TDA_DV_THRESH

Tornado Detection Algorithm Differential Velocity THRESHold. This is an array containing six velocity difference thresholds. These thresholds are used as criteria for building two dimensional circulation features. Note: TDA_DV_THRESH(1) should always be equal to TDA_GTG_THR (See description for TDA_GTG_THR).

Justification for making adaptable/rationale for change: It may be deemed that more appropriate threshold are beneficial to identifying core circulations. For example, 10, 20, 25, 35, 45, 50 m/s. The user may want to identify only stronger circulations (e.g. 25, 30, 35, 40, 45, 50 m/s).

Algorithm response to change: The use of multiple thresholds costs more in terms of computing and memory requirements. However, it is not known exactly how the algorithm would respond to changes in the thresholds. It is recommended that the difference between successive threshold values not exceed 5 m/s (e.g. 20,25,30,35,40,45 m/s).

Default: 11, 15, 20, 25, 30, 35 m/s

Range: TDA_DV_THRESH(1) = 10-75 m/s

TDA_DV_THRESH(2) = 15-80 m/s

TDA_DV_THRESH(3) = 20-85 m/s

TDA_DV_THRESH(4) = 25-90 m/s

TDA_DV_THRESH(5) = 30-95 m/s

TDA_DV_THRESH(6) = 35-100 m/s

TDA_MIN_NSV

Tornado Detection Algorithm MINimum Number of Shear Vectors. The minimum number of pattern vectors required for any 2D circulation.

Justification for making adaptable/rationale for change: Some 2D circulation features are very small scale (e.g. tornadic vortices along a gust front). This allows the detection of smaller scale phenomena (e.g. landspouts) without limiting the algorithm to detecting only the larger scale circulations (e.g. supercell tornadoes).

Algorithm response to change: The lower/higher the threshold the greater/fewer number of 2D circulations constructed leading to a greater/lesser number of false alarms/detections. Furthermore, greater/lesser computing costs result from lower/higher values.

Default: 3 pattern vectors

Range: 1-5 pattern vectors

TDA_GTG_THR

Tornado Detection Algorithm Gate-To-Gate THReshold. The minimum required velocity difference to declare and save one dimensional circulation (pattern vectors) features. This threshold should be equal to TDA_DV_THRESH(1) (See description for TDA_DV_THRESH(1))

Justification for making adaptable/rationale for change: Lower threshold values will allow the detection of weaker circulations (e.g. tornadoes along gust fronts, derechos). The algorithm may be constrained to only identify the stronger circulations.

Algorithm response to change: Lower values require additional memory and computing. Also, lower/higher values may result in more/fewer false alarms/detections.

Default: 11. m/s

Range: 10. - 100. m/s

TDA_RAD_DIST

Tornado Detection Algorithm RADial DISTance. The maximum radial distance allowed for two pattern vectors to be associated into a 2D feature.

Justification for making adaptable/rationale for change: Constrains the construction of 2D circulations to only those pattern vectors which are close together.

Algorithm response to change: A value less than .5 km constrains pattern vectors to be adjacent in range. Values larger than .5 km may result in higher false alarm rate.

Default: .5 km

Range: 0. km to 3. km

TDA_AZM_DIST

Tornado Detection Algorithm AZiMuthal DISTance. The maximum azimuthal distance allowed for two pattern vectors to be associated into a two dimensional circulation.

Justification for making adaptable/rationale for change: Some circulations may be elliptical in nature and span more than two radials. Circulations may be declared to be comprised of only those pattern vectors along the same radial axis.

Algorithm response to change: It is not entirely clear how the algorithm responds to different values. Two dimensional features may tend to span several azimuths, but this is a rare situation.

Default: 1.5

Range: 0.0 to 4.0

TDA_MAX_RATIO

Tornado Detection Algorithm MAXimum Aspect RATIO. The maximum allowable aspect ratio (range/ azimuth) for a two dimensional circulation.

Justification for making adaptable/rationale for change: Azimuthal shear not associated with circulations may be discarded with this constraint. More appropriate values (e.g. 2) may be deemed necessary for identifying bonafide 2D circulations.

