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Richard A. Frey and W. Paul Menzel

Abstract

This paper compares the cloud parameter data records derived from High Resolution Infrared Radiation Sounder (HIRS) and Moderate Resolution Imaging Spectroradiometer (MODIS) measurements from the years 2003 through 2013. Cloud-top pressure (CTP) and effective emissivity (ε f; cloud emissivity multiplied by cloud fraction) are derived using the 15-μm spectral bands in the CO2 absorption band and implementing the CO2-slicing technique; the approach is robust for high semitransparent clouds but weak for low clouds with little thermal contrast from clear-sky radiances. The high-cloud (HiCld; with CTP less than 440 hPa) seasonal cycles of HIRS and MODIS observations are found to be in sync, but the HIRS frequency of detection is about 10% higher than that of MODIS (which is attributed to a lower threshold for cloud detection in the HIRS CO2 bands). Differences are largest during nighttime and at the beginning of the time series (2003–06). Both show Northern Hemisphere (NH) and Southern Hemisphere (SH) seasonal HiClds are out of phase and both agree within 2% on NH–SH HiCld differences. During the summer, maximum HiCld frequency averages 5% more in the NH.

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Richard A. Frey, S. A. Ackerman, and Brian J. Soden

Abstract

An automated method of monitoring various climate parameters using collocated Advanced Very High Resolution Radiometer (AVHRR) and High-Resolution Infrared Sounder-2 (HIRS/2) observations has been developed. The method, referred to as CHAPS (collocated HIRS/2 and AVHRR products) was implemented during the months of July 1993 and January and July 1994. This paper presents the oceanic cloud screening method and analysis of the spectral greenhouse parameter (g λ) for July 1993 and January 1994. In addition, the CHAPS derived clear-sky parameters are compared to the NESDIS historical dataset. There is agreement between NESDIS and CHAPS for the g 6.7 and g 7.3. The NESDIS 8.2-µm data appears to be cloud contaminated. Through comparison with CHAPS, it is suggested that the mode, rather than the mean, provides the better estimate of the central tendency of the NESDIS clear-sky 8.2-µm radiance distribution, particularly for regions with extensive low-level cloud cover.

It is shown that the spectral greenhouse parameter at wavelengths sensitive to middle and upper atmospheric water vapor content is dependent on SST via its connection to large-scale atmospheric circulation patterns. It is also shown that the variability of the spectral greenhouse parameter is strongly a function of latitude at these wavelengths, as well as in spectral regions sensitive to lower-level water vapor. Standard deviations are largest in the Tropics and generally decrease poleward. In contrast, variability in the spectral regions sensitive to upper-tropospheric temperature peaks in the middle latitudes and has its minimum in tropical latitudes.

Variability in the relationship between g λ and SST shows only a weak dependence on season for channels sensitive to water vapor content. A strong seasonal dependence is found in the g 14.2 for the middle-latitude regions associated with changes in the temperature structure of the upper troposphere.

Thee relationship between the spectral greenhouse parameter and the broadband greenhouse parameter is presented and discussed. It is found that the range in broadband g for warm tropical SSTs is driven by spectral changes at wavelengths sensitive to upper-troposheric water vapor. For cooler SSTs associated with the middle latitudes, the range in g is a function of the spectral greenhouse parameter sensitive to the temperature structure of the upper troposphere.

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W. Paul Menzel, Richard A. Frey, Eva E. Borbas, Bryan A. Baum, Geoff Cureton, and Nick Bearson

Abstract

This paper presents the cloud-parameter data records derived from High Resolution Infrared Radiation Sounder (HIRS) measurements from 1980 through 2015 on the NOAA and MetOp polar-orbiting platforms. Over this time period, the HIRS sensor has been flown on 16 satellites from TIROS-N through NOAA-19 and MetOp-A and MetOp-B, forming a 35-yr cloud data record. Intercalibration of the Infrared Advanced Sounding Interferometer (IASI) and HIRS on MetOp-A has created confidence in the onboard calibration of this HIRS as a reference for others. A recent effort to improve the understanding of IR-channel response functions of earlier HIRS sensor radiance measurements using simultaneous nadir overpasses has produced a more consistent sensor-to-sensor calibration record. Incorporation of a cloud mask from the higher-spatial-resolution Advanced Very High Resolution Radiometer (AVHRR) improves the subpixel cloud detection within the HIRS measurements. Cloud-top pressure and effective emissivity (εf, or cloud emissivity multiplied by cloud fraction) are derived using the 15-μm spectral bands in the carbon dioxide (CO2) absorption band and implementing the CO2-slicing technique; the approach is robust for high semitransparent clouds but weak for low clouds with little thermal contrast from clear-sky radiances. This paper documents the effort to incorporate the recalibration of the HIRS sensors, notes the improvements to the cloud algorithm, and presents the HIRS cloud data record from 1980 to 2015. The reprocessed HIRS cloud data record reports clouds in 76.5% of the observations, and 36.1% of the observations find high clouds.

