standard atmospheres, we found that usingTw(U) predicted by LOWTRAN 7, MODTRAN 3, and FASCOD3P leads to an overestimate of CWV by about 18–30%, 7–20%, and 2–18%, respectively. The Thomason model yields good agreement with respect to the experimental data up to medium absorber amounts, whereas at slant path amounts larger than 10 cm, errors up to 60% in retrieved CWV occurred. We also show in this work that a misinterpretation of the LOWTRAN 7 water vapor output counterbalances incorrectly predictedTw, leading to results that agree well with ex

ISSN:0148-0227    

DOI:10.1029/96JD00337

年代:1996

数据来源: WILEY

摘要:

A dynamic‐thermodynamic sea ice model is developed and coupled with the “21 wave 9 level” Melbourne University general circulation model to simulate the seasonal cycle of the global sea ice distribution. We have run the coupled system and obtained a creditable seasonal simulation of global sea ice. When monthly averaged atmospheric data (taken from the mean of the coupled run) are used to force the sea ice model, the seasonal cycle of sea ice extent (to the outer ice edge) is quite similar to that simulated in the interactive run. However, the actual sea ice covered area (i.e., excluding leads) differs considerably between the two simulations. Sea ice is more compact in the monthly averaged forced run than in the interactive run throughout the year in both hemispheres. The sea ice thickness distribution also differs between the two runs. In general, the sea ice is more open and thicker in the seasonal ice zone of the two polar regions for the interactive coupled case than for the mean forcing. We have also run the model forced with daily atmospheric data and the simulated sea ice distribution differs significantly from both the interactive model and the monthly averaged forcing results. These differences highlight the dangers of undertaking studies with sea ice models forced with prescribed atmospheric conditions rather than using a fully interactive atmosphere‐sea ice

ISSN:0148-0227    

DOI:10.1029/95JD03633

年代:1996

数据来源: WILEY

摘要:

Aerosol concentrations and size distributions in the middle and upper troposphere over the remote Pacific Ocean were measured with a forward scattering spectrometer probe (FSSP) on the NASA DC‐8 aircraft during NASA's Global Backscatter Experiment (GLOBE) in May–June 1990. The FSSP size channels were recalibrated based on refractive index estimates from flight‐level aerosol volatility measurements with a collocated laser optical particle counter (LOPC). The recalibrated FSSP size distributions were averaged over 100‐s intervals, fitted with lognormal distributions and used to calculate aerosol backscatter coefficients at selected wavelengths. The FSSP‐derived backscatter estimates were averaged over 300‐s intervals to reduce large random fluctuations. The smoothed FSSP aerosol backscatter coefficients were then compared with LOPC‐derived backscatter values and with backscatter measured at or near flight level from four lidar systems operating at 0.53, 1.06, 9.11, 9.25, and 10.59 μm. Agreement between FSSP‐derived and lidar‐measured backscatter was generally best at flight level in homogeneous aerosol fields and at high backscatter values. FSSP data often underestimated low backscatter values especially at the longer wavelengths due to poor counting statistics for larger particles (>0.8 μm diameter) that usually dominate aerosol backscatter at these wavelengths. FSSP data also underestimated backscatter at shorter wavelengths when particles smaller than the FSSP lower cutoff diameter (0.35 μm) made significant contributions to t

ISSN:0148-0227    

DOI:10.1029/95JD03423

年代:1996

数据来源: WILEY

摘要:

