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Steven A. Ackerman

Abstract

Global analyses of satellite spectral observations indicate the existence of negative brightness temperature differences between 11 and 6.7 µm (BT11 −BT6.7) when cold scenes are viewed. Differences are typically greater than −5 K for the Tropics and midlatitudes but can be smaller than −15 K over high-altitude polar regions during winter. In July, more than 60% of the observations over the Antarctic Plateau had BT11 − BT6.7 < −5 K. In January, over Greenland, the frequency of occurrence is approximately 20%.

Three factors are investigated that may contribute to these observed negative brightness temperature differences: 1) calibration errors, 2) nonuniform scenes within the field of view, and 3) physical properties of the observed phenomena. Calibration errors and nonuniform scenes may generate values of BT11 − BT6.7 that are less than zero; however, these differences are on the order of −2 K and, therefore, cannot fully explain the observations.

A doubling and adding radiative transfer model is used to investigate the physical explanations of the negative differences. Simulations of satellite spectral observations for thick clouds produce negative differences that are comparable to those observed in the Tropics and midlatitudes. The magnitude of the differences is a function of cloud microphysics, cloud-top pressure, view angle, and the cloud optical thickness. The model simulations are also capable of producing large negative differences over high-altitude polar regions.

Distinguishing clear and cloudy regions from satellite infrared radiances is a challenging problem in polar winter conditions. Brightness temperature differences between 11 and 6.7 µm provide a technique to separate cold, optically thick clouds from clear-sky conditions when strong radiation inversions exist at the surface. The presence of a cloud inhibits the development of this inversion and shields its detection using satellite radiance measurements. While physically reasonable, and in agreement with radiative transfer calculations, this technique has not been verified with ground nor with aircraft observations. Further evidence that the large negative values of BT11 − BT6.7 are associated with surface inversions is presented by comparing the satellite observations with surface temperature measurements from an Antarctica automated weather station.

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Steven A. Ackerman
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John A. Knox
and
Steven A . Ackerman

During 2002 and 2003, surveys of introductory meteorology students were conducted at the University of Georgia and the University of Wisconsin—Madison. These surveys asked which one question about weather and climate each student would most like to have answered in the class, as well as other demographic and educational information. The more than 750 responses that were obtained ran the gamut of meteorology and were not overwhelmingly focused on any one topic, including severe weather. Results from the two universities are nearly identical, with the exception of a greater awareness of climate issues at Wisconsin. Several topics that are most commonly noted by students, such as weather forecasting and atmospheric optics, are given inadequate treatment in many introductory meteorology textbooks and classes. The results of the surveys suggest that an instructor could use students' first-day responses to this kind of question to shape a syllabus that would incorporate student interests, while retaining educational integrity.

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Steven A. Ackerman
and
Stephen K. Cox

Abstract

Techniques for normalizing aircraft measurements of solar irradiance to a horizontal surface and a constant solar zenith angle are outlined. The effects of these normalization procedures are a minimum when the data are collected at small solar zenith angles. A method of analysis is discussed which takes into account the effects of the heterogeneous structure of clouds on observations of cloud fractional absorptance in the 0.3–2.8 μm spectral interval. Application of the technique to the observed absorptance, results in a corrected fractional absorptance value which is in better agreement with theoretical calculations than previously reported. In addition, the technique significantly reduces the sampling time required to obtain a representative cloud fractional absorptance.

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Steven A. Ackerman
and
Stephen K. Cox

Abstract

Horizontal cloud coverages derived from a geostationary satellite and all-sky cameras were compared for a 3-month period of the GARP Atlantic Tropical Experiment (GATE). Estimates of total cloud cover using the satellite and all-sky camera were similar for the daytime period. The all-sky cameras also gave reasonable estimates of the 24 h cloud cover due to the small difference in the satellite determined daytime and nighttime total cloud cover in the vicinity of the all-sky cameras. However, other regions in the area of study which were not covered by an all-sky camera revealed large diurnal variations. In these areas the daytime total cloud amount did not yield an accurate representation of the 24 h cloud cover.

A method is presented which enables one to construct a three-dimensional representation of cloud structure by combining surface and satellite observations. The disadvantages of this technique are that it assumes no overlapping cloud tops or cloud bases, as well as the limitations of the satellite and all-sky camera in estimating cloud cover.

