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- Author or Editor: Gérard Szejwach x
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Abstract
The use of simultaneous infrared measurements to derive the temperature and emissivity of semi-transparent cirrus clouds is experimentally investigated. Results from the NASA/CONVAIR-990 Winter Experiment Program, 1977 (WEP) are discussed. It is shown that the mean effective emissivities of cirrus in the water vapor absorption channel 5.7–7.1 μm and in the window channel 10.5–12.5 μm are equal to within 6%. A method is then developed to derive the cirrus temperature and emissivity from simultaneous measurements in the two infrared channels. This method is applied to 14 cirrus cloud cases observed during the WEP experiment. The infrared temperature was found to agree with other aircraft or conventional data. A similar technique is then developed and applied to METEOSAT digital images. The results indicate that using both infrared channels should lead to a major improvement in the determination of the cirrus cloud temperature and height from satellites.
Abstract
The use of simultaneous infrared measurements to derive the temperature and emissivity of semi-transparent cirrus clouds is experimentally investigated. Results from the NASA/CONVAIR-990 Winter Experiment Program, 1977 (WEP) are discussed. It is shown that the mean effective emissivities of cirrus in the water vapor absorption channel 5.7–7.1 μm and in the window channel 10.5–12.5 μm are equal to within 6%. A method is then developed to derive the cirrus temperature and emissivity from simultaneous measurements in the two infrared channels. This method is applied to 14 cirrus cloud cases observed during the WEP experiment. The infrared temperature was found to agree with other aircraft or conventional data. A similar technique is then developed and applied to METEOSAT digital images. The results indicate that using both infrared channels should lead to a major improvement in the determination of the cirrus cloud temperature and height from satellites.
Abstract
A method to estimate surface albedo in the African Sahel is proposed and discussed. This method, which uses METEOSAT imagery and routine surface global radiation measurement is shown to be relevant for climatological studies.
The accuracy in the estimated albedos is analysed with respect to the intervening physical parameters. It is shown that a main source of error lies in the estimate of 0.4–1.1 μm radiances from filtered METEOSAT radiances. This problem limits the expected attainable accuracy in albedo to about 10% for typical land surface albedos.
Abstract
A method to estimate surface albedo in the African Sahel is proposed and discussed. This method, which uses METEOSAT imagery and routine surface global radiation measurement is shown to be relevant for climatological studies.
The accuracy in the estimated albedos is analysed with respect to the intervening physical parameters. It is shown that a main source of error lies in the estimate of 0.4–1.1 μm radiances from filtered METEOSAT radiances. This problem limits the expected attainable accuracy in albedo to about 10% for typical land surface albedos.
Abstract
A statistical classification method based on clustering on three-dimensional histograms is applied to the three channels of the METEOSAT imagery [Visible (VIS)–Infrared Window (IR)–Infrared Water Vapor (WV)]. The results of this classification are studied for different cloud cover cases over tropical regions. For high-level cloud classes, it is shown that the bidimensional histogram IR-WV allows one to deduce the cloud top temperature even for semi-transparent clouds.
Abstract
A statistical classification method based on clustering on three-dimensional histograms is applied to the three channels of the METEOSAT imagery [Visible (VIS)–Infrared Window (IR)–Infrared Water Vapor (WV)]. The results of this classification are studied for different cloud cover cases over tropical regions. For high-level cloud classes, it is shown that the bidimensional histogram IR-WV allows one to deduce the cloud top temperature even for semi-transparent clouds.
Abstract
An analysis of the GOES measurements of a severe thunderstorm anvil on 2 May 1979 presented in Part I (Heymsfield et al.) showed a “V” shaped region of low infrared temperatures (TBB ) and an internal region of high TBB . Several hypotheses have been given in the literature (e.g., dynamical and above-anvil cirrus) concerning the formation of the “V” pattern. In this paper, the radiative characteristics of the cirrus are examined as a partial explanation for the IR observations. Calculations are made relevant to the radiative properties using a plane parallel radiative transfer model which shows the sensitivity of TBB to ice water content (IWC), and an ice particle trajectory model which simulates the horizontal ice particle distribution. A variation in the horizontal distribution of IWC is postulated as an explanation for the “V” shaped area and internal warm region. The radiative model calculations support the hypothesis that the higher TBB values in the internal warm region may result from the radiometer seeing down into the anvil layer. The ice particle trajectory model results indicate that the “V” shape can be produced by the ice particle number distribution, where a higher concentration of particles is found in the arms of the “V". Asymmetry of the “V” results from the inclusion of storm motion in the trajectory calculations. Further, the calculated anvil is oriented along the storm relative wind vector in good agreement with the observations. Based on the results of the models, a variation in horizontal distribution of IWC is postulated as a partial explanation for the “V” shaped area and internal warm region. That is, the lower TBB values in the “V” arms are suggested to result in part from the IWC being higher there than that in the internal warm region in the top few kilometers of the anvil. The proposed mechanism may act together with other plausible mechanisms to produce the observed IR pattern. The relative importance of the proposed mechanism cannot be assessed however, given the uncertainties in the observations and models.