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Jun Li, W. Paul Menzel, Zhongdong Yang, Richard A. Frey, and Steven A. Ackerman

Abstract

A method for automated classification of surface and cloud types using Moderate Resolution Imaging Spectroradiometer (MODIS) radiance measurements has been developed. The MODIS cloud mask is used to define the training sets. Surface and cloud-type classification is based on the maximum likelihood (ML) classification method. Initial classification results define training sets for subsequent iterations. Iterations end when the number of pixels switching classes becomes smaller than a predetermined number or when other criteria are met. The mean vector in the spectral and spatial domain within a class is used for class identification, and a final 1-km-resolution classification mask is generated for such a field of view in a MODIS granule. This automated classification refines the output of the cloud mask algorithm and enables further applications such as clear atmospheric profile or cloud parameter retrievals from MODIS and Atmospheric Infrared Sounder (AIRS) radiance measurements. The advantages of this method are that the automated surface and cloud-type classifications are independent of radiance or brightness temperature threshold criteria, and that the interpretation of each class is based on the radiative spectral characteristics of different classes. This paper describes the ML classification algorithm and presents daytime MODIS classification results. The classification results are compared with the MODIS cloud mask, visible images, infrared window images, and other observations for an initial validation.

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Bryan A. Baum, Richard A. Frey, Gerald G. Mace, Monica K. Harkey, and Ping Yang

Abstract

This study reports on recent progress toward the discrimination between pixels containing multilayered clouds, specifically optically thin cirrus overlying lower-level water clouds, and those containing single-layered clouds in nighttime Moderate Resolution Imaging Spectroradiometer (MODIS) data. Cloud heights are determined from analysis of the 15-μm CO2 band data (i.e., the CO2-slicing method). Cloud phase is inferred from the MODIS operational bispectral technique using the 8.5- and 11-μm IR bands. Clear-sky pixels are identified from application of the MODIS operational cloud-clearing algorithm. The primary assumption invoked is that over a relatively small spatial area, it is likely that two cloud layers exist with some areas that overlap in height. The multilayered cloud pixels are identified through a process of elimination, where pixels from single-layered upper and lower cloud layers are eliminated from the data samples. For two case studies (22 April 2001 and 28 March 2001), ground-based lidar and radar observations are provided by the Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains (SGP) Clouds and Radiation Test Bed (CART) site in Oklahoma. The surface-based cloud observations provide independent information regarding the cloud layering and cloud height statistics in the time period surrounding the MODIS overpass.

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Yinghui Liu, Steven A. Ackerman, Brent C. Maddux, Jeffrey R. Key, and Richard A. Frey

Abstract

Arctic sea ice extent has decreased dramatically over the last 30 years, and this trend is expected to continue through the twenty-first century. Changes in sea ice extent impact cloud cover, which in turn influences the surface energy budget. Understanding cloud feedback mechanisms requires an accurate determination of cloud cover over the polar regions, which must be obtained from satellite-based measurements. The accuracy of cloud detection using observations from space varies with surface type, complicating any assessment of climate trends as well as the understanding of ice–albedo and cloud–radiative feedback mechanisms. To explore the implications of this dependence on measurement capability, cloud amounts from the Moderate Resolution Imaging Spectroradiometer (MODIS) are compared with those from the CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder (CALIPSO) satellites in both daytime and nighttime during the time period from July 2006 to December 2008. MODIS is an imager that makes observations in the solar and infrared spectrum. The active sensors of CloudSat and CALIPSO, a radar and lidar, respectively, provide vertical cloud structures along a narrow curtain.

Results clearly indicate that MODIS cloud mask products perform better over open water than over ice. Regional changes in cloud amount from CloudSat/CALIPSO and MODIS are categorized as a function of independent measurements of sea ice concentration (SIC) from the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E). As SIC increases from 10% to 90%, the mean cloud amounts from MODIS and CloudSat–CALIPSO both decrease; water that is more open is associated with increased cloud amount. However, this dependency on SIC is much stronger for MODIS than for CloudSat–CALIPSO, and is likely due to a low bias in MODIS cloud amount. The implications of this on the surface radiative energy budget using historical satellite measurements are discussed. The quantified ice–water difference in MODIS cloud detection can be used to adjust estimated trends in cloud amount in the presence of changing sea ice cover from an independent dataset. It was found that cloud amount trends in the Arctic might be in error by up to 2.7% per decade. The impact of these errors on the surface net cloud radiative effect (“forcing”) of the Arctic can be significant, as high as 8.5%.

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Irina Gladkova, Fazlul Shahriar, Michael Grossberg, Richard A. Frey, and W. Paul Menzel

Abstract

Distinguishing between clouds and snow is an intrinsically challenging problem because both have similar high albedo across many bands. The 1.6-μm channel (band 6) on the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument provides an essential tool for distinguishing clouds from snow, since snow typically has a much lower albedo in this band. Unfortunately, this band is severely damaged on the MODIS/Aqua platform and is typically not used in either snow or cloud products. An algorithm was previously introduced for quantitative image restoration (QIR) that can restore missing pixels of band 6 using multilinear regression with input from a spatial-spectral window in other bands. Also previously demonstrated was the effectiveness of this restoration for snow products over cloud-free pixels only. The focus of the authors’ previous work was to evaluate the impact of this restoration on the snow product, and they had relied on the current cloud mask, which does not use any information from band 6. In this work the authors propose to apply the QIR-corrected band 6 to directly create a new cloud mask. They demonstrate that this new cloud mask is much more consistent with the one produced using the undamaged MODIS band 6 on Terra. The restoration of MODIS band 6 on the Aqua platform and its impact on the discrimination of clouds, snow, and clear sky is also examined. A comprehensive evaluation was conducted on a global set of granules. The method shows great promise and should be considered for use in NASA’s next reprocessing of Aqua MODIS data.