Algorithm response to change: Large values ( 4) may result in a high false alarm rate due to gust fronts oriented along the beam.

Default: 4.

Range: 1. to 10.

TDA_MAX_RANGE

Tornado Detection Algorithm MAXimum RANGE. The maximum range at which pattern vectors are created.

Justification for making adaptable/rationale for change: It maybe deemed necessary to limit the detection of circulations within a certain range from the radar due to the type of event(s) anticipated or lack of confidence in the algorithm's ability to identify tornadic circulations beyond a certain range.

Algorithm response to change: Additional computing and memory requirements will result for large values of this parameter as more detections and their components are identified and processed. Of course, events beyond TDA_MAX_RANGE will not be identified.

Default: 100 km

Range: 230 km

TDA_CRADIUS1 (Ignore for SSAP 3.4)

Tornado Detection Algorithm Circulation RADIUS. The maximum allowable horizontal distance between the centroids of two consecutive 2D circulations comprising a 3D circulation. This radius is used in the vertical association process to search for the strongest (maximum velocity difference) two dimensional circulation at the next elevation angle. Note that TDA_CRADIUS1 is envoked if the range of the 2D circulation at the higher tilt is TDA_CRADIUS_RNG.

Justification for making adaptable/rationale for change: The distance between two consecutive 2D circulations is a function of both vortex tilt and vortex translation between elevation angles. Also, the distance between two consecutive 2D circulations may become quite large at larger ranges due to spreading of the radar beam, and artificial shifting of azimuthal shear by improper velocity dealiasing.

Algorithm response to change: Small values may limit the algorithm's ability to fully construct 3D circulations. Large values may result in a higher false alarm rate by associating unrelated and/or false circulation signatures.

Default: 2.5 km

Range: 0.0 - 10.0 km

TDA_CRADIUS2 (Ignore for SSAP 3.4)

Tornado Detection Algorithm Circulation RADIUS. Same as TDA_CRADIUS1. Invoked when the range of the 2D circulation at the higher tilt is TDA_CRADIUS_RNG.

Justification for making adaptable/rational for change:

Algorithm response to change:

Default: 4.0 km

Range: TDA_CRADIUS1 - 10. km

TDA_CRADIUS_RNG (Ignore for SSAP 3.4)

Tornado Detection Algorithm Circulation RADius RaNGe. The range beyond which TDA_CRADIUS2 is invoked. Otherwise, TDA_CRADIUS1 is used.

Justification for making adaptable/rationale for change: It may be deemed necessary to increase or decrease the value as

Algorithm response to change:

Default: 80.0 km

Range: 230.0 km

TDA_MIN_HT

Tornado Detection Algorithm MINimum HeighT. The minimum height AGL which the base of a 3D circulation may extend to be declared a TVS. Used in conjunction with TDA_MIN_ELEV (See description TDA_MIN_ELEV).

Justification for making adaptable/rationale for change: TVS detections at near ranges may not extend to the lowest elevation angle. Allow for circulations aloft to be identified as TVSs.

Algorithm response to change: Large values may limit the detectability of tornadic circulations. The false alarm rate may increase with an increase in TDA_MIN_HT.

Default: .6 km

Range: 0. - 10.0 km

TDA_MIN_ELEV

Tornado Detection Algorithm MINimum ELEVation. The lowest elevation angle to which the base of a three-dimensional circulation must extend to declare a TVS unless the circulation extends at or below TDA_MIN_HT (See description for TDA_MIN_HT). Three dimensional circulations which do not extend to the prescribed lowest elevation angle are declared ETVSs.

Justification for making adaptable/rationale for change: Close range phenomena may be detected more easily with higher values of TDA_MIN_ELEV.

Algorithm response to change: The false alarm rate may increase with an increase in TDA_MIN_ELEV.

Default: 1.0

Range: 0.0 to 10.0

TDA_MAX_HT

Tornado Detection Algorithm MAXimum HeighT. The maximum height at which the velocity data are processed in TDA_VECT.

Justification for making adaptable/rationale for change: Allow or dismiss the processing of velocity data above TDA_MAX_HT. If computing resources are limited then this value may be set lower.

