INTENSITY ANALYSIS PROCEDURES AND RULES
STEP 1. LOCATE THE CLOUD SYSTEM CENTER (CSC)
The cloud system center is defined as the focal point of all the curved lines or bands of the cloud system. It can also be thought of as the point toward which the curved lines merge or spiral.
(1) The CSC is located at the center of the eye or at the center of curvature of a partial eye wall when one of these features is observed.
(2) When the CSC is not obvious, locate the model expected CSC.
Draw a line along the "curved band axix" through the most dense (coldest) por-tion of the bandm The axis should roughly parallel the concave (inner) over-cast boundary of the band. Locate the model expected center location in relation the the curved band. (See plus symbols in diagram in Step 2A.) The center is located near the inner (concave) edge of the band on the coun-terclockwise end (comma head) portion of the band. Locate tightly curved lines, merging lines, or CDO near the point where the center is expected to fall. The CSC is located at the center of curvature, near the point of mergence or at the center of the CDO (for CDO of _< 1 1/2° latitude in size). For large CDO’s, the center is sometimes defined by an arc of overshooting cloud tops or in an isolated cluster of convective tops. When not visible, use (3) below.
(3) When features are not visible at the expected CSC, or when the curved band is not apparent, use the circle method. The method consists of first drawing lines following the cloud line curvature or curved boundaries that fall within the curve of the curved band axis, and then fitting circles to the lines with tightest curvature.The CSC is located at the center of the area common to the circles. For relatively circular embedded center patterns of >T3.5 intensity, fit a log 10° spiral overlay to the curved band axis to locate center.
(4) When a cloud minimum wedge is visible on the concave side of the band near its middle, the CSC is located at the midpoint of a line drawn bet-ween the deepest cloud minimum incursion of the wedge and the counterclockwise extremity of the curved band axis. This method is frequently used with EIR pictures. In EIR pictures, the center is often located in the tight gradient near the coldest part of the pattern.
(5) When the location of the CSC is unclear, or could be placed at different locations, use all the methods above along with an extrapolation from the past track positions in making the final decision.
(6) When more than one well-defined CSC is apparent, use the one defined by the strongest appearing, lowest level cloud lines that best fits the past track of the storm. When strong vertical shear is apparent, remember that the upper level (dense) clouds will not be centered directly over the low-level center, but will be displaced with the CSC on the tight temperature gradient (sharp boundary) side of the dense cloud pattern.
Step 1A. Initial Development
The earliest signs of tropical cyclone development are observed about 1 1/2 days before a disturbance reaches tropical storm strength. At this time, the disturbance is classified a T1. A T1 is first used when a cluster of deep layer convective clouds showing line or band curvature has the following three properties.
(1) It has persisted for 12 hours or more.
(2) It has a cloud system center defined within an area having a diameter of 2 1/2° latitude or less which has persisted for 6 hours.
(3) It has an area of dense, cold (DG or colder) overcast* of >1 1/2° in extent that appears less than 2° from the center. The overcast may also appear in cumulonimbus lines the curve around the center. The cloud system center will be defined in one of the following ways:
(1) Curved band, a dense (DG or colder) overcast band that shows some curvature around a relatively warm (cloud minimum) area. It should curve at least one-fifth the distance around a 10° log spiral. Cirrus, when visible, will indicate anticyclonic shear across the expected CSC. (See diagrams, Step 6, PT 1.5 pattern types.)
(2) Curved cirrus lines indicating a center of curvature within or near a dense, cold (DG or colder) overcast. (See Figure 4, Step 6, PT 1.5b.)
(3) Curved low cloud lines showing a center of curvature within 2° of a cold (DG or colder) cloud mass. (See diagrams, Step 2B, DT 1.5 pattern.) In many cloud clusters that eventually develop, the northern boundaries show a straightening about 1 1/2 days prior to the T1 classifications. During the organizing stage of the T1 pattern, there may be extreme variability in the cloud pattern. In most developments at the T1 stage, strong upper-level hori-zontal anticyclonic shear will be indicated across the disturbance center when curved cirrus lines are present to reveal the shear. These upper level clouds. may indicate patterns far more advanced than T1 at the time of the initial classification. These patterns do not involve deep tropospheric circulations at this time and will be short lived. This means that the Day-2 data T-number may at times be less the Day-1’s, but still development is indicated as long as theDT is 2 or more. There may also be times during the first two days of development when cirrus or convective clouds are almost absent, showing little pattern during the nighttime hours. This usually does not mean the storm is weakening. The rule is to never lower the T-number at night during the first 24 hours of development. A flat boundary rotating clockwise across the north side of the pattern throughout the period is a good sign of development. Note that a classification of T1 forecasts tropical storm
*The amount of cold overcast may decrease during the subsequent nighttime hours making it crucial that the analyst watch for the required amount of overcast when it occurs. intensity (T2.5) 36 hours after the T1 observation only when the environment is expected to remain favorable. A minus symbol is used after the T1 to indi-cate a T1 pattern that is not expected to develop. (See step 11. )
STEP 2. DETERMINE THE PATTERN TYPE THAT BEST DESCRIBES YOUR DISTURBANCE AND MEASURE CLOUD FEATURES AS INDICATED
The manner in which the cloud system center is defined determines the pattern to be analyzed. The pattern types listed below are described on the following pages. When the cloud pattern being analyzed does not resemble one of the patterns, proceed to Step 3.
