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 techniques 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

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. following sunrise for a GOES sounder time period. This is important because the defined threshold, listed 2. Background in Eq. (1) below, can be easily surpassed in clear-sky situations for SZEN values less than 70°. Given these a. Theory conditions, an FOV is defined cloudy if the brightness The low thermal contrast between clear skies and low temperature differences all meet the following criteria: cloud and fog makes detection of these clouds at night Ϫ Ͼ ͑ ͒ challenging (Ellrod 1995). Techniques that make use of BT3.7 BT11 5.0 K, 1 the shortwave infrared (i.e., 3.5–4-␮m region) and long- Ϫ Ͼ ͑ ͒ BT3.7 BT4.0 5.0 K, and 2 wave infrared (i.e., 10–12 ␮m) observations exploit the Ͼ ͑ ͒ emissivity differences at these wavelength regions for BT11 260.0 K. 3 stratus and fog conditions. These clouds have a lower The elevation check was added as a result of contin- emissivity in the shortwave infrared window than the ued failure of this check in elevated terrain, especially longwave infrared window, resulting in negative bright- in the late fall and early spring when vegetation is quite ness temperature differences when using the following sparse over the mountainous regions. Thermal checks relationship: BT Ϫ BT . This negative difference is a 3.7 11 (2) and (3) were included to minimize the effects of function of the cloud droplet size and the underlying snow, especially fresh snow cover at all elevations. surface. Baum et al. (2003) provide some radiative transfer calculations for both ice and water clouds at b. Data night that provide insight into the behavior of this brightness temperature difference as a function of par- Twenty-one different clear and cloudy cases were ex- Ϫ ticle size and optical thickness, but new calculations are amined to determine whether the BT3.7 BT11 bright- not provided herein. Over clear-sky vegetative surfaces ness temperature difference [Note: This difference is and water the differences are generally larger than Ϫ0.5 defined as the window difference (WNDF).] for a given K. Clear-sky desert scenes have brightness temperature FOV at a clear versus cloudy scene just after sunrise differences that range between approximately Ϫ5 and correctly satisfied the difference threshold (5 K) to Ϫ1 K, making detection of optically thin low-level properly identify the FOV as clear or cloudy. For the 21 clouds at night more difficult because of the emissivity cases ranging from clear (i.e., 1) and low cloudy (i.e., 3) differences of soil. During the day the brightness tem- conditions over water to clear (i.e., 10) and cloudy or perature difference between BT3.7 and BT11 is large foggy conditions (i.e., 7) over land at various locations, OCTOBER 2007 NOTES AND CORRESPONDENCE 1803