Algorithm response to change: The range of detection decreases with a decrease in TDA_MAX_HT (decrease probability of detection).

Default: 10. km

Range: 1. - 15. km

TDA_MAX_DIST (SSAP 3.2 and before)

Tornado Detection Algorithm MAXimum DISTance. The distance used to time associate TVSs and ETVSs. Previous detections which fall within this distance are time associated.

Justification for making adaptable/rationale for change: The expected storm motion changes on a daily basis. It may be deemed necessary to change this value according to the assessment of the storm motion for a particular period.

Algorithm response to change: Values that are too small will result in detections not being time associated. Tracking, position forecasts, and trends will be lost. Large values may result in incorrect tracking.

Default: 6.5 km

Range: 0. - 20. km

TDA_MIN_ASSOC_RAD (Ignore for SSAP 3.4)

Tornado Detection Algorithm MINimum ASSOCiation RADius. The minimum distance within which to search for a current detection for time association.

Justification for making adaptable/rationale for change: User may determine that a larger or smaller distance is more appropriate.

Algorithm response to change: As threshold is increased, fewer iterations are made during time association process.

Default: 1.0 km

Range: 0.0 - 10.0 (1.0 km increments ONLY)

TDA_MAX_ASSOC_RAD (Ignore for SSAP 3.4)

Tornado Detection Algorithm MAXimum ASSOCiation RADius. The maximum distance within which to search for a current detection for time association.

Justification for making adaptable/rationale for change: Same as for TDA_MIN_ASSOC_RAD .

Algorithm response to change: As threshold is decreased, fewer iterations are made during time association process.

Default: 10.0

Range: TDA_MIN_ASSOC_RAD - 20.0 (1.0 km increments ONLY)

TDA_MAX_DIST (SSAP 3.2 version and before)

Tornado Detection Algorithm MAXimum DISTance. The distance used to time associate TVSs and ETVSs. Previous detections which fall within this distance are time associated.

Justification for making adaptable/rationale for change: The expected storm motion changes on a daily basis. It may be deemed necessary to change this value according to the assessment of the storm motion for a particular period.

Algorithm response to change: Values that are too small will result in detections not being time associated. Tracking, position forecasts, and trends will be lost. Large values may result in incorrect tracking.

Default: 6.5 km

Range: 0. - 20. km

TDA_MIN_FPD

Tornado Detection Algorithm MINimum Features Per Detection. The minimum number of vertically correlated 2D features required to declare a TVS or elevated TVS (ETVS).

Justification for making adaptable/rationale for change: Permits the detection of circulations (especially shallow circulations) at far ranges ( 80 km).

Algorithm response to change: A lower value may result in a higher false alarm rate at near ranges but greater probability of detection at greater ranges.

Default: 3. features

Range: 1. -10. features

TDA_MAX_DIST_ASSOC

Tornado Detection Algorithm MAXimum DISTance ASSOCiation. The maximum distance used to associate three-dimensional circulations (TVS and/or ETVS) with storm centroids.

Justification for making adaptable/rationale for change: The number and type of expected storms may change on a daily basis and from region to region.

Algorithm response to change: Smaller values may result in not associating with a nearby storm. No impact on detection of tornadic circulations.

Default: 20. km

Range: 0. to 25. km

TDA_MESO_ASSOC_BUFF

Tornado Detection Algorithm MESOcyclone ASSOCiation BUFFer. The maximum distance from the mesocyclone perimeter within which to associate TVS detections with MESO detections. Association is not required to declare a TVS or ETVS detection.

Justification for making adaptable/rationale for change: Permit the user to determine whether a mesocyclone has been detected in order to give greater confidence of a bonafide TVS detection.

Algorithm response to change: Large values may result in an incorrrect association if there are closely spaced mesocyclones. Values that are too small may result in no mesocyclone association. This parameter has no impact on the detection of tornadic circulations.

Default: 2. km

Range: 0. to 10. km

TDA_MIN_DEPTH (SSAP 2.0 and after)

Tornado Detection Algorithm MINimum DEPTH. The minimum 3D circulation depth required to declare a TVS or an ETVS.

