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NSSL Mesocyclone Detection Algorithm (MDA)

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NSSL Mesocyclone Detection Algorithm (MDA) Home The WATADS system utilizes special computer meteorological algorithms to process and analyze data, including algorithms developed at the National Severe Storms Laboratory and algorithms designed for the WSR-88D system (Build 10.0). NSSL ALGORITHMS The NSSL algorithms are designed to detect and analyze severe weather events. By processing Doppler radar data, the NSSL algorithms produce product information for display in RADS. Text files are also produced for more quantitative evaluation. See Appendix H for samples. The various NSSL algorithms included in WATADS are: • Mesocyclone Detection Algorithm (MDA) • Tornado Detection Algorithm (TDA) • Storm Cell Identification and Tracking (SCIT) Algorithm • Hail Detection Algorithm (HDA) Each of these meteorological algorithms is discussed below. NSSL Mesocyclone Detection Algorithm (MDA) The NSSL MDA addresses several shortcomings of the WSR-88D Mesocyclone Algorithm (88D-Meso), namely the high false alarm ratio and the failure to detect unconventional mesocyclones, such as low-topped mesocyclones. The algorithm also attempts to detect storm-scale vortices which fail to meet defined criteria for "mesocyclones," but still produce severe weather or tornadoes. Therefore, the new paradigm used in the NSSL MDA is to have the algorithm detect storm- scale vortices of various dimensions and strengths --- not only those meeting mesocyclone criteria. It then diagnoses those detections further to determine whether they may be asso- ciated with severe weather or tornadoes on the ground. The NSSL MDA algorithm diagnoses in two ways. The first way uses "Rule Bases," or decision trees, which attempt to classify the detections into one of several categories. The second way is via a Neural Network (NN) which computes the Probability of Tornado (POT) and Probability of Severe Weather (POSW) associated with each detected vortex. Other strength diagnostic parameters which are available in the NSSL MDA are Strength Rank and Mesocyclone Strength Index (MSI). These parameters are described in further detail in the next section. Page 6-1 Home By detecting vortices from their incipient signatures, through maturity, and on to demise, the algorithm can accurately associate the detections in time. This produces more valuable diagnostic information, such as tracking information, trends and time-height trends of various strength parameters associated with each vortex. This additional information provides the warning forecaster with valuable guidance in making warning decisions. Comparison to WSR-88D Mesocyclone Algorithm (88D-Meso) and use of output in making warning decisions. The NSSL MDA detects a wider spectrum of vortices than the 88D-Meso on the storm-scale (1-10 km from the radar), and diagnoses them in many ways. Two strength parameters are computed: 1) Strength Rank: A non-dimensional number based on various strength param- eters (rotational velocity, shear, maximum gate-to-gate velocity difference). Strength Rank is range-dependent, e.g., the farther a mesocyclone is from the radar, the lower the strength values needed to have the same strength rank). This parameter is similar to the WSR-88D Mesocyclone Recognition Guideline nomogram, except that the NSSL MDA has five strength classifications rather than four. The following general categories should assist in making warning decisions (See viewing NSSL Algorithm Output; 6-19): Rank 1 very weak Rank 3 weak to minimal meso Rank 5 mesocyclone Rank 7 strong mesocylone Rank 9+ very strong mesocyclone The highest rank observed in previous tests is 15, but higher ranks are not improbable. Note that Rank 5 and higher delineates a mesocyclone, and represents a three-dimensional vortex in which all two-dimensional features meet mesocyclone strength criteria over a continuous depth of 10,000 feet. Note also that Rank 5 and higher detections are coded in red or purple ("hot" colors) in the RADS Mesocyclone Table. Within 60 nm of the radar, the above Strength Ranks are achieved when vortices have a base below 18,000 feet and meet one or more of the following thresholds across a continu- ous depth of 10,000 feet: Page 6-2 Home Rank Shear (m s-1 km-1) Velocity difference or Maximum gate-to-gate (m s-1) 1 3.0 10. 3 4.5 20. 5 6.0 30. 7 7.5 40. 9 9.0 50. Intermediate ranks (e.g., 2, 4, 6, 8, 10, 11, 12, ....), are determined using linear interpolation or extrapolation from these values. Strength Rank values are also range dependent. At ranges beyond 120 nm, shear thresh- olds are 50% of the thresholds above, and the velocity difference/gate-to-gate thresholds are 75% of the above thresholds. Between the ranges of 60-120 nm, the thresholds decrease linearly from the values in the table above to the lower values at 120 nm. 2) Mesocyclone Strength Index (MSI): A non-dimensional number derived from vertical integration of the three strength parameters which define the Strength Rank (above) for each 2D feature comprising a 3D detection. The vertical integration is divided by the depth of the vortex, such that strong shallow vortices are not misrepresented as being weaker than strong deep vortices. The integration is weighted by density so that measurements closer to the surface are given more weight. In the RADS Mesocyclone Table, the MSI is coded in the following way: MSI Color 0 - 2300 (weak) green 2300-3600 (moderate) yellow >3600 (strong) red The highest value seen with the strongest mesocyclones is about 7500. Since the NSSL MDA uses a greater range of strength characteristics to detect mesocyclones than the 88D-Meso algorithm, it also has the flexibility for more vortex classifications. The 88D-Meso only classifies detections as uncorrelated shear, 3-D correlated shear and mesocyclones. The NSSL MDA uses the following classifications, listed from least to most significant: Type Table Code Table Color Description Weak Circulation WKCIRC (white) Any detection with a Rank less than 5. Note: some of these can be significant. Page 6-3 Home Type Table Code Table Color Description Couplet CPLT (orange) A "new" detection of Rank 5 or greater, having a depth of at least 10,000 feet, with no time history. Mesocyclone MESO (red) A Couplet with time history. (Rank 5 or greater) TVSMESO TUSMES (purple) A Mesocyclone associated with a TVS detection from the NSSL TDA. In addition, there are two "special" classifications: Low-topped LOWTOP (red) A detection of Rank 5 or greater, with a Mesocyclone base below 10,000 feet, which does not meet the >10,000 feet depth criteria of a mesocyclone, but instead meets a 25% relative depth criteria (relative to the depth of the storm). Shallow Vortex SHALLO (yellow) A detection of Rank 5 or greater, with a top below 10,000 ft. and a base at the 0.5˚ tilt, whose depth is between 3,000 and 10,000 feet and less than the 25% low-topped criteria. RADS allows the user to "filter" the icon display by strength rank. If the user feels there are too many low-ranked detections on the screen at the same time there are more significant vortices, the user can filter the weaker detections off the display. (These weak detections will remain in the Meso Table.) Suggested filter values would be 1 through 5, depending on the nature of the convective situation. The colors above are as they appear in the Meso Table. Detections in the table are sorted based on severity, such that all TVSMESOs are first, followed by MESOs and LOWTOPs, CPLTs, and then everything else. Secondary and tertiary sorting are done using Strength Rank and MSI, respectively. The color conventions for the icons that are overlaid on the radar images are: thick yellow with red: Mesocyclone, TVSMESO, or Low-Topped Meso with base at 0.5˚ tilt. Note that the BASE column in the table will also be colored red. Page 6-4.
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