threshold value. Seven of the nine are cloudy or foggy cases (solid circles or solid triangles) and two were clear snow (hollow diamonds). Eleven of the cases fell below the WNDF threshold. All but one was either clear (hollow squares) or clear snow cases. In total, Fig. 2 shows that 18 of the 21 cases were correctly identified using the WNDF. Three of the 21 cases depicted in Fig. 2 were clear sky FIG. 2. Comparison of the shortwave window (3.7 ␮m) minus the longwave window (11.0 ␮m) in the ordinate vs solar zenith with snow on the surface. They ranged from fresh snow angle in the abscissa. Included are 8 cloudy (solid dots) or foggy (1 day old) to 4- and 7-day-old snow. Two of the three sites (solid triangles), 3 clear snow-covered locations (open dia- initial failures were snow cases (1- and 4-day-old snow). monds), and 10 additional clear sites (open squares). The thick Both snow cases were correctly flagged with the addi- solid line at 5 K denotes the WNDF threshold cutoff between tion of the additional thermal checks (BT Ϫ BT Ͼ clear (WNDF Յ 5K) and cloudy (WNDF Ͼ 5K). 3.7 4.0 Ͼ 5.0 K and BT11 260.0 K) and the elevation check. The third failure in Fig. 2 was a low cloud case over Texas the differences were tabulated. The numbers in paren- during November in which the WNDF value was less theses indicate number of cases for each set of condi- than 4.0 K. No modifications could be made that would tions. The cloudy cases were selected due to the inabil- uniquely capture this situation and not negatively affect ity of the old technique to correctly identify them as the other 20 cases, primarily the clear cases. This par- such. “Clear” and “cloudy or foggy” were determined ticular case exposes a weakness in the technique. in two ways. First, sites were verified by inspecting Of the 18 successful cases 11 were clear sites and 7 hourly surface observations. Second, to ensure that lo- were low cloud or fog examples. A typical example of a cations were not affected by mid- and high clouds, successful clear-versus-cloudy comparison is shown in GOES visible and longwave window imagery were also Fig. 3. This figure describes the 3.7 Ϫ 11.0 ␮m value at used. It should be noted that foggy locations were pri- different SZEN values based on GOES-12 sounder marily determined by the Automated Surface Observa- data for two sites (one clear and one cloudy) over Texas tion System (ASOS) hourly observation (National Oce- on 14 November 2005. Note the differential rate of in- anic and Atmospheric Administration/Department of crease for the 3.7 Ϫ 11.0-␮m band difference just after Defense/Federal Aviation Agency/U.S. Navy 1998). At sunrise for the clear versus the cloudy case. (SZEN each case (or ASOS site) a 3 ϫ 3 FOV box was defined decreasing from 92.0° to 80.0°, corresponding to the and a simple average of the nine FOVs for the three IR nominal 1200 and 1300 UTC time periods.) For the bands (3.7, 4.0, and 11.0 ␮m) was determined. eight cloudy locations the average 3.7 Ϫ 11.0 ␮m value A variety of locations were made ranging from the is 8.04 K, where the maximum difference is 16.6 K and Gulf of Mexico (“water”) and surface hourly sites along the minimum difference is 3.8 K. For all (10) clear cases the East Coast to as far west as Kansas, south to (not including clear where the ground is covered with Florida, and north to Minnesota. Spatially, these locations represent a sampling of the area the WNDF technique is expected to be the most effective, namely, the eastern half of the United States. Finally, these cases span from the beginning of September 2005–March 2006, a temporal range that goes from late Northern Hemisphere summer to early spring. Figure 2 shows the distribution of the differences for Ϫ the BT3.7 BT11 bands at various solar zenith angles between 70° and 89° for all 21 cases. It should be noted that no cases were found near the solar zenith limits. Given the predefined thresholds, it is very possible that cloudy cases very near the solar zenith angle of 89° will FIG. 3. Comparison of clear (dashed) vs low cloud or fog (solid) still be missed. The thick solid line at 5 K represents the difference for the 3.7- and the 11-␮m bands aboard the GOES-12 cutoff threshold for the WNDF. The range of local ze- sounder over TX on 14 Nov 2005. Along the abscissa is the solar zenith angle (deg) and the ordinate is the temperature difference nith angles for the 21 locations vary from 32° to 57° for (K). At each location a 3 ϫ 3 FOV box was defined and a simple the clear comparisons and 37° to 57° for the cloudy average of the nine FOVs for the two IR bands (3.7 and 11.0 ␮m) cases. Nine of the cases are plotted above the WNDF was determined for each time period. 1804 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 24

