World Wind Regimes - Eastern Pacific Ridgelines Tutorial

	NRL Monterey, Marine Meteorology Division http://www.nrlmry.navy.mil

World Wind Regimes - Eastern Pacific Ridgelines Tutorial

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Introduction

Use of Satellite Data in Ridge Line Analysis

Ridgelines on surface charts are drawn to denote the location of regions of maximum anticyclonic vorticity emanating from high-pressure centers. These lines also denote the region of weakest winds over oceanic regions where sea state, in terms of capillary wave action, is calm or almost calm. Such regions in polar-orbiting satellite data reflect solar energy brilliantly to the satellite visible sensor when the region is viewed near the Primary Specular Point (PSP) (Fett, et al., 1977). A short distance east and west of the PSP unusually dark tones are observed. The dark tones indicate the region where the sun's rays are diverted totally away from the spacecraft sensor, as from a mirror-like surface. This effect can be very usefully employed in detecting ridgelines over oceanic regions. An example of such an effect is provided in Fig. 1 and in Fig. 2, below.

Background

Fig.1. TIROS-N Visible Image, 9 Jul 1979, 2331Z	Fig. 2. FNOC Surface Analysis, 10 Jul 1979 0000Z

Fig. 2 shows a region of sunglint south of a frontal band in EASTPAC. The linear dark-toned region running through the sunglint pattern indicates a calm sea region that should define a high pressure center or ridge line in advance of the front. Fig. 3 is an FNMOC surface analysis near the time of the TIROS-N visible data, which confirms that a ridgeline and high pressure center exactly conforms to the position of the dark-toned streak in the TIROS-N data.

Fig.1. TIROS-N Visible Image, 9 Jul 1979, 2331Z

Fig. 2. FNOC Surface Analysis, 10 Jul 1979 0000Z

Fig. 2 shows a region of sunglint south of a frontal band in EASTPAC. The linear dark-toned region running through the sunglint pattern indicates a calm sea region that should define a high pressure center or ridge line in advance of the front.

Fig. 3 is an FNMOC surface analysis near the time of the TIROS-N visible data, which confirms that a ridgeline and high pressure center exactly conforms to the position of the dark-toned streak in the TIROS-N data.

Advantages

Today new sensors such as the Special Sensor Microwave Imager (SSM/I) of DMSP and the scatterometer "QuikSCAT", aboard NASA'S Ocean Wind Satellite, can often be used to provide information relative to ridgeline location. Knowledge of ridgeline location is important, not only for accurate synoptic analysis, but also for use in predicting the most likely regions for development of low clouds and fog, which have a high correlation of occurrence in such regions. From a tactical perspective, aircraft carriers try to avoid such regions for aircraft landing and takeoffs because of the necessity of the carrier to build up speed to safely support such operations by increasing airflow across the carrier's deck. This can be quite expensive when compared to operating the carrier in a naturally occurring higher wind speed region. Some examples are provided below to demonstrate the new capability with SSM/I data.

Examples

Fig. 3. SSM/I Winds. 11 Sep 2001, 0610Z	Fig. 4. Same as Fig. 1 (Ridge Line Locations Superimposed)

Fig. 3 shows SSM/I data that reveal elongated minimum wind speed east and west of a frontal cloud band in EASTPAC north of Hawaii. Logical interpretations of these minima suggest ridgeline locations as drawn in Fig. 4. Note that the locations suggested are quite definitive. Accuracy of ridgeline location derived from pressure readings by ship reports in the region are dependent on the aerial density of such reports and might not be as definitive.

Fig. 3. SSM/I Winds. 11 Sep 2001, 0610Z

Fig. 4. Same as Fig. 1 (Ridge Line Locations Superimposed)

Fig. 3 shows SSM/I data that reveal elongated minimum wind speed east and west of a frontal cloud band in EASTPAC north of Hawaii. Logical interpretations of these minima suggest ridgeline locations as drawn in Fig. 4. Note that the locations suggested are quite definitive. Accuracy of ridgeline location derived from pressure readings by ship reports in the region are dependent on the aerial density of such reports and might not be as definitive.

Fig. 5. GOES-10 Infrared Image (Surface Pressure Analysis Superimposed). 11Sep 2001, 0630Z	Fig.6. Same as Fig. 1 (Ridge Line Locations Superimposed)

Fig. 5 shows GOES infrared data with a surface pressure analysis superimposed. Fig. 6 is the same figure with ridgelines drawn, as suggested by the pressure analysis. The SSM/I data do not cover as large an area as the GOES image; however, agreement of ridgeline locations in the region covered by SSM/I is excellent. The point of this comparison is to show the usefulness of SSM/I data for such determinations. In the event of communication problems, when model output might not be available, satellite readout of SSM/I might "save the day". REFERENCES Fett, R. W., W. A. Bohan and W. F. Mitchell, Navy Tactical Applications Guide, Vol. 1, Techniques and Applications of Image Analysis, Naval Research Laboratory, Monterey, CA, 93940, pp176.

Fig. 5. GOES-10 Infrared Image (Surface Pressure Analysis Superimposed). 11Sep 2001, 0630Z

Fig.6. Same as Fig. 1
(Ridge Line Locations Superimposed)

Fig. 5 shows GOES infrared data with a surface pressure analysis superimposed. Fig. 6 is the same figure with ridgelines drawn, as suggested by the pressure analysis. The SSM/I data do not cover as large an area as the GOES image; however, agreement of ridgeline locations in the region covered by SSM/I is excellent. The point of this comparison is to show the usefulness of SSM/I data for such determinations. In the event of communication problems, when model output might not be available, satellite readout of SSM/I might "save the day".

REFERENCES

Fett, R. W., W. A. Bohan and W. F. Mitchell, Navy Tactical Applications Guide, Vol. 1, Techniques and Applications of Image Analysis, Naval Research Laboratory, Monterey, CA, 93940, pp176.

Author: Bob Fett
Last Updated: Mon Dec 9 10:48:23 2002
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NRL Monterey, Marine Meteorology Division http://www.nrlmry.navy.mil