Step 2C. Eye Pattern

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Step 2C. Eye Pattern

Eye patterns are analyzed in this step only when the eye falls near the point of the expected cloud system center, and after a T2 or greater pattern has been observed 24 hours prior to the current observation.

The eye is defined as one of the following:

(1) A warm (dark) spot in a dense, cold (OW or colder) overcast. (When more than one dark spot appears near the CSC, use the center closest to the expected center location.)

(2) A point in a dense, cold (OW or colder) overcast centered within the curvature of a colder (denser) band that curves at least halfway around the point with a diameter of curvature of 1 1/2° latitude or less.

(3) A spiral band wrapped around a relative warm (dark) spot with a diameter of curvature of 1 1/2° latitude or less. The band must curve at least 1.0 the distance around the 10° log spiral curve. (See pattern labeled DT 4 in figure 3, 2A.

The analysis of the eye pattern involves three computations: The eye number (E), the eye adjustment factor (Eye Adj), and the banding feature (BF) Eye Adj. number. The equation is: CF + BF = DT {data T-number), where CF = E no. + 1. EIR only (See 2. for VIS)

a. E (eye) number. To get the E or eye number, first determine the coldest gray shade that surrounds the relatively warm spot. Make certain that the minimum width of this grav shade meets the ""narrowest width" requirement shown in the diagram. When a spiral eye is defined, use the average width of the spiral band to determine the narrowest width criteria.

b. Eye Adjustment Factor. The eye adjustment factor is determined by using the graph in Figure 6. The graph is a plot of eye temperatures versus the temperature of the coldest ring or spiral that completely encircles the eye. This provides an adjustment of +_0.5, +_1, or 0 to the "E" number. No plus adjustment can be made for large eyes (>_ 3/4° diameter within the surrounding gray shade) or elongated eyes. When no previous subtraction was made, .5 is subtracted for elongated eyes having E numbers of > 4.5. Elongated eyes are defined as those having a short axis of <2/3 the long axis within the surrounding gray shade.

c. Banding Feature (BF). The BF addition is used with EIR pictures only when the T-number estimate without the BF is lower than the model expected T-number. It is defined only for patterns of CF4 or more that contain a clear-cut comma tail band that:

(1) curves 1/4 or more of the distance around the central features or comma head,

(2) is cold (MG or colder), and

(3) has a warm wedge (DG or warmer) between the tail and the central features that cuts at least halfway through the pattern for patterns a and b, Figure 7, and at least 2/3 the way for pattern c.

2. VIS only (See 1 above for EIR)

a. The E (eye) number is obtained by measuring the distance the eye is embedded in dense overcast clouds. The embedded distance of the eye is measured outward from the center of the eye to the nearest outside edge of the dense overcast for small (<30nm) eyes. For large eyes, measure outward from the inner wall of the eye. When a banding-type eye is indicated, the arc length of the band around the eye and the average width of the band surrounding the eye are important to the intensity determination, as indicated in the diagram. See analysis diagram (Figure 3, 2C) for the relationships between E-number and embedded distance (eye in CDO), and for band width (banding eye).

Figure 6. Eye Adjustment Graph. Rules: (1) For large or elongated eyes, use values to the right of the diagonal line only; (2) for elongated eye patterns >4.5, subtract .5 when no other subtraction was made.

Figure 7. EIR Banding Feaatures. Add to the CF only when the data T-no. is lower than the MET.

Figure 8. VIS Banding Features.

b. The eye adjustment factor is determined by the definition, shape, and size of the eye. The eye is well-defined by either its blackness or by a well-defined boundary. To be well-defined, the eye should be dark or black. Remember that a very high or very low sun angle may reduce the eye definition unrealistically, and that high-resolution pictures may show a poorly defined eye that would not appear in the low-resolution pictures for which the tech-nique was designed. A poorly defined eye is one that is barely visible. A ragged eye is one with a very uneven boundary with little circularity. VIS eye adjustment rules are as follows. (1) For poorly defined or ragged eyes, subtract 1/2 number for E numbers of < 4.5 and subtract 1 number when E > 5. When analyzing patterns with poorly defined eyes especially in high-resolution pictures, also check the CDO size. Use the estimate which is most consistent with the MET. (2) For large eyes, limit the maximum T-number to T6 for round, well-defined eye patterns, and to T5 or lower for all other large-eyed pat-terns. And, (3) the E-number may also be adjusted upward by either .5 or 1.0 when the eye is well-defined, circular and embedded in a very smooth, very dense appearing canopy. The addition is made only when the data T-number is lower than the MET and the storm’s past history gives an expected T-number of T-6 or more. The general rule for the eye adjustment factor is: When an adjustment is not clearcut, use the guidance of the MET to make the final decision.

c. The BF adjustment is often an important factor when VIS pictures are used. It is defined as a dense, mostly overcast band that curves quasi-circularly at least 1/4 the distance around the central feature. Bands that curve evenly around an inner BF may also be counted. The amount of the BF term ranges from .5 to 2.5. It depends on the width of the band and the amount the band curves evenly around the central features, as shown in Figure 8. A BF term is not used for pre-hurricane patterns when the curved dense band concept in Step 2A is used. However, it is still needed for CDO patterns and all hurricane patterns when indicated. For banding eye patterns use the central coil (once around the eye) as the CF and add the BF as indicated. This pattern type is rarely used for DT of greater than 4.5.