of the one snow case noted above, the defined thresh- Ϫ olds in this study for both the WNDF and BT3.7 BT4.0 were satisfactory. After the SIMDIF was included in the cloud detection subroutine for the GOES sounder cloud (Schreiner et al. 2001) processing at the University of Wisconsin— Madison Cooperative Institute for Meteorological Sat- ellite Studies (CIMSS), it was found that an elevation/ terrain-type check was also required, as well as an additional thermal check [11.0 ␮m Ͼ 260.0 K, Eq. (3) FIG. 4. Same as in Fig. 3, but the differences refer to the 3.7-␮m above]. The additional thermal check helped in detect- band minus the 4.0-␮m band aboard the GOES-12 sounder. The ing “old” snow (Ͼ1 day). The elevation/terrain-type sites are located in TX on 14 Nov 2005. At each location a 3 ϫ 3 check assisted in discriminating between low cloud/fog FOV box was defined and a simple average of the nine FOVs for the two IR bands (3.7 and 4.0 ␮m) was determined for each time and clear-sky and arid scenes during the late fall and period. early spring in mountainous regions. As was noted ear- lier when these tests were applied to the three failing snow, i.e., clear snow), the average difference is 2.1 K. cases of the original 21 cases, 2 of the 3 failures were For these 10 clear cases the range of difference is 3.3– correctly identified. 0.9 K. When the three “clear snow” sites are added, the average difference increases to 2.8 K (6.6-K maximum 3. Case studies difference and 3.5-K minimum difference for clear a. 9 March 2006 (cloud mask only) snow sites only). Using this distribution, a 3.7 Ϫ 11.0 ␮m value of 5 K was defined as a threshold to delineate Figures 5a–d detail an example comparing the GOES clear from cloudy scenes as a first step in this technique. sounder cloud mask with (labeled “NEWMSK”) and It should be noted that the 3.7- and 11.0-␮m difference without (labeled “OLDMSK”) the SIMDIF for (WNDF) was independent of local zenith angle. 4-hourly time periods (1246–1546 UTC) on 9 March With a threshold of 5 K for the WNDF 3 of the 21 2006. This test was applied to GOES-12 sounder data cases or locations failed to be correctly identified as covering the eastern portion of the United States. Of either clear or cloudy. Therefore, a second test was interest is the cloud cover at two locations (Fargo, included. Figure 4 details the comparison between the North Dakota, and Bemidji, Minnesota) corresponding 3.7–4.0-␮m band and SZEN for the same case as in Fig. to the gray and white dots, respectively, within the 3. Similar to the WNDF, the 3.7–4.0-␮m bands differ- white and stippled ovals. ence is greater for a cloudy location (5.2 K for this site) At 1246 UTC (Fig. 5a) both the NEWMSK and the than for a clear example (2.4 K). For each of the cloudy OLDMSK are correctly indicating cloud for both sur- cases (except the one cloudy failure noted above, 2.4 face locations, as is indicated in Table 1. The SZEN at K), both the WNDF and the 3.7- and 4.0-␮m bands this time is greater than 89°; therefore, the SIMDIF (SWNDF) difference were greater than 5.0 K for the does not affect the region within the stippled oval. The first GOES sounder time period after sunrise. The re- 1346 UTC time period shows a difference between the verse was true for the clear cases with the exception for NEWMSK and OLDMSK, with the NEWMSK detect- the locations that included fresh snow (4 days old or ing cloud at both Fargo and Bemidji, while OLDMSK less). For these locations the WNDF was greater than indicates cloud only at Bemidji, which upon examina- 5.0 K, but the SWNDF was less than 5.0 K. By adding tion of the longwave window image (top right) is de- this additional check, the remaining two clear snow termined to be high-level cloud. The visible image for cases were correctly flagged. this time period denotes cloudy conditions for both lo- It should be noted that, typically, for the 21 cases no cations and is supported by the surface reports for the more than two sites were tested in the “zone of inter- same time period. It appears the NEWMSK is better est” (89.0° Ͻ SZEN Ͻ 70.0°). A more rigorous ap- able to identify clouds in northern Minnesota and proach would be to investigate the thresholds at numer- North Dakota than the OLDMSK. ous locations or sites for varying SZENs and note the At 1446 UTC once again the OLDMSK is apparently threshold differences. Although not performed, it is as- only detecting the upper-level cloud (see the longwave sumed that as one more closely approaches SZEN ϭ window image), while the NEWMSK is able to identify 89° the threshold difference criteria for Eqs. (1) and (2) both the high and low cloud deck in the region of Be- will break down. But for all 21 cases, with the exception midji and Fargo. Surface observations for this time re- OCTOBER 2007 NOTES AND CORRESPONDENCE 1805

FIG. 5. (a) A comparison of the cloud mask (lower left) with the new simple difference technique and (lower right) without for 1246 UTC 9 Mar 2006. (top left) The GOES-12 sounder visible band (0.65 ␮m) and (top right) is the longwave window band (11.0 ␮m). The dark gray dot (located in the white and stippled ovals) is the location of Fargo, ND, and the white dot (also in the white and stippled ovals) is the location of Bemidji, MN. The solid lines represent SZEN ϭ 89° (white) and SZEN ϭ 70° (orange). (b) Same as in (a), but at 1346 UTC. (c) Same as in (a), but at 1446 UTC. (d) Same as in (a), but at 1546 UTC. veal that there may be an extensive low cloud deck Tables 2a,b refer to the average 3 ϫ 3 brightness passing over both surface sites in addition to high cloud temperatures based on the observed GOES-12 sounder over Bemidji. At 1546 UTC (Fig. 5d) the NEWMSK radiance data at Fargo and Bemidji, respectively. This Ϫ and OLDMSK are in agreement again. By 1546 UTC table details the differences (WNDF and BT3.7 BT4.0), the SZEN is less than 70° within the oval of interest and the SZEN and BT11 used in the SIMDIF. At both sites the SIMDIF is not applicable. The time surface reports for both time periods (1346 and 1446 UTC) all four indicate that Bemidji is still under low clouds and this tests, including the two difference thresholds, are seen situation is captured by both cloud mask versions. to indicate clouds, in this case low clouds. However, the cloud cover at Fargo has cleared off, al- One of the weaknesses of the technique is also ob- though haze is reported. The cloud masks show that served in this case study. By 1446 UTC additional Fargo is right on the edge of what is apparently a low “cloud” is observed over the extreme eastern Colorado/ cloud deck. Closer investigation of the visible image western Nebraska region. In higher elevations solar re- reveals that the highly reflective region over Fargo is in flectance in the 3.7- and 4.0-␮m bands is locally high in fact snow. This feature is confirmed in subsequent time clear scenes, even when snow is not present. An in- periods (visible images not shown). For this particular crease in solar reflectance is a result of barren vegeta- case the SIMDIF more accurately depicts the cloud tion at this elevation during the late winter early spring mask for the northern Minnesota and North Dakota time frame in this region. Dead vegetation, especially at region using the GOES sounder radiance information. high elevations, tends to demonstrate similar thresholds

