The Mesocyclone Detection Algorithm (MDA) is the rotation algorithm from GR2Analyst. The MDA produces a Rotation (ROT) product which is invisible to the user and a Normalized Rotation (NROT) product which is visible to the user. The algorithm is described below, and also in more detail in the powerpoint presentation GR2Analyst Algorithms in WFO Operations (Lincoln, 2012), which is the source of this page's images/tables.
The MDA algorithm is based upon:
Smith, T.M., and Elmore, K.L., 2004: The Use of Radial Velocity Derivative to Diagnose Rotation and Divergence. 11th Conf. on Aviation, Range, and Aerospace. Hyannis, MA. Amer. Meteor. Soc.
TVS/meso detections based on magnitude of rotation.
The LLSD is calculated by the equation:
*The equation and its usage is described further in Smith & Elmore (2004).
The LLSD shear technique was compared to the traditional peak-to-peak shear technique using two synthetic circulations of 0.01/s mangitude: one with a diameter of 5km and another with a diameter of 8km. The 95% confidence interval for both circulations are shown below, wtih green indicating the LLSD shear technique and orange indicating the peak-to-peak shear technique.
Base velocity data is dealiased based upon automated settings which are independent of those in the GR2Analyst "Dealias Settings" box. The LLSD technique is applied over a 9x9 pixel area (5x5 pixel area for legacy resolution) to create the Rotation (ROT) product. For the example below, the ROT value for the white cell is based on the velocity gradient across the darkened area. ROT is then divided by a distance scaling factor (ROT Threshold) to create the Normalized Rotation (NROT) product. The distance scaling factor is used to correct for radar distance issues like increased beam width vs. a fixed-size phenomena.
The center bin (white pixel) has a calculated ROT value of 0.0398/s, or 39.8/ks. It is ~21km from the radar site. From the default GR-MDA Settings, the ROT Threshold would be about 24.9/ks. Dividing ROT by the ROT Threshold produces an NROT value of 1.6, which is dimensionless.
*An example set of radar data including a walk through of a MDA detection can be found in the associated powerpoint presentation.
Because the GR2Analyst MDA algorithm requires a base elevation of 10,000ft or less to trigger a detection and a base elevation of 5,000ft or less to trigger a TVS, there are some areas that are not close enough to a NEXRAD site for the MDA algorithm to indicate either. Using the GR2Analyst equation for estimating beam centerline elevation above radar level, we can see the effective ranges for a TVS detection (green) and a mesocyclone detection (yellow).
Accurate calcuation of ROT, and thus, NROT, requires properly dealiased velocity data. MDA detections near the radar or in heavy clutter should receive more scrutiny. Check dealiasing algorithm by comparing data with/without algorithm. Although dealiased through an independent algorithm than the one used for BV/SRV data, the presence of substantial failures of the BV/SRV dealiasing algorithm might suggest issues with the MDA algorithm and subsequent Meso/TVS detections.
Very low-topped cells may not trigger any MDA detection regardless of strength because of VILs near or below 5kg/m^2 or rotation height not reaching "ROT count" in GR-MDA Settings. Nearby circulations might be dropped leaving only one MDA detection if too close together.