Multiresolution feature analysis has been applied to data from NOAA's Phoenix II dual Doppler radar observations of a convective boundary layer near Boulder, Colorado, during June 1984 and to large eddy simulation (LES) results (using the regional atmospheric modeling system (RAMS) developed at Colorado State University) corresponding to a subset of that data. The data and LES results provide winds on a three‐dimensional grid with spacing of ∼200 m to a depth of about 2 km over a 9 × 9 km square for the observations. LES results were for an elongated (in the east‐west direction) volume. The prevailing circulation maintained a strong shear (synoptic westerlies aloft above upslope easterlies at the surface) despite strong afternoon heating. The analysis defined preferred motion patterns (for both the observed and the simulated data) over 3 × 3 × 3 grid points using empirical orthogonal functions. In the case of the observations the patterns strengthened or weakened the shear locally for the most stable cases, but other patterns became relatively more important with increasing convection. Among these were a vortical pattern tilted in the shear direction. Differences in the peak intensity statistics for two different smoothings were used to estimate their support dimension from the observed data during moderately unstable conditions; most fell between 2.2 and 2.6. When the LES results corresponding to a moderately unstable observed atmosphere were analyzed, they showed that the preferred motion patterns were more like the observed stable case than the unstable cases, and the support dimension estimates were all greater than 2.6, indicating less intermittency than was

ISSN:0148-0227    

DOI:10.1029/96JD00094

年代:1996

数据来源: WILEY

摘要:

Since cross‐isentropic mass fluxes are directly related to net diabatic heating, mass fluxes into the “overworld” across the 385 K and 444 K isentropes are calculated using a comprehensive radiative transfer code. Ozone, methane, water vapor, and nitrous oxide data from instruments aboard the Upper Atmospheric Research Satellite (UARS) and temperature data from United Kingdom Meteorological Office (UKMO) are used for this study. Water vapor budget for the stratosphere is calculated as an independent check on the validity of the calculated mass exchange rate. The total upward mass flux is found to be strongest in northern winter and weakest in northern summer, with an annual mean flux of 73 and 119 × 108kg/s across the 444 K and 385 K isentropes, respectively, giving a turnover time of 1.6 years for the ove

ISSN:0148-0227    

DOI:10.1029/96JD00057

年代:1996

数据来源: WILEY

摘要:

We study in this paper characteristics of both wind velocity and temperature fluctuations due to a 2‐day wave near the mesopause, by means of meteor echo measurements with the MU (middle and upper atmosphere) radar in Shigaraki, Japan (35°N, 136°E) and a meteor wind radar, newly constructed near Jakarta, Indonesia (6°S, 107°E). A 2‐day wave was conspicuously enhanced on July 12–19, 1993, at both Shigaraki and Jakarta. Fluctuations of temperature and northward wind were inphase in Shigaraki, and they became antiphase in Jakarta, consistent with a prediction assuming a Rossby‐gravity wave. The phase lag of the fluctuations between the two sites suggested that this wave propagated westward with a zonal wavenumber of 3. Seasonal variations of the 2‐day wave in Jakarta were examined by using a frequency spectrum and complex demodulation technique for temperature and wind fields, finding clear enhancements during periods corresponding to summer months in each hemisphere, especially in January 1993. The dominant wave period was fairly stable and was confined in a narrow range centered around 47 and 49 hours during summer months of the southern and northern hemispheres, respectively. The phase variations of temperature preceded those for the northward wind by 180°–225° when an enhanced 2‐day wave was seen in the northward component, indicating their antiphase relation in the southern hemisphere. These results strongly support the interpretation of the 2‐day wave near the mesopause as the (3,0) mode of a Rossby‐gravity wave wit

ISSN:0148-0227    

DOI:10.1029/95JD03579

年代:1996

数据来源: WILEY

摘要:

Timescales for mixing of midlatitude air into the tropical lower stratosphere are deduced from observations of long‐lived tracers N2O and CCl3F. Bulk mixing between tropical and midlatitude regions is assumed to be isentropic and relatively slow compared with local mixing within each region. The mean value of the mixing timescale ranges from 12 to 18 months near 20 km. There is a tendency for shorter mixing times at higher and lower altitudes, although vertical profiles of mixing cannot be definitively established by the data. A more robust quantity is given by the fraction of midlatitude air entrained into the tropical upwelling region. Implied mixing fractions exceed 50% above 22 k

ISSN:0148-0227    

DOI:10.1029/96JD00335

年代:1996

数据来源: WILEY