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C. PRABHAKARA

Abstract

The high resolution (5 cm−1) measurements of the outgoing infrared energy in the region of the 9.6µ ozone band offer a means of determining the vertical distribution and total amount of ozone in the earth's atmosphere. With the application of radiative transfer theory and perturbation technique a method is developed to deduce such information. The method hinges on a two-parametric representation of the ozone distribution in the earth's atmosphere.

An error analysis based on four case studies is presented to show how well the atmospheric ozone could be determined. It is found that a small error in radiance value is magnified considerably in the inferred atmospheric ozone.

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C. Prabhakara
and
Joseph S. Hogan Jr.

Abstract

Recent high-resolution infrared spectroscopic investigations by Kaplan, Münch and Spinrad (1964) have resulted in estimates of the surface pressure and atmospheric composition of the planet Mars which differ considerably from those previously given. In the light of these findings, the radiative equilibrium temperature structure of the atmosphere of the planet has been re-examined. The absorption of solar energy in the ultraviolet (UV) and visible by O2 and O3 and in the near infrared (IR) by CO2 has been included in the calculation of atmospheric heating.

The transmission functions of CO2 were theoretically calculated making use of a “statistical” model for band absorption. These transmission functions were then used to evaluate the absorption of solar energy in the near IR and to investigate the radiative transfer in the far IR. The theoretical band parameters, involving the line intensity and the mean ratio of line half-width to line spacing, were derived using the transmittance tables of CO2 presented by Stull, Wyatt and Plass (1963).

The basic photochemical theory of O3 production was used to determine a vertical O3 distribution consistent with the radiative equilibrium temperature structure.

The equation of radiative transfer was numerically integrated avoiding the empirical relationships commonly involved in the pressure dependence of CO2 absorption. The IR flux transmittance was also calculated without any simplifying assumptions.

Our approach to the radiative transfer problem was not a time-marching one in which a final solution requires the rate of heating to become zero. Instead, we have treated it as a steady state problem in which we require, at each step, equality between absorbed and emitted energies for all levels.

We have calculated radiative equilibrium temperatures from the surface to the 100-km level. For surface temperatures ranging from 230 K to 270 K, surface pressures from 10 mb to 50 mb, and CO2 amounts from 40 m atm to 70 m atm, the “tropopause” is found at levels below 10 km. Within these limits of surface temperature and pressure and CO2 amounts, the temperature above the tropopause steadily decreases toward a value of ∼155 K in the upper layers. The results indicate definitely that no temperature maximum is produced by the absorption of solar energy in the UV by O3 or in the near IR by CO2 in the Martian atmosphere. The maximum O3 number density is found at the surface of Mars with a gradual decrease upward. The total amount of O3 present is about one-tenth of the amount found in the Earth's atmosphere (∼0.3 cm atm). The total UV energy absorbed in the Martian atmosphere by O2 and O3 is comparable to the near IR energy absorbed by CO2. However, the vertical distribution of absorbed energy shows that, below ∼30 km, O2 and O3 absorption is comparable to CO2 absorption, while above this level CO2 absorption becomes considerably larger.

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C. Prabhakara
,
G. Dalu
,
R. C. Lo
, and
N. R. Nath

Abstract

From the depth of the water vapor spectral lines in the 8–9 μm window region, measured by the Nimbus 4 Infrared Interferometer Spectrometer (IRIS) with a resolution of about 3 cm−1, the precipitable water vapor w over the oceans is remotely sensed. In addition the IRIS spectral data in the 11–13 μm window region have been used to derive the sea surface temperature (SST). Seasonal maps of w on the oceans deduced from the spectral data reveal the dynamical influence of the large-scale atmospheric circulation. With the help of a model for the vertical distribution of water vapor, the configuration of the atmospheric boundary layer over the oceans can be inferred from these remotely sensed w and SST. The gross seasonal mean structure of the boundary layer inferred in this fashion reveals the broad areas of trade wind inversion and the convectively active areas such as the ITCZ. The derived information is in reasonable agreement with some observed climatological patterns over the oceans.

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C. Prabhakara
,
H. D. Chang
, and
A. T. C. Chang

Abstract

Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) brightness temperature measurements in the 21 and 18 GHz channels are used to sense the precipitable water in the atmosphere over oceans. The difference in the brightness temperature (T 21T 18), both in the horizontal and vertical polarization, is found to be essentially a function of the precipitable water in the atmosphere. An equation, based on the physical considerations of the radiative transfer in the microwave region, is developed to relate the precipitable water to (T 21T 18). It is shown from theoretical calculations that the signal (T 21T 18) does not suffer severely from the noise introduced by variations in sea surface temperature, surface winds and liquid water content in non-raining clouds. The rms deviation between the estimated precipitable water from SMMR data and that given by the closely coincident ship radiosondes is about 0.25 g cm−2.

Global maps of precipitable water over oceans derived from SMMR data reveal several salient features associated with ocean currents and the large-scale general circulation in the atmosphere.

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C. Prabhakara
,
I. Wang
,
A. T. C. Chang
, and
P. Gloersen

Abstract

The Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) brightness temperature measurements over the global oceans have been examined with the help of statistical and empirical techniques. Such analyses show that zonal averages of brightness temperature measured by SMMR, over the oceans, on a large scale are primarily influenced by the water vapor in the atmosphere. Liquid water in the clouds and rain, which has a much smaller spatial and temporal scale, contributes substantially to the variability of the SMMR measurements within the latitudinal zones. The surface wind not only increase the surface emissivity but through its interactions with the atmosphere produces correlations, in the SMMR brightness temperature data, that have significant meteorological implications. It is found that a simple meteorological model can explain the general characteristics of these data. With the help of this model, methods are developed for investigation of surface temperature, liquid water content in the atmosphere, and surface wind speed over the global oceans. Monthly mean estimates of the sea surface temperature and surface winds are compared with ship measurements. Estimates of liquid water content in the atmosphere are consistent with earlier satellite measurements.