Step 2D. CDO Patterns (VIS only)

CDO patterns are defined when a dense, solid-looking mass of clouds covers the cloud system center and lies within the curve of the system’s comma band. Both its size and the sharpness of its boundary are important to the analysis. A well-defined CDO has an abrupt edge on at least one side of the cloud mass. An irregular CDO appears within the curve of the comma band but has ragged boundaries and uneven texture. Generally, well-defined CDO’s that measure about 1° latitude in their narrowest width are associated with tropi-cal storm intensities while those measuring 2° latitude or more are associated with hurricanes. The size-CF number relationship is given in the analysis diagram, Figure 3. Examples of CDO’s are shown in Figure 4, Step 6b. For CDO patterns, the analysis equation is CF + BF = DT. Banding features (BF) are usually added to the CF term for CDO patterns. The BF’s are described above in 2C,2C.

Step 2E. Embedded Center Patterns (EIR only)

Embedded center patterns are analyzed when the storm has had a previous history of a T3.5 or greater intensity and when the CSC is clearly indicated to be within a cold overcast (OW or colder). Curved cloud lines or bands within the cold overcast as well as the outer curved bands will indicate the location of the CSC within the overcast. A 10° logarithmic spiral can often be fitted to the system’s pattern to help locate the CSC in patterns of hurricane intensity. (See Step 2A for fitting spiral.)

The analysis of this pattern is similar to the eye pattern analysis except that no eye adjustment factor is added. Determine the coldest overcast in which the CSC is embedded the required distance. This yields the central feature number (CF). Then add a banding feature (BF) adjustment when indi-cated. The equation being CF + BF = DT.

STEP 3. CENTRAL COLD COVER (CCC) PATTERN

The CCC pattern is defined when a more or less round, cold overcast mass of clouds covers the storm center or comma head obscuring the expected signs of pattern evolution. The outer curved bands and lines usually weaken with the onset of CCC. When using VIS pictures, substitute the word "dense" for "cold." It is only rarely that the CCC pattern is used with VIS pictures since the CDO or curved lines are usually visible through the thin cirrus clouds. When the CCC persists (see rules in diagram, Step 3), development has been arrested until signs of development or weakening once again appear in the cloud features. Care should be exercised under the following conditions:

(1) Do not confuse a CCC pattern with a very cold comma pattern. A very cold (usually white) pattern is indicated by a very cold (very smooth texture) comma tail and head with some indication of a wedge in between. Curved cirrus lines or boundaries usually appear around the cold pattern and not around the CCC pattern. The very cold pattern for T-numbers of T3 or less warrant an additional 1/2 number in intensity estimate and often indicates rapid growth.

(2) Do not assume weakening in a CCC pattern when the comma tail begins to decrease in size. It is common to observe the tail decreasing in size at the onset of the CCC. Also the CCC often warms as the eye of the T4 pattern begins to be carved out by a warm incursion into the side of the cold over-cast. This signals the resumption of pattern evolution (intensification) even though some warming is evident.

STEP 4. DETERMINE THE TREND OF THE PAST 24-HOUR INTENSITY CHANGE

The trend of the past 24-hour intensity change is determined qualitati-vely by comparing the cloud features of the current picture with those in the 24-hour old picture of the storm. In general, a disturbance has developed when its center appears better defined with no change in the relation to the dense clouds of the disturbance or is more involved with dense overcast clouds. More precise definitions for development, weakening or steady state changes are given below.

The storm has developed (D):

(1) Curved band pattern: Curved band coils farther around the CSC.

(2) CDO pattern: CDO becomes larger or an increase in banding features is noted.

(3) Shear pattern: CSC becomes more tightly defined in curved cloud lines or appears closer to the dense overcast.

(4) Eye pattern: Eye is more embedded, more distinct (warmer), less ragged, or is surrounded by colder (smoother textured) clouds, or more banding features.

(5) No significant warming (darkening) of the cloud system is noted. By significant, it is meant that a change that is not diurnal (near sunset), which lasts for more than 3 hours, and is great enough to lower the T-number.) The storm has weakened (W):

(1) The storm has weakened when its cloud pattern indicates a persistent trend opposite to those listed in (1)-(5) above. Watch in particular for pat-terns that become sheared out (elongated with time) or for patterns undergoing nondiurnal warming (lowering) of their cloud tops.

The storm has become steady state (S):

(1) When a central cold cover appears in a T3.5 or greater storm or has persisted for more than 12 hours in a weaker storm; or

(2) When the CSC’s relationship to the cold clouds has not changed significantly; or

(3) When there are conflicting indications of both development and weakening.

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