Fig 5 live 4/C 1806 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 24

TABLE 1. ASOS hourly observations for Fargo, ND (KFAR), and Bemidji, MN (KBJI), from 0955 to 1755 UTC (approximately 4:00 A.M. to 12:00 P.M. local time) 9 Mar 2006 (2006068). The remaining column labels are defined as follows. The times of the observation are in UTC. WX, with respect to ASOS, can be defined as a visibility obstruction and is limited to clear (blank), fog (F), haze (H), and freezing fog (IF). WX, again with respect to ASOS, can refer to certain weather types such as rain (R) and snow (S). Temperature (T) and dewpoint (TD) in K. Visibility (VIS) in km. C refers to cloud cover, where 0 is clear, 1 is scattered, 2 is broken, and 3 is overcast. CIGH and ZCL1 are the height of the ceiling and lowest cloud base, respectively, in km.

ID State Day UTC WX T TD VIS C CIGH C ZCL1 KFAR ND 2006068 0953:00 F 271.5 270.4 4.8 3 0.64 KFAR ND 2006068 1053:00 F 272.0 270.4 6.4 3 0.70 KFAR ND 2006068 1153:00 F 271.5 269.8 8.0 3 0.70 KFAR ND 2006068 1253:00 F 270.9 269.8 8.0 3 0.70 KFAR ND 2006068 1353:00 F 271.5 270.4 4.8 3 0.70 KFAR ND 2006068 1453:00 F 272.0 270.4 6.4 3 0.70 KFAR ND 2006068 1553:00 H 273.7 270.9 9.7 0 0.00 KFAR ND 2006068 1653:00 275.9 270.4 12.9 0 0.00 KFAR ND 2006068 1753:00 277.0 270.4 14.5 0 0.00 ID State Day UTC WX T TD VIS C CIGH C ZCL1 KBJI MN 2006068 0955:00 272.0 270.4 16.1 3 0.58 KBJI MN 2006068 1055:00 272.0 270.4 16.1 3 0.52 KBJI MN 2006068 1155:00 272.0 270.4 16.1 3 0.52 KBJI MN 2006068 1255:00 272.0 270.4 16.1 3 0.52 KBJI MN 2006068 1355:00 272.0 270.4 16.1 3 0.52 KBJI MN 2006068 1455:00 273.2 269.3 16.1 3 0.46 KBJI MN 2006068 1555:00 273.2 269.3 16.1 3 0.40 KBJI MN 2006068 1655:00 273.2 269.3 16.1 3 0.40 KBJI MN 2006068 1755:00 273.2 270.4 16.1 3 0.40

for the WNDF and BT3.7–BT4.0 as low clouds at lower algorithm is a two-step process. The first step defines a elevations (Ackerman et al. 1998). The SIMDIF will cloud mask (where the SIMDIF is applied). The second fail in these instances because the 5-K threshold is sur- step then will determine a cloud-top pressure only if a passed. The region of interest is below the elevation FOV has been determined to be cloudy. The effect of threshold (999 m) used in the logic, and therefore cloud the SIMDIF is shown for the nominal time periods is being incorrectly identified. from 0946 to 1247 UTC 9 May 2006 using the GOES-12 sounder data in Figs. 6a–d, which are essentially the cloud mask augmented with cloud-top pressures. b. 9 May 2006 (cloud mask and cloud-top Figure 6a (0946 UTC) shows the cloud coverage ap- pressure) proximately 1 h prior to sunrise along the east coast of A second case over Texas and the southeastern the United States with the exception of the extreme United States also demonstrates the effectiveness of the northern portion of the east coast. The synoptic condi- WNDF technique. In this case the SIMDIF was incor- tions include a low pressure system located over the porated in the GOES sounder Cloud Height Program Central Plains generating some convective activity over (Schreiner et al. 2001). This cloud height processing eastern Kansas, Missouri, and eastern portions of Iowa.

