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NOTES and CORRESPONDENCE a Multispectral Technique For 1800 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 24 NOTES AND CORRESPONDENCE A Multispectral Technique for Detecting Low-Level Cloudiness near Sunrise ANTHONY J. SCHREINER,STEVEN A. ACKERMAN, AND BRYAN A. BAUM Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin—Madison, Madison, Wisconsin ANDREW K. HEIDINGER National Oceanic and Atmospheric Administration/National Environmental Satellite, Data, and Information Service, Center for Satellite Applications and Research, Madison, Wisconsin (Manuscript received 19 July 2006, in final form 6 February 2007) ABSTRACT A technique using the Geostationary Operational Environmental Satellite (GOES) sounder radiance data has been developed to improve detection of low clouds and fog just after sunrise. The technique is based on a simple difference method using the shortwave (3.7 ␮m) and longwave (11.0 ␮m) window bands in the infrared range of the spectrum. The time period just after sunrise is noted for the difficulty in being able to correctly identify low clouds and fog over land. For the GOES sounder cloud product this difficulty is a result of the visible reflectance of the low clouds falling below the “cloud” threshold over land. By requiring the difference between the 3.7- and the 11.0-␮m bands to be greater than 5.0 K, successful discrimination of low clouds and fog is found 85% of the time for 21 cases from 14 September 2005 to 6 March 2006 over the GOES-12 sounder domain. For these 21 clear and cloudy cases the solar zenith angle ranged from 87° to 77°; however, the range of solar zenith angles for cloudy cases was from 85° to 77°. The success rate further improved to 95% (20 out of 21 cases) by including a difference threshold of 5.0 K between the 3.7- and 4.0-␮m bands, requiring that the 11.0-␮m band be greater than 260 K, and limiting the test to fields of view where the surface elevation is below 999 m. These final three limitations were needed to more successfully deal with cases involving snow cover and dead vegetation. To ensure that only the time period immediately after sunrise is included the solar zenith angle threshold for application of these tests is between 89° and 70°. 1. Introduction is also described as the day–night terminator) occur in orbiting and geostationary remote sensing platforms. In A noted difficulty in cloud detection using remotely the case of geostationary satellites, as will be discussed sensed radiances occurs when attempting to detect low here and in particular the Geostationary Operational clouds and fog just after sunrise, during the transition Environmental Satellite (GOES) sounder (Menzel and from nighttime cloud-detection techniques to daytime Purdom 1994; Menzel et al. 1998), the “disappearing methods. In practice, the thermal difference between clouds syndrome” can at times be seen to “move” from the longwave window (11.0 ␮m) brightness tempera- east to west over the course of 3 or 4 h. ture and the surface skin temperature is frequently For the GOES sounder cloud mask algorithm within the noise limitations of the observed brightness (Schreiner et al. 2001), as the sun rises over a region, temperatures when low clouds and fog are present. As the daytime series of tests become the primary means a result cloud detection errors near sunrise (this region for identifying clouds. Reasons for the transition from “nighttime” to “daytime” are twofold. First, the tech- niques for detecting low clouds at night strongly depend Corresponding author address: Anthony J. Schreiner, CIMSS, ␮ University of Wisconsin—Madison, 1225 W. Dayton St., Madison, on the differences between the infrared (IR) 11.0- m WI 53706. and shortwave window (3.7 ␮m) bands (Eyre 1984; E-mail: [email protected] d’Entremont 1986; Saunders and Kriebel 1988; DOI: 10.1175/JTECH2092.1 © 2007 American Meteorological Society JTECH2092 OCTOBER 2007 NOTES AND CORRESPONDENCE 1801 FIG. 1. Composite of the GOES sounder cloud mask showing 1200–1400 UTC images for 6 Jun 2005. Note the undetected and, then again, detected clouds occurring (area within the yellow oval) after local sunrise. (lower-right-hand panel) The 1346 UTC sounder visible image. Kleespies 1995; Lee et al. 1997; Ackerman et al. 1998). to be brighter and can be detected by comparison of the At night for clear-sky scenes, the 3.7-␮m minus 11.0- 11.0-␮m brightness temperature to a skin temperature ␮m brightness temperature difference over land ranges or modified surface observed temperature. Also, as from approximately Ϫ4toϩ1 K, depending on surface with the current GOES imager, the visible band of the emissivity and the atmospheric water vapor distribu- sounder is not calibrated once the satellite achieves or- tion. However, the