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Steven A. Ackerman
and
Graeme L. Stephens

Abstract

In this paper we demonstrate that the anomalous diffraction theory of van de Hulst with some modifications, provides a reasonable approximation of the volume extinction and absorption coefficients. We also show how the shortwave radiative properties of a water cloud can be derived in terms of 1) cloud liquid water path, 2) the effective radius of the droplet distribution, and 3) the balk absorption coefficient of water. With the aid of the approximate diffraction theory we describe how cloud albedo and shortwave absorption depend on the droplet size. We demonstrate this dependence to be somewhat complex and show that the variation of absorption with variation of droplet size depends also on the cloud liquid water path. For “deep” or semi-infinite clouds, absorption increases monotonically with increasing effective radius, but the reverse dependence is established for thin clouds. The implications of these results to the so-called absorption paradox and to the possibility of determining droplet size information from remotely measured reflectance spectra are also discussed.

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Steven A. Ackerman
and
Stephen K. Cox

Abstract

Obsemations of temperature moisture, cloud amount, cloud height and soil-derived aerosols are incorporated into radiative transfer models to yield estimates of the tropospheric and surface radiative energy budgets for the summer Monsoon of 1979. Results are presented for six phases of the monsoon for the region 30°S to 40°N latitude and 30°E to 100°E longitude. The derived radiative fields are significantly different from climatological estimates. The evolution of the radiative energy budgets are discussed in relation to monsoon activity. Total tropospheric convergence (TTC) for the January and February phases exhibits a minimum cooling over the southern Indian Ocean and a maximum tropospheric radiative energy loss over the Arabian Sea and Bay of Bengal. The early May, pre-onset, onset and post-onset periods exhibit cellular patterns in TTC, with maximum cooling over the cloud-free oceanic regions, and minimum cooling associated with continental regions and areas with large amounts of cloud. This cellular structure is still evident when TTC is averaged over 10° regions. Large seasonal variations in TTC are observed over the deserts, due to the presence of dust in the summer. Regions with large seasonal variations in cloud cover (e.g., the Arabian Sea) also display large variations in TTC. Regionally averaged radiative heating profiles also change significantly with period. These variations result primarily from changes in the cloud distribution associated with the evolution of the monsoon.

The net surface radiative flux varies markedly from period to period, and within the same period. As expected, all six periods have a maximum surface radiative energy gain for the cloud-free oceanic regions, while cloudy and continental regions tend to have relative minimae. Large spatial and temporal variations exist in the net surface flux.

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Steven A. Ackerman
and
Toshiro Inoue

Abstract

Changes in the energy balance at the top of the atmosphere are specified as a function of atmospheric and surface properties using observations from the Advanced Very High Resolution Radiometer (AVURR) and the Earth Radiation budget Experiment (ERBE) scanner. By collocating the observations from the two instruments, flown on NOAA-9, the authors take advantage of the remote-sensing capabilities of each instrument. The AVHRR spectral channels were selected based on regions that are strongly transparent to clear sky conditions and am therefore useful for characterizing both surface and cloud-top conditions. The ERBE instruments make broadband observations that an important for climate studies. The approach of collocating these observations in time and space is used to study the radiative energy budget of three geographic regions: oceanic, savanna, and desert.

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Steven A. Ackerman
and
Hyosang Chung

Abstract

The effects of dust on the radiative energy budget at the top of the atmosphere were investigated using model calculations and measurements from the Earth Radiation Budget Experiment (ERBE). Estimates of the dust optical depth were made from observations of the Advanced Very High-Resolution Radiometer (AVHRR). Model calculations of the radiative fluxes at the top of the atmosphere were compared with ERBE measurements made during a dust outbreak that occurred over the Saudi Arabian peninsula during July 1985.

Measurements of the ERBE over the oceanic regions indicated that the presence of the dust increased the clear-sky shortwave radiative exitance (SWRE) at the top of the atmosphere (TOA) by 40–90 W m−2. Over the desert regions the differences in the SWRE between clear and dust-laden regions were difficult to determine from the satellite observations. In contrast, the presence of dust over the ocean decreased the observed longwave radiative exitance (LWRE) at the TOA by 5–20 W m−2, while over the desert regions its reduction was 20–50 W m−2. The major discrepancy between the observations and calculations occurred for the SWRE over the desert.

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Steven A. Ackerman
and
Stephen K. Cox

Abstract

Tropospheric radiative convergence profiles are derived for an easterly wave composite during Phase III of the GATE. The easterly waves observed during this period were generally well developed. The profiles also represent the magnitudes and the spatial distribution of atmospheric radiative convergence of the Intertropical Convergence Zone in the GATE area. The 12 h mean daytime and nighttime profiles are presented. Cloud-top pressure distributions as a function of wave position are also presented.

The results of this research indicate three possible radiative induced mechanisms which contribute to the observed diurnal cycle in large-scale mass convergence: 1) radiative convergence differences between the ITCZ and the surrounding regions; 2) mesoscale radiative convergence differences between clear and cluster regions, and 3) a nighttime upper level tropospheric cooling maximum that is centered one-half a wavelength from the region of maximum convective activity.

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