␮ ␮ TABLE 2a. Hourly brightness temperatures based on GOES-12 sounder radiances for band 8 (11.0 m, BT11), band 17 (4.0 m, ␮ BT4.0), and band 18 (3.7 m, BT3.7) for six consecutive hourly time periods (time in UTC) on 9 Mar 2006 for Fargo, ND. The visible reflectance (VIS in %), solar zenith angle (SZEN), and difference between the shortwave window and the longwave window (WNDF)

and the shortwave bands (BT3.7–BT4.0 in K) are shown in the rightmost four columns.

UTC BT11 BT4.0 BT3.7 VIS SZEN WNDF BT3.7 – BT4.0 1146:00 265.88 262.41 261.67 0 102.25 Ϫ4.21 Ϫ0.74 1246:00 265.82 261.56 261.03 0 91.97 Ϫ4.79 Ϫ0.53 1346:00 266.58 270.17 283.15 6 81.89 16.57 12.98 1446:00 269.30 278.46 292.92 15 72.40 23.62 14.46 1546:00 270.77 272.28 276.85 24 64.01 6.08 4.57 1646:00 273.84 277.09 281.78 25 57.26 7.94 4.69 OCTOBER 2007 NOTES AND CORRESPONDENCE 1807

TABLE 2b. Same as in Table 2a, but the location coincides with Bemidji, MN.

UTC BT11 BT4.0 BT3.7 Vis SZEN WNDF BT3.7 – BT4.0 1146:00 263.61 260.42 260.39 0 101.01 Ϫ3.22 Ϫ0.03 1246:00 253.08 254.55 255.05 1 90.84 1.97 0.50 1346:00 262.48 263.05 269.37 6 80.92 6.89 6.32 1446:00 263.91 279.22 295.35 20 71.64 31.44 16.13 1546:00 263.79 285.64 304.32 29 63.51 40.53 18.68 1646:00 263.99 292.26 313.99 38 57.12 50.00 21.73

There are also convective storms over the Florida Pen- At 1046 UTC (Fig. 6b), the sun is just above the insula. The southeastern portion of the country is over- horizon for the eastern coast of the United States. cast due to low-level clouds and/or fog throughout the When this time period is compared to the previous im- time period 1000–1500 UTC. These conditions are in- age (Fig. 6a), there appears to be a clear region en- dicated in Table 3. compassing both North and South Carolina in the

FIG. 6. (a) A comparison of the derived CTP image based on GOES-12 sounder radiance data (bottom left) with and (bottom right) without the new simple difference technique at 0946 UTC 9 May 2006. The color levels refer to various levels of CTP (hPa), and the gray shades in the CTP images indicate cloud-free regions. (top left) The GOES-12 sounder visible band (0.65 ␮m) and (top right) the longwave window band (11.0 ␮m). The four square dots (white) refer to the location of the surface observation sites (see Table 3) used to verify the existence of low cloud. The surface observations sites are Jacksonville, NC; Albany, GA; Houston, TX; and Corsicana, TX. The solid lines represent SZEN ϭ 89° (white) and SZEN ϭ 70° (orange) (b) Same as in (a), but at 1046 UTC. The white dot indicates the location of Jacksonville. (c) Same as in (a), but at 1146 UTC. The white dot indicates the location of Albany. (d) Same as in (a), but at 1247 UTC. The white dot indicates the location of Houston.

Fig 6 live 4/C 1808 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 24

TABLE 3. Surface hourly reports for the four locations on 9 May 2006 (day 2006129) noted in Figs. 6a–d. KNCA is the ID for Jacksonville, NC; KABY is Albany, GA; and KHOU is Houston, TX. The remaining column labels are the same as defined in Ta- ble 1.

