C. Prabhakara

Global warming: Evidence from satellite observations

Observations made in Channel 2 (53.74 GHz) of the Microwave Sounding Unit (MSU) radiometer, flown on-board sequential, sun-synchronous, polar-orbiting NOAA operational satellites, indicate that the mean temperature of the atmosphere over the globe increased during the period 1980 to 1999. In this study, we have minimized systematic errors in the time series introduced by satellite orbital drift in an objective manner. This is done with the help of the onboard warm-blackbody temperature, which is used in the calibration of the MSU radiometer. The corrected MSU Channel 2 observations of the NOAA satellite series reveal that the vertically-weighted global-mean temperature of the atmosphere, with a peak weight near the mid troposphere, warmed at the rate of 0.13±0.05 Kdecade−1 during 1980 to 1999. The global warming deduced from conventional meteorological data that have been corrected for urbanization effects agrees reasonably with this satellite-deduced result.

Rainfall Estimation over Oceans from SMMR and SSM/I Microwave Data

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 Tb 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 devel...

Global warming deduced from MSU

Microwave Sounding Unit (MSU) radiometer observations in Channel 2 (53.74 GHz) made from sequential, sun-synchronous, polar-orbiting NOAA operational satellites have been used to derive global temperature trend for the period 1980 to 1996. Christy et al. (1998) emphasize that they find a tropospheric cooling trend (−0.046 K decade−1) from 1979 to 1997 with these MSU data, although their analysis of near nadir measurements yields a near zero trend (0.003 K decade−1). Using an independent method to analyze the MSU Ch 2 nadir data separately over global ocean and land, we infer that the temperature trends over both these regions are about 0.11 K decade−1, during the period 1980 to 1996. This result is in better agreement with trend analyses based on conventional surface data.

Deep optically thin cirrus clouds in the polar regions. I, Infrared extinction characteristics

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.

TRMM Precipitation Radar and Microwave Imager Observations of Convective and Stratiform Rain Over Land and Their Theoretical Implications

Observations of brightness temperature, Tb, made over land regions by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) radiometer are analyzed with the help of nearly simultaneous measurements of the vertical profiles of reflectivity factor, Z, made by the Precipitation Radar (PR) onboard the TRMM satellite. Furthermore, this analysis is done separately over convective and stratiform rain regions. This examination reveals a clear relationship between TMI and PR data. Possible explanation for this relationship is explored with the help of radiative transfer calculations. With this approach, we demonstrate that the 85 GHz observations of TMI can be simulated crudely from the observations of Z. However, the 37 and 19 GHz observations are not as well simulated, possibly because of horizontal non-uniformity in the hydrometeor distribution in the broad footprints of these channels and contamination introduced by land-surface emissivity. On the other hand, from TMI and PR observations, we find that the brightness temperature difference (T19-T37) minimizes these sources of error. Our simulations of (T19-T37) over convective rain regions are in reasonable agreement with this finding. This investigation indicates that the TMI 85 GHz channel yields the best information about rain over tropical land, because it has minimal surface contamination, strong extinction, and a fine footprint. The brightness temperature difference (T19-T37) can supplement the information given by the 85 GHz channel.

Rainfall over oceans: Remote sensing from satellite microwave radiometers

SummaryMicrowave radiometer brightness temperature (Tb) measurements obtained from satellites over the oceans in dual polarization, at frequencies ranging from 6.6 to 85 GHz, reveal information about the rain and precipitation sized ice. These multifrequency measurements are composited from observations made by the Scanning Multichannel Microwave Radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I). TheTb measurements at 37 GHz, having a field of view (fov) of about 30 km, show relatively strong emissions due to rain, reaching values as large as 260 K over the tropical and mid-latitude rainbelts. Only marginal effects due to scattering by ice above the rain clouds are revealed. At frequencies below 37 GHz, where the fov is much larger than 30 km and the extinction is weaker,Tb is significantly smaller than 260 K. Additional information content about rain, at these low frequencies, is not appreciable. On the other hand, at 85 GHz (fov ≅15 km), where the extinction is very strong, the sea surface below the clouds is often masked and scattering due to ice above the rain clouds is vividly noticed. However, these high frequency measurements do not yield direct information about rain below the clouds.Recognizing the above merits inherent in the 37 GHz observations the SMMR and SSM/I data at this frequency are utilized to develop and empirical method to retrieve rain rate over oceans. In this method it is assumed that over an oceanic area, the statistics of the observedTb must be derivable from the statistics of the corresponding rain rates. Furthermore, the underestimation of rain rate, arising from the inability of the radiometer to respond sensitively to rain above a given threshold is empirically rectified with the help of two parameters that depend on the total water vapor content in the atmosphere. Rain rates deduced over the oceans around Japan using the SSM/I data, when compared with those measured by radars that are calibrated against rain gauges, show a good correlation; there is, however, a systematic overestimation. Seasonal mean maps of the rainfall over the global oceans based on SMMR data compare favorably with climatological rain maps over the Atlantic and Pacific Oceans developed by Dorman and Bourke (1979, 1981).