Step 2A. "Curved Band" Pattern
Step 2B. "Shear" Pattern
Step 2C. "Eye" Pattern
Step 2D. Central Dense Overcast (CDO) Pattern
Step 2E. Embedded Center Pattern
General Analysis Rules:
1. When short-interval pictures are available, use the average measure-ment of all of the pictures with well-defined features taken within the 3 hour period ending at analysis time.
2. When two or more T-number estimates are made from the same picture, use the estimate closest to the MET.
3. When in doubt concerning ambiguous features, bias the analysis toward the MET.
Step 2A. Curved Band Pattern
The intensity estimate determined from this pattern type is derived by measuring the arc length of the curved band fitted to a 10° logarithm spiral overlay. (A circle will give the same answer most of the time.) The inten-sity values that relate to the curved band length are given in the analysis diagrams, Figure 1,3. Curved band measurements may be used with both VIS and EIR pictures until an intensity of DT 4.5 is reached. For EIR patterns greater than DT3.5 use measurements from VIS diagram.
The spiral overlay is fitted to the curvature of the dense (cold) band by first drawing a line along the "curved band axis" and then fitting the spiral curve to the line drawn. The curved band axis is defined as the axis of the coldest overcast gray shade (most dense clouds) within the cloud band. The line should roughly parallel the overcast edge on the concave side of the band. When the band indicates two possible axes, use the one with tightest curvature. Cellular cold globs that do not fall in line with the curve of the comma band are ignored when drawing the line. Fit the spiral to the line drawn on the picture and measure the spiral arc length of the dense (cold) band that follows the spiral curve.
In EIR patterns (like those in Figure 2, Step 6, Row b), the cold comma band will often show warm breaks through its middle. These breaks will appear to be almost clear in the VIS picture, When this occurs, draw the comma axis as though it were continuous through the breaks paralleling the edge of the cloud minimum incursion into the concave side of the band. As the curved band pattern evolves it will usually be defined by the dark gray shade of the BD curve, but may at times appear defined in warmer or colder shades of gray. At times the boundaries of the band must be interpreted from its form in previous pictures.
During the first 2 days of development (T1 to T2), the amount of overall band curvature may change excessively, very little, or even decrease somewhat for short periods even though typical development is occurring. For this reason, the tendency should be to raise the T-number by one during the first 24 hours of development as long as the band remains curved enough for T2 and clear signs of weakening or rapid development are not apparent. It is also important to allow at least 24 hours to pass between a T2 and a T4 classifica-tion. Even though the coiling process has been observed to be faster than this at times, the surface pressure does not fall accordingly.
During the T2.5 or T3 stage, a tightly curved band < 1 1/4° diameter of curvature observed within the curve of the broad curved band can also be, used as an indicator of tropical storm intensity. This is evidence that the wall cloud is forming. This tight curvature at weak tropical storm inten-sity is often ragged in appearance but will have deep-layer convective cloudi-ness on nearly opposite sides of a system center.
Step 2B. Shear Patterns
Shear patterns appear in pre-hurricane stages of development when vertical shear prevents the cold clouds from bending around the cloud system center as they do in the curved band patterns. The pattern may also appear after the hurricane stage has weakened to a pre-hurricane pattern because of increasing vertical shear.
The intensity estimate determined from this pattern type is derived by (1) the way in which the cloud system center is defined and (2) the distance between the low cloud center and the dense, cold overcast. For shear patterns associated with tropical storm intensity (T2.5 to T3.5), the center will be defined by parallel, circularly curved low cloud lines with a diameter of about 1.5° latitude or less. They indicate a center either near the edge or under the edge of a dense, cold (DG or colder) overcast cloud mass (see patterns in Step 2B, Figures 1,3. During the weaker stages of development (T1.5 + .5), the low cloud center will either be poorly defined in spiral lines within 1.25° of the cold overcast, circularly defined but some distance (>1.25° latitude) from the cold overcast clouds, or circularly defined near a small amount (<1 1/2° diameter) of dense overcast.