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Elisabeth Weisz, W. Paul Menzel, Nadia Smith, Richard Frey, Eva E. Borbas, and Bryan A. Baum

Abstract

The next-generation Visible and Infrared Imaging Radiometer Suite (VIIRS) offers infrared (IR)-window measurements with a horizontal spatial resolution of at least 1 km, but it lacks IR spectral bands that are sensitive to absorption by carbon dioxide (CO2) or water vapor (H2O). The CO2 and H2O absorption bands have high sensitivity for the inference of cloud-top pressure (CTP), especially for semitransparent ice clouds. To account for the lack of vertical resolution, the “merging gradient” (MG) approach is introduced, wherein the high spatial resolution of an imager is combined with the high vertical resolution of a sounder for improved CTP retrievals. The Cross-Track Infrared Sounder (CrIS) is on the same payload as VIIRS. In this paper Moderate Resolution Imaging Spectroradiometer (MODIS) and Atmospheric Infrared Sounder (AIRS) data are used as proxies for VIIRS and CrIS, respectively, although the approach can be applied to any imager–sounder pair. The MG method establishes a regression relationship between gradients in both the sounder radiances convolved to imager bands and the sounder CTP retrievals. This relationship is then applied to the imager radiance measurements to obtain CTP retrievals at imager spatial resolution. Comparisons with Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) cloud altitudes are presented for a variety of cloud scenes. Results demonstrate the ability of the MG algorithm to add spatial definition to the sounder retrievals with a higher accuracy and precision than those obtained solely from the imager.

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Richard A. Frey, Steven A. Ackerman, Yinghui Liu, Kathleen I. Strabala, Hong Zhang, Jeffrey R. Key, and Xuangi Wang

Abstract

Significant improvements have been made to the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask (MOD35 and MYD35) for Collection 5 reprocessing and forward stream data production. Most of the modifications are realized for nighttime scenes where polar and oceanic regions will see marked improvement. For polar night scenes, two new spectral tests using the 7.2-μm water vapor absorption band have been added as well as updates to the 3.9–12- and 11–12-μm cloud tests. More non-MODIS ancillary input data have been added. Land and sea surface temperature maps provide crucial information for mid- and low-level cloud detection and lessen dependence on ocean brightness temperature variability tests. Sun-glint areas are also improved by use of sea surface temperatures to aid in resolving observations with conflicting cloud versus clear-sky signals, where visible and near-infrared (NIR) reflectances are high, but infrared brightness temperatures are relatively warm. Day and night Arctic cloud frequency results are compared to those created by the Advanced Very High Resolution Radiometer (AVHRR) Polar Pathfinder-Extended (APP-X) algorithm. Day versus night sea surface temperatures derived from MODIS radiances and using only the MODIS cloud mask for cloud screening are contrasted. Frequencies of cloud from sun-glint regions are shown as a function of sun-glint angle to gain a sense of cloud mask quality in those regions. Continuing validation activities are described in Part II of this paper.

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Bryan T. Smith, Richard L. Thompson, Douglas A. Speheger, Andrew R. Dean, Christopher D. Karstens, and Alexandra K. Anderson-Frey

Abstract

A sample of damage-surveyed tornadoes in the contiguous United States (2009–17), containing specific wind speed estimates from damage indicators (DIs) within the Damage Assessment Toolkit dataset, were linked to radar-observed circulations using the nearest WSR-88D data in Part I of this work. The maximum wind speed associated with the highest-rated DI for each radar scan, corresponding 0.5° tilt angle rotational velocity V rot, significant tornado parameter (STP), and National Weather Service (NWS) convective impact-based warning (IBW) type, are analyzed herein for the sample of cases in Part I and an independent case sample from parts of 2019–20. As V rot and STP both increase, peak DI-estimated wind speeds and IBW warning type also tend to increase. Different combinations of V rot, STP, and population density—related to ranges of peak DI wind speed—exhibited a strong ability to discriminate across the tornado damage intensity spectrum. Furthermore, longer duration of high V rot (i.e., ≥70 kt) in significant tornado environments (i.e., STP ≥ 6) corresponds to increasing chances that DIs will reveal the occurrence of an intense tornado (i.e., EF3+). These findings were corroborated via the independent sample from parts of 2019–20, and can be applied in a real-time operational setting to assist in determining a potential range of wind speeds. This work provides evidence-based support for creating an objective and consistent, real-time framework for assessing and differentiating tornadoes across the tornado intensity spectrum.

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