Justification for making adaptable/rationale for change: Depths of 3D circulations may vary according to expected storm type. Also, studies at NSSL show that the majority of tornadic circulations meet or exceed 1.5 km depth.

Algorithm response to change: Large values may result in a lower probability of detection. Small values may result in an increased number of false detections.

Default: 1.5 km

Range: 0. km to 5. km

TDA_MIN_ LADV (SSAP 3.1 and after)

Tornado Detection Algorithm MINimum Low-Altitude Delta Velocity. The minimum radial velocity difference at the lowest elevation angle required to declare a TVS detection.

Justification for making adaptable/rationale for change: Allow freedom to consider weaker circulations or consider only the stronger circulations based upon expected type of events (e.g. supercells, squall line, bow-echo, etc.).

Algorithm response to change: Lower values will allow weaker circulations to be processed. However, this could result in a higher false alarm rate. Also, lower values allows the detection and trending of tornadic circulations in their incipient stages.

Default: 25.0 m/s

Range: 0. - 100. m/s

TDA_MIN_ MLDV (SSAP 3.1 and after)

Tornado Detection Algorithm MINimum Max-Level Delta Velocity. The minimum radial velocity difference at the lowest elevation angle required to declare a TVS detection.

Justification for making adaptable/rationale for change: See TDA_MIN_LADV.

Algorithm response to change: See TDA_MIN_LADV.

Default: 36.0 m/s

Range: 0. - 100. m/s

TDA_USE_STRM_MOTION

Tornado Detection Algorithm USE SToRM MOTION. A logical flag that determines whether the storm motion vector from the NSSL SCIT is to be used to compute forecast positions.

Justification for making adaptable/rationale for change: The SCIT storm motion may or may not be available or an alternative method may be desired to determine an appropriate motion vector.

Algorithm response to change: If .TRUE. then the nearest storm motion will be used. If .FALSE. then alternative motion vectors are used.

Default: TRUE

Range: N/A

TDA_USE_DEF_MOTION (SSAP 3.2 and before)

Tornado Detection Algorithm USE DEFault MOTION. A logical used to invoke a specified motion vector (specified within a parameter file) to compute forecast positions of three-dimensional circulations.

Justification for making adaptable/rationale for change: See TDA_USE_STRM_MOTION, except for default motion.

Algorithm response to change: See TDA_USE_STRM_MOTION, except for default motion.

Default: TRUE

Range: N/A

TDA_WRITE_VECT

Tornado Detection Algorithm WRITE VECTors. A logical flag used to determine whether the pattern vector information is to be written to an output file.

Justification for making adaptable/rationale for change: Allow the user to inspect the pattern vector output. Primarily used offline to evaluate algorithm performance.

Algorithm response to change: If .TRUE., the pattern vector information is written to an output file. If .FALSE., the pattern vector information is not written to an output file. If a large number of pattern vectors are output, then computing time is lost to I/O processing. However, this loss is minimal.

Default: FALSE

Range: N/A

TDA_PLOT_TVS_ONLY

Tornado Detection Algorithm PLOT TVS ONLY. A logical flag used to plot TVS detections ONLY on image display.

Justification for making adaptable/rationale for change: This allows the display of the TVS and ETVS detections. Currently, this is for display purposes ONLY within the RADS.

Algorithm response to change: If .TRUE. then only TVS detections will be displayed. If .FALSE. then both TVS and ETVS detections will be displayed. Minimal impact on processing.

Default: TRUE

Range: N/A

TDA_RAPID_UP (SSAP 2.61 and after)

Tornado Detection Algorithm RAPID UPdate. A logical flag used to invoke vertical association prior to the end of a volume scan (see Functional Description of NSSL TDA). Note that no product is currently displayed to the user. Text output only is written to an output file. Thus, it is recommended that this parameter remain .FALSE.

Justification for making adaptable/rationale for change: Allow the user the option of obtaining detection information prior to the end of a volume scan.

Algorithm response to change: This process requires more computing time and memory.

Default: FALSE

Range: N/A