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C. Prabhakara
,
Jung-Moon Yoo
,
Giuseppe Dalu
, and
R. S. Fraser

Abstract

The spectral data obtained by the Infrared Interferometer Spectrometer (IRIS) flown on Nimbus 4 satellite in 1970 indicated the existence of optically thin ice clouds in the upper troposphere that probably extended into lower stratosphere, in the polar regions, during winter and early spring. The spectral features of these clouds differ somewhat from that of the optically thin cirrus clouds in the tropics. From theoretical simulation of the infrared spectra in the 8–25 μm region, we infer that these polar clouds have a vertical stratification in particle size, with larger particles (∼12 μm) in the bottom of the cloud and smaller ones (≲1 μm) aloft. Radiative transfer calculations also suggest that the equivalent ice-water content of these polar clouds is of the order of 1 mg cm−2.

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C. Prabhakara
,
David A. Short
, and
Bruce E. Vollmer

Abstract

Atmospheric water vapor over the global oceans is remotely sensed from the Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) measurements for about five years—January 1979 to September 1983. Based on the data for three years, 1979–81, preceding the recent El Niño event, we have derived monthly and seasonal mean maps of the water vapor over the oceans. These seasonal mean maps show five prominent features in the water vapor distribution throughout the year with some seasonal changes. Four of these features are associated with the subtropical anticyclones in the Pacific and Atlantic Oceans in the Northern and Southern Hemispheres. The fifth feature is apparently associated with the Walker circulation in the tropical Pacific Ocean. This feature is a cell of maximum values in the water vapor stretched along the equator from about 60°E to the dateline. Interannual variability of the water vapor over the oceans occurs mostly on the peripheries of these atmospheric circulation features.

Anomalies, with respect to the three year monthly mean, in the water vapor distribution over the oceans during the 1982–83 El Niño event lead us to infer significant changes in the circulation of the lower layers of the atmosphere at that time. The intense phase of the El Niño is accompanied by well-organized subsidence to the west, north, and south of the convectively active zone that is over the near-equatorial regions of the central Pacific. As the convective zone intensifies and moves eastward the associated subsidence intensifies and moves with it. These observations imply, in a simplistic sense, that the “Walker circulation” that is normally present in the equatorial Pacific region appears to be reversed at the time of the intense phase of El Niño. It appears that the decay phase of El Niño is marked by decoupling of the subsidence zone from the convective zone.

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C. Prabhakara
,
G. Dalu
,
G. L. Liberti
,
J. J. Nucciarone
, and
R. Suhasini

Abstract

Passive microwave measurements made by the Scanning Multichannel Microwave Radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I) reveal information about rain and precipitation-sized ice in the field of view (FOV) of the instruments. The brightness temperature T b measured at 37 GHZ, having an FOV of about 30 km, shows relatively strong emission from rain and only marginal effects caused by scattering by ice above the rain clouds. At frequencies below 37 GHz, where the FOV is larger and the volume extinction coefficient is weaker, it is found that the observations made by these radiometers do not yield appreciable additional information about rain. At 85 GHz (FOV ≈ 15 km), where the volume extinction coefficient is considerably larger, direct information about rain below the clouds is generally masked.

Based on the above considerations, 37-GHz observations with a 30-kin FOV from SMMR and SSM/I are selected for the purpose of rain-rate retrieval over oceans. An empirical method is developed to estimate the rain rate in which it is assumed that over an oceanic area the statistics of the observed T b 's at 37 GHz in a rain storm are related to the rain-rate statistics in that storm. Also, in this method, the underestimation of rain rate, arising from the inability of the radiometer to respond sensitively to rain rate above a given threshold, is rectified with the aid of two parameters that depend on the total water vapor content in the atmosphere. The rain rates retrieved by this method compare favorably with radar observation. Monthly mean global maps of rain derived from this technique over the oceans are consistent with climatology.

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C. Prabhakara
,
D. A. Short
,
W. Wiscombe
,
R. S. Fraser
, and
B. E. Vollmer

Abstract

Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) measurements at five frequencies in the region 6.6 to 37 GHz, at a resolution of 155 km, are analyzed to infer precipitation over the global oceans. The microwave data show, on this spatial scale, that the combined liquid water in the clouds and rain increases the brightness temperature almost linearly with frequency in the 6.6 to 18 GHz region, while at 37 GHz such a simple relationship is not noticed. Further, as the atmospheric water vapor absorption and the effects of scattering by precipitation particles are relatively weak at 6.6 and 10.7 GHz, a technique to remotely sense the liquid water content in the atmosphere is developed based on the brightness measurements at these two frequencies. Seasonal mean patterns of liquid water content in the atmosphere derived from SMMR over global oceans relate closely to climatological patterns of precipitation. Based on this, an empirical relationship is derived to estimate precipitation over the global oceans, with an accuracy of about ±30 percent, on a seasonal basis from satellite measurements made during the three years (1979–81) before the recent El Niño event. The deviations from these three-year means in the precipitation, produced by the 1982–83 El Niño event are then deduced from the SMMR measurements. In the Pacific one notices from these deviations that the precipitation over the ITCZ in the north, the South Pacific Convergence Zone, and the oceans around Indonesia is drastically reduced. At the same time a substantial increase in precipitation is observed over the normally dry central and eastern equatorial Pacific Ocean.

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