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Comparing Ship-Track Droplet Sizes Inferred from Terra and Aqua MODIS Data 230 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 52 Comparing Ship-Track Droplet Sizes Inferred from Terra and Aqua MODIS Data BURCU KABATAS Eurasian Institute of Earth Sciences, Istanbul Technical University, Istanbul, Turkey W. PAUL MENZEL Space Science and Engineering Center, University of Wisconsin–Madison, Madison, Wisconsin ATA BILGILI Maritime Faculty, Istanbul Technical University, Istanbul, Turkey LIAM E. GUMLEY Space Science and Engineering Center, University of Wisconsin–Madison, Madison, Wisconsin (Manuscript received 7 November 2011, in final form 3 July 2012) ABSTRACT In this study of ship tracks, Moderate Resolution Imaging Spectroradiometer (MODIS) measurements from late-morning (Terra) and early-afternoon (Aqua) Earth Observing System platforms are analyzed in five separate geographically distributed cases to compare estimates of the sizes (and their changes in time) of droplets associated with ship exhaust. Ship tracks are readily detected in near-infrared imagery as bright features, especially in 2.13-mm observations. The Terra ‘‘MOD06’’ and Aqua ‘‘MYD06’’ cloud products are used to determine the effective radius of the ship-track droplets; droplet age (time in the atmosphere) is estimated as a function of the distance from the ship. Terra and Aqua MODIS estimates of droplet sizes in ship-track plumes are found to be in agreement, with a correlation greater than 0.90; for the cases studied, droplet sizes in the ship plumes are between 6 and 18 mm. Moreover, the droplets’ size growth rates inferred 2 from the length of the ship track were found to average between 0.5 and 1.0 mmh 1. 1. Introduction balance (Durkee et al. 2000a). The exhaust released by ships has been found to increase the number of cloud The effect of anthropogenic aerosols on the earth’s droplets, to reduce the droplet size (Coakley et al. 1987), energy budget can either be direct through scattering and thus to contribute to a cooling effect on the earth’s and absorbing the incoming solar and infrared radiation surface (Twomey 1974; Albrecht 1989). Platnick and or indirect by acting as cloud condensation nuclei (CCN). Twomey (1994) used ship tracks to define cloud suscep- Ship tracks, which were first observed by the Television tibility and explained how the increase in cloud-droplet and Infrared Observation Satellite VII (TIROS VII)in number also increases the cloud reflectivity (albedo). 1965 (Conover 1966), are linear clouds seen in near- In this paper, Moderate Resolution Imaging Spec- infrared images of marine stratocumulus that are caused troradiometer (MODIS) cloud products (King et al. by emissions from ships (Segrin et al. 2007). The Monterey 1997) derived from near-infrared (NIR) MODIS spectral- Area Ship Track experiment studied ship tracks to un- band measurements are used to study ship-track droplet derstand the role of anthropogenic aerosols in modifying sizes and their changes over time in five case studies. the cloud reflectivity and hence the earth’s radiation MODIS cloud-particle-size estimates from the Terra and Aqua platforms of the National Aeronautics and Space Administration (NASA) Earth Observing System Corresponding author address: W. Paul Menzel, Space Science and Engineering Center, University of Wisconsin—Madison, 1225 (EOS) are compared. The following sections present a West Dayton St., Madison, WI 53706. summary of detection of ship tracks in MODIS imagery E-mail: [email protected] (section 2), a description of the data and algorithms DOI: 10.1175/JAMC-D-11-0232.1 Ó 2013 American Meteorological Society Unauthenticated | Downloaded 10/02/21 05:56 PM UTC JANUARY 2013 K A B A T A S E T A L . 231 (section 3), results from the five case studies (section 4), between reflection from a ship track’s smaller droplets and some conclusions (section 5). and nearby cloudy larger droplets. How much greater depends on droplet size and optical depth, which are inferred in the MOD06 algorithm (King et al. 1997; 2. Ship-track formation and detection Platnick et al. 2000). Ship tracks form in the low stratus and stratocumulus clouds off the western coasts of large continents. The dominant areas for stratiform-cloud formation are the 3. Data and algorithms eastern ocean basins between 208 and 508 latitude and at a. MODIS data overview high latitudes above about 608 (Durkee et al. 2000a). Schreier et al. (2007) reported on ship tracks in the sub- MODIS is a scanning radiometer on the EOS Terra tropical latitudes along the west coasts of southern platform (equator crossing at ;1030 local standard time) Africa, South America, and North America. In the and EOS Aqua platform (equator crossing at ;1330 local Northern Hemisphere, cold upwelling ocean currents standard time) that has a long-term science mission to cause a stable