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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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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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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

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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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Yoonkyung Lee, Grace Wahba, and Steven A. Ackerman
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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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Yoonkyung Lee, Grace Wahba, and Steven A. Ackerman

Abstract

Two-category support vector machines (SVMs) have become very popular in the machine learning community for classification problems and have recently been shown to have good optimality properties for classification purposes. Treating multicategory problems as a series of binary problems is common in the SVM paradigm. However, this approach may fail under a variety of circumstances. The multicategory support vector machine (MSVM), which extends the binary SVM to the multicategory case in a symmetric way, and has good theoretical properties, has recently been proposed. The proposed MSVM in addition provides a unifying framework when there are either equal or unequal misclassification costs, and when there is a possibly nonrepresentative training set.

Illustrated herein is the potential of the MSVM as an efficient cloud detection and classification algorithm for use in Earth Observing System models, which require knowledge of whether or not a radiance profile is cloud free. If the profile is not cloud free, it is valuable to have information concerning the type of cloud, for example, ice or water. The MSVM has been applied to simulated MODIS channel data to classify the radiance profiles as coming from clear sky, water clouds, or ice clouds, and the results are promising. It can be seen in simple examples, and application to Moderate Resolution Imaging Spectroradiometer (MODIS) observations, that the method is an improvement over channel-by-channel partitioning. It is believed that the MSVM will be a very useful tool for classification problems in atmospheric sciences.

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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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