difference between these two bands bit. The degradation of the visible (imager) band has can be a strong indicator of low-level cloudiness and/or been observed by Hillger et al. (2003) and Daniels et al. fog. For the case of clouds having small effective radii (2001), since the launch of GOES-11 and GOES-12. and high optical thicknesses, the 11.0-␮m value tends to The net result of this cloud-detection shortcoming be greater than that of the 3.7-␮m value (Baum et al. can frequently be seen when a loop of derived imagery 2003). As the sun rises above the horizon this difference is set in motion (e.g., Fig. 1). Prior to sunrise low clouds becomes positive because of the contribution of solar are correctly depicted (1146 UTC), as noted by the area reflection in the 3.7-␮m band. highlighted within the oval in the southwestern portion A second reason for the “disappearance” of low of the figure. For the 1246 UTC scan line start time clouds just after sunrise is the failure of the visible re- image, the first image after sunrise, nearly the entire flectance tests for low-altitude clouds and fog. Visible region of low cloudiness is not detected by the algo- thresholds for cloud/no-cloud detection during the day- rithm currently employed operationally. Then, once time are higher over land than over water, as well as a again at 1346 UTC the cloud bank along the southern function of terrain type. These terrain types are depen- portion of the CONUS is correctly depicted. In addi- dent on the time of year, elevation, type of vegetation, tion to incorrectly portraying cloudiness in the loops of snow, and roughness of the landscape. Just after sunrise derived imagery these derived data may have a nega- the visible reflectance of clouds, especially low clouds tive impact when used in the initialization step for nu- and fog, is below the threshold for cloudiness over the merical weather prediction models (Bayler et al. 2000). conterminous United States (CONUS), and thus are By exploiting three of the IR bands of the GOES incorrectly flagged as clear. Mid- and high-level clouds sounder for a particular field of view (FOV), a tech- are not as sensitive to this visible threshold, as they tend nique for identifying these low clouds just after sunrise Fig 1 live 4/C 1802 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 24 has been developed. The IR window bands for the because of the reflection of solar energy at 3.7 ␮m. This GOES sounder are the longwave window (11.0 ␮m), brightness temperature difference technique is very shortwave window (3.7 ␮m), and a second shortwave successful at detecting low-level water clouds during window (4.0 ␮m). In essence, the simple difference the day. The approach is generally not applied over (SIMDIF) technique looks at the difference between deserts during daytime, as bright desert regions with the 3.7- and 11.0-␮m bands and the difference between highly variable emissivities tend to be classified incor- the 3.7- and 4.0-␮m bands. If the differences fall within rectly as cloudy with this test. In general, the emissivity the predetermined thresholds and some additional cri- differences of the same stratiform water cloud being teria based on 11.0-␮m temperature, surface elevation, observed simultaneously by the 3.7- and 4.0-␮m bands and solar zenith angle (SZEN), the FOV is defined as is small as is the emissivity differences with varying cloudy. The SIMDIF technique defines “just after sun- surface types (Hunt 1973; Sutherland 1986; Ellrod rise” as the first time period of the GOES sounder data 2006). Here we explore using the differences between Ն at a particular location where the SZEN is 89° SZEN BT3.7 and BT4.0 for cloud detection, since over clouds Ն Ͻ Ͼ 70°. The cutoff is set at 70° because at SZEN 70° during the day, BT3.7 BT4.0 because there is more certain difference thresholds begin to break down. This reflected solar energy at 3.7 ␮m. will be demonstrated in the following section. The following approach has been developed to as- The purpose of this note is to detail the criteria certain whether the observed FOV is either clear or needed to satisfy the SIMDIF technique and to define obscured by low clouds. The logic is applied to a given why the SIMDIF is successful. The background section FOV when the SZEN is within the following window: briefly describes the reasoning. Two case studies will be 89.0° Ͻ SZEN Ͻ 70.0°, but is limited to FOVs with a examined in section 3 showing both the success and the surface elevation (EL) threshold less than 999 m. limitations of the SIMDIF technique. The summary The SZEN window described above was chosen be- and future work will summarize the note and introduce cause it roughly defines no more than one time period future goals.
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