atmospheric layer that is due to Ekman study global changes in land, ocean, and atmosphere pumping by northerly winds along the coast; in the (King et al. 1992). Each MODIS provides worldwide Southern Hemisphere, the necessary winds are south- datasets in 36 spectral bands every 2 days from a polar- erly winds. This causes the marine boundary layer to orbiting, sun-synchronous, platform at an altitude of become saturated and thus provides an ideal environ- 705 km with a 2330-km swath width. ment for the formation of stratiform clouds (Evans 1992; The MODIS cloud products (denoted by ‘‘MOD06’’ Klein and Hartmann 1993). When particles from ship for Terra and ‘‘MYD06’’ for Aqua) have been described engine exhaust enter a stratiform cloud layer in the by King et al. (1997); these include the effective particle boundary layer, they can act as CCN and form cloud radius, which is used to examine the droplet size along droplets (Hobbs et al. 2000). the lengths of ship tracks. Not every ship produces a ship track. Ambient con- b. Estimating droplet size ditions of a well-mixed boundary layer, low numbers of CCN, and near-constant surface temperature and rela- The dependence of cloud reflectance on droplet size is tive humidity in the marine atmosphere enhance the related to the ratio of total volume of the drops to their probability of ship-track formation (Conover 1966). The total surface area (Rosenfeld and Woodley 2001). The high static stability associated with these atmospheric incoming radiation is scattered from the surface of conditions confines the clouds to the boundary layer, the droplets, and therefore the scattered radiation is enabling the ship-emitted aerosols to reach the cloud proportional to the total surface area of the droplets base. The boundary layer depth must be low; ship tracks (proportional to the droplet radius squared: ;r2). The rarely occur in low-level clouds having altitudes of absorbed radiation is proportional to the total volume greater than 1 km. A small rise in low-level cloud alti- of the droplets (proportional to the droplet radius cu- tude can cause the disappearance of ship tracks from one bed: ;r3) since it occurs inside the droplet. Hence, in day to the next (Coakley et al. 2000). Fuel type also plays approximately constant amounts of liquid water, the a significant role in whether a ship track is produced. It scattering-to-absorption ratio increases in ship-track has been reported (Hobbs et al. 2000; Noone et al. 2000) clouds with smaller and more numerous droplets; the that diesel ships burning low-grade marine fuel oil emit increased scattering is evident in the higher reflection larger particles than do ships burning navy distillate fuel, from the ship-track plumes than from the surrounding and these particles serve as CCN at lower supersatura- clouds with larger and fewer droplets. tions (and will therefore be more likely to produce ship Reflection in the NIR wavelengths is sensitive to tracks). both cloud optical thickness and cloud-particle size In ship tracks, the smaller and more numerous droplet (Nakajima and King 1990; Platnick et al. 2000). MOD06 sizes make the plume brighter and more reflective to and MYD06 use the bands at 0.86 and 2.13 mm. For incoming sunlight, especially in the NIR part of the a given optical depth, the 2.13-mm reflected radiance is spectrum at 1.64 and 2.13 mm (King et al. 1992). These more sensitive to the cloud-particle size; conversely, NIR spectral bands exhibit little water vapor absorption for a given particle size, the reflected radiance of the and reflect more from smaller droplets. Moreover, the 0.86-mm band is more sensitive to the cloud optical decrease in reflectivity with increased droplet radius depth. (ranging from 5 to 20 mm) is more pronounced at longer In this work, the droplet effective radius at a given wavelengths; hence, 2.13 mm shows greater contrast location within a ship-track plume is estimated from Unauthenticated | Downloaded 10/02/21 05:56 PM UTC 232 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 52 FIG.1.Terra MODIS 2.13-mm reflectance for a scene exhibiting ship tracks off the coast of California at 1920 UTC 30 Sep 2005. Five plumes are identified with numbers. Two transects are indicated across exhaust plume 1, one nearer the ship (in red) and another farther away (in green). MODIS Terra MOD06 and Aqua MYD06 determi- Figure 1 shows the reflectance values for a scene nations along a transect perpendicular to the ship track, with several ship tracks off the coast of California on away from ship-track intersections. Working from the 30 September 2005 in Terra MODIS band 7 (2.13 mm). 2.13-mm radiance data, one selects a given part of the The reflectance values within a plume are larger (;0.20– ship track, and a pixel near the center of the plume, 0.25) than the values outside the plumes (;0.10–0.15).
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