ID State Day UTC WX T TD VIS C CIGH C ZCL1 KNCA NC 2006129 0956:00 F 285.9 284.3 8.0 2 0.30 3 1.22 KNCA NC 2006129 1056:00 F 285.9 284.3 9.7 2 0.30 2 0.61 KNCA NC 2006129 1156:00 F 286.5 284.3 9.7 2 0.30 2 0.91 KNCA NC 2006129 1256:00 287.0 283.7 11.3 2 0.30 KNCA NC 2006129 1356:00 288.7 283.7 11.3 2 0.61 2 3.05 KNCA NC 2006129 1456:00 289.3 283.7 11.3 2 0.61 3 0.91 ID State Day UTC WX T TD VIS C CIGH C ZCL1 KABY GA 2006129 0953:00 F 291.5 290.9 4.8 3 0.12 KABY GA 2006129 1053:00 F 291.5 290.4 4.0 3 0.12 KABY GA 2006129 1153:00 F 292.0 290.4 4.0 3 0.12 KABY GA 2006129 1225:00 F 292.0 290.9 4.8 3 0.12 KABY GA 2006129 1253:00 F 292.0 290.4 4.8 3 0.15 KABY GA 2006129 1353:00 F 292.6 290.4 4.8 3 0.15 KABY GA 2006129 1453:00 H 293.2 290.4 6.4 3 0.21 ID State Day UTC WX T TD VIS C CIGH C ZCL1 KHOU TX 2006129 0953:00 F 296.5 295.4 9.7 2 0.46 1 0.24 KHOU TX 2006129 1053:00 297.0 295.9 11.3 2 0.61 1 0.34 KHOU TX 2006129 1153:00 F 297.0 295.9 8.0 2 0.34 2 0.70 KHOU TX 2006129 1253:00 F 298.2 296.5 8.0 2 0.43 2 0.61 KHOU TX 2006129 1353:00 H 299.3 296.5 8.0 2 0.49 1 0.30 KHOU TX 2006129 1453:00 H 300.4 296.5 9.7 2 0.70 1 0.46

“OLDCTP” image in the lower-left-hand corner. This does show the SIMDIF correctly detecting fog at Jack- clear area is also observed in the “NEWCTP” (lower- sonville at 1046 UTC. By 1146 UTC (Fig. 6c) the sun is right-hand corner), but the NEWCTP is not as void of far enough above the horizon that in the visible band, clouds as the OLDCTP. Both the visible imagery for clouds and clear regions are distinctly visible as far west this time period (upper-right-hand corner) and the sur- as Texas and Wisconsin. Despite these conditions, low face observation for Jacksonville, North Carolina (the clouds over Georgia and Alabama are not properly de- white dot in Fig. 6b), at 1100 UTC strongly contradict tected in the OLDCTP. Based on comparisons to the the OLDCTP and also indicate that the NEWCTP is visible imagery for this time period and comparisons to not cloudy enough either. surface observations of clouds at Albany (see Table 3 The primary reasons for the lack of cloudiness in the for details), the NEWCTP correctly identifies low NEWCTP version of the cloud-top pressure (CTP) are clouds over this region. And, at 1247 UTC the trend of twofold. First, the difference between 3.7- and 11.0-␮m the OLDCTP not detecting low clouds near the termi- bands falls below the criteria threshold (i.e., 5 K). A nator continues to move westward (Fig. 6d). For this second reason for the failure of the technique to cor- time period all of eastern Texas is devoid of clouds for rectly detect cloud is due to the terrain elevation limi- the OLDCTP, while the NEWCTP more correctly de- tation for the SIMDIF. Table 4a (similar to Table 2, but fines the cloudiness over this region based on compari- that the locations are Jacksonville; Albany, Georgia; sons with the visible image for the same time period Houston, Texas; and Corsicana, Texas; for 9 May 2006) and the surface observation at Houston (Table 3). In

TABLE 4a. Same as Table 2a, but for 9 May 2006. The surface location coincides with Jacksonville, NC.

UTC BT11 BT4.0 BT3.7 VIS SZEN WNDF BT3.7 – BT4.0 0946:00 276.14 274.42 274.62 0 93.14 Ϫ1.52 0.20 1046:00 279.16 279.54 285.16 5 81.43 6.00 5.62 1146:00 278.82 284.20 293.28 12 69.29 14.46 9.08 1246:00 277.10 286.89 298.49 24 56.80 21.39 11.60 1346:00 278.74 294.97 309.76 33 44.67 31.02 14.79 1446:00 279.77 296.99 311.52 39 32.80 31.75 14.53 OCTOBER 2007 NOTES AND CORRESPONDENCE 1809

TABLE 4b. Same as in Table 4a, but for Albany, GA.

