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Satellite Derived Offset Technique
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Satellite-derived Barnes Wind Analysis





In order to account for cloud motion in the time interval between satellite images, this study incorporates the UW-CIMSS satellite-derived cloud-motion VIS and IR AMV algorithm (Velden et al. 1997, 1998) toward the formation of satellite-derived offset vectors (SOVs) for evaluating cloud-top TB and multi-spectral band difference trends. A SOV is a measure of the distance that a cumulus cloud pixel has traveled (in pixels) between successive satellite images.

Initialization settings associated with AMV algorithm were manipulated in a way that allows for the identification of both synoptic- and meso-scale flows (Bedka and Mecikalski 2005). The AMV algorithm used in operational settings preferentially identifies synoptic-scale flow in geostrophic balance. Adjustments were made to 1) reduce the impact of the NWP model background, 2) increase the number of cloud features tracked, 3) change the AMV editing methodology for optimal meso-scale AMV detection. These changes all serve to enhance detection of meso-scale cloud motions associated with boundary layer (upper-tropospheric) convergence (divergence) that can be used to more accurately obtain cloud-top trend information.

Once the AMVs are obtained, a Barnes objective analysis (Barnes 1964) is performed to produce a satellite wind analysis at 1-km resolution for three atmospheric layers, 100-70 kPa, 70-40 kPa, and 40-10 kPa. Convectively-induced cloud pixels are assigned an AMV from one of the three layers depending on a comparison of the pixel 10.7 µm TB and a NWP model temperature profile. Cloud-top trends are calculated if a satellite wind is present at the appropriate height within the near vicinity of a cumulus cloud (~0.25º radius). The wind speed, direction, and the time interval between images are used to obtain the SOV and thus the approximate pixel/cloud location in previous images.

After applying the SOV, a check is performed to ensure that the past pixel location does in fact represent a convectively-induced cloud. Assuming a pixel passes both checks (wind availability and past cloud presence) for both the 15- and 30-min time lags, cloud-top cooling/multi-spectral band difference trends are calculated. The passage of these checks therefore indicates that a convectively induced cloud is being tracked, back to a reasonable prior location, across successive images.

Despite use of the above checks, and every effort to obtain accurate SOVs (in terms of direction and magnitude), errors in estimating cloud TB trends using SOVs are inevitably present in the analysis below. These errors have been determined to range from ~1-2 km at 15 mins, to as large as 5 km by 30-45 mins. In fact, by the 45 min time lag (and beyond), use of the SOV technique to track clouds is not reasonable, except in conditions of uniform flow and nearly unidirectional vertical shear. Nonetheless, SOVs obtained from satellite winds are perhaps the only means of tracking clouds with reasonable accuracy over large geographical regions in real-time, thereby offering a substantial improvement over simple per-pixel differencing techniques. Other more sophisticated methods exist for tracking clouds in satellite data (e.g., Papin et al. 2000; Carvalho and Jones 2001), and these will be tested as part of future improvements to SOV calculation for this project. (back to top-->)

6.5-10.7 um Temporal Trends
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30 minute trend in the 6.5-10.7 micron difference technique

As described in the "Infrared Products Description" section, the 6.5-10.7 um band difference is useful for inferring cloud top height. The images above provide examples of temporal trends in this band subtraction technique. Values of this band subtraction technique typically range from around -50 to +3 K. The highest (postive) values of this technique indicate the presence of high clouds at or above ("over-shooting" cumulus tops) the tropopause whereas the lowest (negative) values are indicative of clear sky. Therefore, the highest temporal change in this technique can be +/- 53 K. A highly negative (-20 or greater) temporal change indicates one of two things: 1) high cloud movement away from a location (currently negative - formerly postive = negative value) or 2) rapid high cloud decay (currently highly negative - formerly slightly negative (positive) = negative value). A highly positive value can also mean one of two things: 1) high cloud movement into a location (i.e. cirrus expansion, currently positive - formerly negative = positive value) or 2) cumulus growth. Cumulus growth is what we are most interested in determining. Trend values of 3 K/15 mins were found to precede CI in several CI events syudied in the development of the CI nowcast algorithm. Hence, this value is used as an interest field for CI nowcasting. (back to top-->)

13.3-10.7 um Temporal Trends
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30 minute trend in the 13.3-10.7 micron difference technique

As described in the "Infrared Products Description" section, the 13.3-10.7 um band difference is useful for inferring cloud height and cloud type. The image above provides an example of the 30 min temporal trend in this band subtraction technique. Values of this band subtraction technique typically range from around -30 to +3 K. The highest (postive) values of this technique indicate the presence of high cirrus or mature cumulus clouds whereas the lowest (negative) values are indicative of low clouds such as immature cumulus, stratus, and fog. Therefore, the highest temporal change in this technique can be +/- 33 K. A highly negative temporal change indicates cirrus expansion (currently negative - formerly postive (or 0)= negative value). A positive value indicates cumulus growth. (currently positive - formerly negative (or 0) = positive value) Trend values of 3 K/15 mins were also found to precede CI in several CI events syudied in the development of the CI nowcast algorithm. Hence, this value is used as an interest field for CI nowcasting. (back to top-->)

10.7 um Temporal Trends
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GOES 10.7 micron: 5/04/03, 1930-2000 UTC


15 minute change of 10.7 micron TB < -4 K

This technique, documented in Roberts and Rutledge (2003), involves examination of time trends in 10.7 um TB (cloud top temp.). In this work, the authors noted that "the onset of vigorous cloud growth leading to storm development was characterized by cloud tops that reached sub-freezing temperatures and exhibited large cooling rates at cloud top 15 min. prior to the first detection of 10 dBZ radar echoes aloft and 30 min before 35 dBZ. The rate of cloud top temperature change was found to be important for descriminating between weakly precipitating storms (<35 dBZ) and vigorous convective storms (>35 dBZ)." The cloud top cooling rate for "weak, limited growth" was 4 K/15 mins and was 8 K/15 mins for "vigorous" growth. The product above illustrates locations exhibitng "weak, limited" growth" (i.e. the 4 K/15 mins threshold). We utilized the "satellite-derived offset vector" technique (described above) to perform the cloud top trend estimates. We have also calculated the cloud top cooling rate over a 30 minute interval, which was not performed in the forementioned paper. This allows us to identify clouds exhibiting sustained growth.

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