SATELLITE
INTERPRETATION
DISCUSSION

NOAA/NESDIS  Regional and Mesoscale Meteorology Team Cooperative Institute for Research in the Atmosphere (CIRA) Colorado State University   Fort Collins, Colorado

January 9, 2002

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Figure 1

Abstract:  On the evening of 17 December 2001, rain fell over much of the eastern United States and snow fell north of Massachusetts. Members of the RAMMT examined this precipitation event during the daily weather discussion. RAMMT members noted the cold cloud tops to the west of the Appalachians and the warmer clouds tops to the east (Figure 1). The following question was raised: “based on satellite and surface temperature observations, where is it raining and snowing and with what intensity?”  This question is treated as an exercise in satellite interpretation. Click on image to enlarge.

1) clouds cover most areas east of the Great Plains, except for Florida, Georgia, the Carolinas, and Virginia. 2) large parts of this cloud area exhibit tops below freezing (indicated by light gray, white, and color enhancements) 3) a contiguous line of cold cloud extends northeast from Tennessee to the eastern Great Lakes and beyond (cold cloud refers to areas of color enhancement with temperatures less than –30C) 4) this line of cold cloud exhibits transverse banding (i.e., cloud elements oriented perpendicular to the line) 5) this line of also extends south from Tennessee to the Gulf of Mexico as a contiguous line of clouds, sometimes exhibiting color enhancement 6) to the east of this line, more isolated and transitory cold cloud elements are observed.

Figure 2 Click on image to start loop.

Surface observations show freezing temperatures north of Massachusetts (at the end of this exercise, a national surface map/radar mosaic is presented).

Based on the infrared loop and surface temperature observations, the following distribution of precipitation is inferred:
1) light precipitation under all areas with persistent and/or cooling cloud tops well below freezing (–10C or colder; white and color enhancements)
2) moderate precipitation under the contiguous line of cold cloud, with more intense precipitation under areas with cooling and/or banded tops and less intense precipitation under areas with warming tops
3) moderate precipitation under isolated and transitory cold cloud elements
4) snow north of Massachusetts where surface temperatures are below freezing and one or more of the above conditions exists.

Light precipitation is expected beneath persistent and/or growing clouds with tops colder than –10C. These clouds contain high concentrations of supercooled liquid water droplets that are ready to nucleate, grow by deposition / aggregation / riming, and precipitate. Stated more generally, these clouds are primed for mixed phase precipitation production involving supercooled droplets and ice particles. Mixed phase precipitation production is important since it is much more capable of forming precipitation during the wintertime than warm rain precipitation production (i.e., collision-coalescence). Note that these precipitating clouds delineate a region of synoptic-scale rising motion.

Moderate precipitation is expected under the contiguous line of cold cloud since these deeper clouds are assumed to contain active mixed phase regions where supercooled droplets and ice particles coexist and interact. More intense precipitation is expected under areas with cooling and/or banded tops since these features indicate enhanced vertical motions. Less intense precipitation is expected under areas with warming cloud tops since the warming suggests local subsidence. Moderate precipitation is expected under isolated and transitory cloud elements since these clouds are assume to contain active mixed phase regions.

The precipitation distribution inferred from satellite and surface temperature observations is verified with a national surface map/radar mosaic at 24:00 UTC (Figure 3). The correspondence between cloud and precipitation features can be seen in greater detail by comparing an infrared image at 23:15 UTC (Figure 4) with a regional radar mosaic at 23:30 UTC (Figure 5). Click on images (Figures 3 - 5) to enlarge. 
 

Figure 3

Figure 4

Figure 5

The interested reader should refer to Bader et al. (1995) for more information on satellite interpretation.