UTC BT11 BT4.0 BT3.7 Vis SZEN WNDF BT3.7 – BT4.0 0946:00 286.52 283.44 283.90 0 98.83 Ϫ2.62 0.46 1046:00 285.82 283.53 284.58 2 86.93 Ϫ1.24 1.05 1146:00 286.26 288.94 294.76 12 74.51 8.50 5.82 1246:00 287.34 291.99 298.87 22 61.66 11.53 6.88 1346:00 287.49 294.31 302.60 32 48.83 15.11 8.29 1446:00 286.01 296.20 306.38 47 35.67 20.37 10.18

this case Table 4b does point out a weakness of the visible band of the GOES sounder as deep convective SIMDIF. Note the 1046 UTC GOES-12 sounder obser- clouds. vations and differences. At Albany with the SZEN ϭ The SIMDIF uses the brightness temperature differ- 86.93°, the SIMDIF does not accurately detect the ob- ences between the longwave window (11.0 ␮m), the served fog. This problem is rectified in the following shortwave window (3.7 ␮m), and also employs another time period. shortwave window (4.0 ␮m) for solar zenith angles be- As in Table 4b, Table 4c once again points out a tween 89.0° Ͻ SZEN Ͻ 70.0°, and surface elevations weakness. For the first time period immediately after less than 999 m. The strengths in this technique lie in its sunrise (SZEN Ͼ 83°) at the Houston location the ability to identify low-level water clouds or fog over SIMDIF fails to detect fog. Although for the following low, flat terrain over the eastern portion of the conter- time period (75° Ͼ SZEN Ͼ 70°) fog is correctly iden- minous United States (CONUS). Testing of the SIM- tified. Despite these very specific shortcomings, by in- DIF was not performed over the western sections of the corporating the SIMDIF a more complete definition of CONUS because of the terrain elevation limitations. this low cloud is delineated (NEWCTP). Figure 6f is This technique has been shown to be effective during included to show that 1) the two cloud product versions the late fall, winter, and early spring seasons. A weak- once again converge on the same answer once the ness of this method is mistakenly identifying low cloud SZEN is less than 70° and 2) the cloudiness for the when there are clear skies over fresh snow and dead southeastern portion of the United States is continuous vegetation, especially at high elevations. for the six time periods included in this particular In summary, for both the cloud mask and the CTP case. case study the SIMDIF was able to more correctly identify low-level cloudiness and fog that up to now was not 4. Summary and future work identified. The SIMDIF is currently being applied to the GOES sounder cloud product algorithm from A technique has been developed and tested to detect CIMSS. Routine hourly process incorporating this tech- low-level clouds and fog just after sunrise over the east- nique has shown a significant improvement in the iden- ern portion of the United States, based on analysis of 21 tification of low cloud and fog. The goal is to apply this cases from mid-September 2005 to early March 2006 technique to the GOES sounder temperature/moisture using GOES sounder radiance data. Frequently these retrieval algorithm. (Ma et al. 1999; Schmit et al. 2002). clouds are not properly flagged as the nighttime cloud (In fact, the latest iteration of this processing system mask is replaced with a daytime version of the mask at generates temperature/moisture products and cloud large solar zenith angles. Low clouds and fog just after products, CTP, and the effective cloud amount from sunrise are missed as they are not as reflective in the one processing system.)

TABLE 4c. Same as in Table 4a, but for Houston, TX.

UTC BT11 BT4.0 BT3.7 Vis SZEN WNDF BT3.7 – BT4.0 0946:00 289.88 290.38 290.95 0 107.41 1.07 0.57 1046:00 292.80 291.28 291.71 0 95.78 Ϫ1.09 0.43 1146:00 292.27 290.03 292.28 3 83.44 0.01 2.25 1246:00 292.38 295.59 301.11 9 70.55 8.73 5.52 1346:00 293.34 299.69 306.28 16 57.52 12.94 6.59 1446:00 294.23 303.69 310.51 17 44.09 16.28 6.82 1810 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 24