TRMM Observations of Polarization Difference in 85 GHz : Information about Hydrometeors and Rain Rate

Observations made by the Precipitation Radar (PR) and the Microwave Imager (TMI) radiometer on board the Tropical Rainfall Measuring Mission (TRMM) satellite help us to show the significance of the 85 GHz polarization difference, PD85, measured by TMI. Rain type, convective or stratiform, deduced from the PR allows us to infer that PD85 is generally positive in stratiform rain clouds, while PD85 can be markedly negative in deep convective rain clouds. Furthermore, PD85 increases in a gross manner as stratiform rain rate increases. On the contrary, in a crude fashion PD85 decreases as convective rain rate increases. From the observations of TMI and PR, we find that PD85 is a weak indicator of rain rate. Utilizing information from existing polarimetric radar studies, we infer that negative values of PD85 are likely associated with vertically-oriented small oblate or wet hail that are found in deep convective updrafts.

Global warming estimation from MSU

In this study, we develop an independent method to derive global temperature trend from Microwave Sounding Unit (MSU) radiometer observations in Ch 2, made from sequential, sun- synchronous, polar-orbiting NOAA operational satellites. Also, a detailed examination of the systematic errors in these data is performed with the objective to improve this method. Partitioning these data from 75 N to 75 S into global land and ocean sets, and further subdividing them into AM and PM subsets with the help of the LECT, enables us to perform this examination. The systematic errors in the MSU Ch 2 data are mainly related to differences in the MSU instrument calibration and the Local Equatorial Crossing Times (LECT) of successive satellites. They can be removed from these data with the help of the overlapping observations made by successive satellites. Errors in the MSU Ch 2 data are also introduced by orbital drift, which is the progressive change in the LECT of a satellite. Changes of the AM and PM observation times due to orbital drift causes Ch 2 brightness temperatures from each satellite to be affected by the diurnal cycle. In addition, orbital drift alters satellite illumination by sun, and thereby the instrument calibration. These errors introduced by orbital drift cannot be eliminated objectively. However, in this study, the uncertainty in the global temperature trend resulting from the cumulative error generated by drifts of all the satellites is inferred with an indirect approach. Based on our method of analysis of the MSU Ch 2 data, we find a global temperature trend from 1980 to 1996 of 0.11 K decade -1 with an uncertainty of 0.06 K decade -1.

Beam-filling effect in the microwave remote sensing of precipitation

The passive microwave radiometers have a field of view that is much larger than the size of the raining cells, and within the rain cell the brightness temperature can reach saturation. These problems are generally referred to as the beam filling effect (BFE). We developed a technique to retrieve rain rates from the Special Sensor Microwave/Imager (SSM/I) data, which inherently incorporates the BFE. However an overestimation of rain rates was evident when the data from the Global Precipitation Climatology Project (GPCP) program became available. The procedure followed to obtain the algorithm was revised and modified, and new coefficients were calculated. This modified algorithm was tested by comparing the SSM/I derived rainfall with the corresponding values obtained by radar and rain gauges in the framework of the GPCP program. This comparison reveals two separate clusters of points, which can be related to the fact that the BFE is larger for convective precipitating clouds and smaller for stratiform clouds. An examination of the data at 19 and 85 GHz gives a clue to discriminate between these two types of rain. Then two different algorithms are proposed for the retrieval of rain from microwave data.

Rainfall estimation over the oceans from 37 GHz radiometric measurements

SummaryAn improved algorithm to estimate rainfall over the oceans, utilising the observations made by the Special Sensor Microwave/Imager (SSM/I) radiometer at 37 GHz, is developed. The algorithm is based on an earlier analysis that included the satellite microwave data from SSM/I and the Scanning Multichannel Microwave Radiometer (SMMR), and the radar data obtained during the GATE experiment. The modified algorithm was tested by comparing the SSM/I-derived rainfall with the corresponding values obtained by radar and rain gauges in the framework of the GPCP programme, over the seas around Japan. While good agreement of ±25% is obtained in this test, the analysis of the results suggests that further improvements are possible. An attempt to separate rain originated from convective and stratiform clouds is made, based on multichannel measurements obtained from SSM/I.