Abstract
An analysis of the GOES measurements of a severe thunderstorm anvil on 2 May 1979 presented in Part I (Heymsfield et al.) showed a “V” shaped region of low infrared temperatures (TBB ) and an internal region of high TBB . Several hypotheses have been given in the literature (e.g., dynamical and above-anvil cirrus) concerning the formation of the “V” pattern. In this paper, the radiative characteristics of the cirrus are examined as a partial explanation for the IR observations. Calculations are made relevant to the radiative properties using a plane parallel radiative transfer model which shows the sensitivity of TBB to ice water content (IWC), and an ice particle trajectory model which simulates the horizontal ice particle distribution. A variation in the horizontal distribution of IWC is postulated as an explanation for the “V” shaped area and internal warm region. The radiative model calculations support the hypothesis that the higher TBB values in the internal warm region may result from the radiometer seeing down into the anvil layer. The ice particle trajectory model results indicate that the “V” shape can be produced by the ice particle number distribution, where a higher concentration of particles is found in the arms of the “V". Asymmetry of the “V” results from the inclusion of storm motion in the trajectory calculations. Further, the calculated anvil is oriented along the storm relative wind vector in good agreement with the observations. Based on the results of the models, a variation in horizontal distribution of IWC is postulated as a partial explanation for the “V” shaped area and internal warm region. That is, the lower TBB values in the “V” arms are suggested to result in part from the IWC being higher there than that in the internal warm region in the top few kilometers of the anvil. The proposed mechanism may act together with other plausible mechanisms to produce the observed IR pattern. The relative importance of the proposed mechanism cannot be assessed however, given the uncertainties in the observations and models.
Abstract
Thunderstorm top structure is examined with high spatial resolution radiometric data (visible and infrared) from aircraft overflights together with other storm views, including geosynchronous satellite observations. Results show that overshooting cumuliform towers appear as distinct cold areas in the high resolution 11 μm infrared (IR) aircraft images, but that the geosynchronous satellite observations significantly overestimate the thunderstorm top IR brightness temperature (TB ) due to field-of-view effects. Profiles of cloud top height and TB across overshooting features indicate an adiabatic cloud surface lapse rate. However, one-dimensional cloud model results indicate that when comparing thunderstorm top temperature and height at different times or different storms, a temperature-to-height conversion of ∼7 K km−1 is appropriate.
Examination of mature storm evolution indicates that during periods when the updraft is relatively intense the satellite IR “cold point” is aligned with the low-level radar reflectivity maximum, but during periods of updraft weakening and lowering cloud top heights, the satellite TB minimum occurs downwind with cirrus anvil debris. The growth period of a relatively weak cumulonimbus cluster is also examined with aircraft and satellite data.
Abstract
Thunderstorm top structure is examined with high spatial resolution radiometric data (visible and infrared) from aircraft overflights together with other storm views, including geosynchronous satellite observations. Results show that overshooting cumuliform towers appear as distinct cold areas in the high resolution 11 μm infrared (IR) aircraft images, but that the geosynchronous satellite observations significantly overestimate the thunderstorm top IR brightness temperature (TB ) due to field-of-view effects. Profiles of cloud top height and TB across overshooting features indicate an adiabatic cloud surface lapse rate. However, one-dimensional cloud model results indicate that when comparing thunderstorm top temperature and height at different times or different storms, a temperature-to-height conversion of ∼7 K km−1 is appropriate.
Examination of mature storm evolution indicates that during periods when the updraft is relatively intense the satellite IR “cold point” is aligned with the low-level radar reflectivity maximum, but during periods of updraft weakening and lowering cloud top heights, the satellite TB minimum occurs downwind with cirrus anvil debris. The growth period of a relatively weak cumulonimbus cluster is also examined with aircraft and satellite data.