Although the SIMDIF was not tested for cases near ——, 2006: Evaluation of Moderate-Resolution Imaging Spectro- sunset (because none were found during the test period radiometer (MODIS) shortwave infrared bands for optimum of 14 September 2005 and 6 March 2006 over the east- nighttime fog detection. Preprints, 14th Conf. on Satellite Me- teorology and Oceanography, Atlanta, GA, Amer. Meteor. ern portion of the CONUS), there is no reason to be- Soc., CD-ROM, P3.24. lieve the SIMDIF will not work for this time period as Eyre, J. R., 1984: Detection of fog at night using Advanced Very well. By incorporating the SIMDIF into the operational High Resolution Radiometer (AVHRR) imagery. Meteor. production of GOES sounder products, the effective- Mag., 113, 266–271. ness of this technique near sunset will also be deter- Hillger, D. W., T. J. Schmit, and J. M. Daniels, 2003: Imager and mined. Some experimentation using this method in a sounder radiance and product validations for the GOES-12 science test. NOAA Tech. Rep. 115, U.S. Department of temporal mode has shown favorable results in addition Commerce, Washington, DC, 70 pp. to being more robust in its application. Future work Hunt, G. E., 1973: Radiative properties of terrestrial clouds at along this line will continue. visible and infrared thermal window wavelengths. Quart. J. Roy. Meteor. Soc., 99, 346–369. Acknowledgments. The authors wish to acknowledge Kleespies, T. J., 1995: The retrieval of marine stratiform cloud ␮ Mr. Timothy J. Schmit, Mr. Richard A. Frey, Dr. Jun properties from multiple observations in the 3.9- m window under conditions of varying solar illumination. J. Appl. Me- Li, Dr. W. Paul Menzel, Ms. Leanne Avila, and Mr. teor., 34, 1512–1524. Thomas H. Achtor for their insightful comments and Lee, T. F., F. J. Turk, and K. Richardson, 1997: Stratus and fog helpful suggestions. In addition, the authors wish to products using GOES-8–9 3.9-␮m data. Wea. Forecasting, 12, thank three anonymous reviewers who provided con- 664–677. structive criticism and helpful remarks. The Space Sci- Ma, X. L., T. J. Schmit, and W. L. Smith, 1999: A nonlinear physi- ence Engineering Center supplied the data for this cal retrieval algorithm—Its application to the GOES-8/9 sounder. J. Appl. Meteor., 38, 501–513. study. Funding for this work was provided by NOAA Menzel, W. P., and J. F. W. Purdom, 1994: Introducing GOES-I: Grant NA06NES4400002. The first of a new generation of Geostationary Operational Environmental Satellites. Bull. Amer. Meteor. Soc., 75, 757– REFERENCES 782. ——, F. C. Holt, T. J. Schmit, R. M. Aune, A. J. Schreiner, G. S. Ackerman, S. A., K. I. Strabala, W. P. Menzel, R. A. Frey, C. C. Wade, and D. G. Gray, 1998: Application of GOES-8/9 Moeller, and L. E. Gumley, 1998: Discriminating clear-sky soundings to weather forecasting and nowcasting. Bull. from clouds with MODIS. J. Geophys. Res., 103, 32 141– Amer. Meteor. Soc., 79, 2059–2077. 32 158. National Oceanic and Atmospheric Administration/Department Baum, B. A., R. A. Frey, G. G. Mace, M. K. Harkey, and P. Yang, of Defense/Federal Aviation Administration/U.S. Navy, 2003: Nighttime multilayered cloud detection using MODIS 1998: ASOS user’s guide. NOAA/DOD/FAA/U.S. Navy, 72 and ARM data. J. Appl. Meteor., 42, 905–919. pp. Bayler, G. M., R. M. Aune, and W. H. Raymond, 2000: NWP cloud initialization using GOES sounder data and improved Saunders, R. W., and K. T. Kriebel, 1988: An improved method modeling of nonprecipitating clouds. Mon. Wea. Rev., 128, for detecting clear sky and cloudy radiances from AVHRR 3911–3920. data. Int. J. Remote Sens., 9, 123–150. Daniels, J. M., T. J. Schmit, and D. W. Hillger, 2001: GOES-11 Schmit, T. J., W. F. Feltz, W. P. Menzel, J. A. Jung, A. P. Noel, Science Test: GOES-11 imager and sounder radiance and J. N. Heil, J. P. Nelson III, and G. S. Wade, 2002: Validation product validations. NOAA Tech. Rep. NESDIS 103, U.S. and use of GOES sounder moisture information. Wea. Fore- Department of Commerce, Washington, DC, 49 pp. casting, 17, 139–154. d’Entremont, R. P., 1986: Low and midlevel cloud analysis using Schreiner, A. J., T. J. Schmit, and W. P. Menzel, 2001: Trends and nighttime multispectral imagery. J. Climate Appl. Meteor., 25, observations of clouds based on GOES sounder data. J. Geo- 1853–1869. phys. Res., 106, 20 349–20 363. Ellrod, G. P., 1995: Advances in the detection and analysis of fog Sutherland, R. A., 1986: Broadband and spectral emissivities (2– at night using GOES multispectral infrared imagery. Wea. 18 ␮m) of some natural soils and vegetation. J. Atmos. Oce- Forecasting, 10, 606–619. anic Technol., 3, 199–202.