MEMORANDUM FOR:	WSR-88D Focal Points
FROM:	W/OSO4 - James D. Belville Chief
SUBJECT:	FAXBack Distribution: Recommended Parameter Changes to Improve WSR-88D Hail Detection Algorithm Estimates

The purpose of this memorandum is to notify WSR-88D Site Focal Points that the OSF is recommending Unit Radar Committees (URC) make changes to two Hail Detection Algorithm (HDA) Adaptable Parameters, the POSH Offset and Warning Threshold Selection Model (WTSM) Offset as specified in the attached letter. The letter minus attachments has been faxed to you to hasten the distribution process. The letter with attachments was mailed on April 17th.

1 Attachment: Adaptable Parameter Change Letter

	Office of the Director
	WSR-88D Operational Support Facility
	April 14, 1998

MEMORANDUM FOR:	WSR-88D URC Chairpersons
FROM:	W/OSO4 - James D. Belville Chief
SUBJECT:	Recommended Parameter Changes to Improve WSR-88D Hail Detection Algorithm Estimates

The purpose of this letter is to inform Unit Radar Committees (URC) of changes to the Hail Detection Algorithm (HDA) Adaptable Parameters. The OSF authorizes the URCs to change their site's POSH Offset downward to 30% when they are experiencing a summertime environment characterized by high melting levels and low vertical shear. In addition, the OSF and the WSR-88D Adaptable Parameter Working Group (APWG) strongly urge all sites to adjust their Warning Threshold Selection Model (WTSM) Offset as specified below. Finally, please let the OSF know how these changes affect your POSH estimates. In addition, the OSF Applications Branch would appreciate copies of any research or studies resulting from the changes.

Over the past year, the OSF has received reports from a number of sites that the HDA exhibits an over-forecasting bias at high elevation sites and during the summer. In response to these reports, the National Severe Storms Laboratory (NSSL) conducted studies to improve the HDA. Attachment I is the report summarizing these studies and is the primary scientific bases for these changes. In addition to the report, the articles referenced within provide further supportive evidence for the changes. Regarding the WTSM Offset change, although the sample size in the report is not large, the change is physically derived, not empirically derived. Regarding the POSH Offset change, two other studies, Wyatt and Witt (1997) and Barjenbruch and LaPlante (submitted 1997) are referenced as additional evidence.

The two recommended changes will improve the HDA's Probability of Severe Hail (POSH) estimates; the changes will have no effect on the Maximum Expected Hail Size (MEHS) or Probability of Hail (POH) estimates. Both POSH Offset and WTSM Offset are on the Hail Detection Menu of the UCP. To modify these values first type

AD,<password1>,M,<password2>,HA

POSH Offset is labeled POSH OFFSET, and WTSM Offset is labeled, WARN THRESH SELECT MODEL OFFSET.

POSH OFFSET

Adjusting POSH Offset should be done whenever your site experiences a summertime environment characterized a high melting level and low vertical shear to alleviate an over-forecasting bias. Wyatt and Witt (1997) define a high melting level as > 4km (or 13 kft) MSL and characterize low vertical shear as having winds at 500 mb < 30 kts. When these conditions occur, the POSH Offset should be lowered to 30%. When the summertime environment is no longer in effect, it is important that the POSH Offset be reset to the default value of 50%. Thererfore, the OSF suggests adding POSH Offset to UCP Shift Change Checklists

WTSM OFFSET

Adjusting WTSM Offset will ensure the Warning Threshold in the POSH computation uses the height of the melting level relative to MSL (instead of ARL). Each site will have a new value based on their radar height; Attachment II has the new WTSM Offset values for all sites in alphabetical order. This change needs to be done only once and should eliminate most of the bias caused by high radar elevations and will create a common, accurate frame of reference (MSL) in the Warning Threshold for all sites.

Attachment III has the values for all sites in order of radar height and new WTSM Offset from highest to lowest. The higher the radar, the more the WTSM Offset will change and the more impact the change will have on the POSH estimates. Therefore, it is most critical for sites near the top of the list to adjust their WTSM Offset.

If you have any questions regarding these changes, please call Mark Fresch of the OSF. The OSF greatly appreciates your cooperation in this process.

3 Attachments     [In PDF format at Attachment I, Attachment II, and Attachment III]

cc:
W/OSO42 - N. Bernigaud
AOS-254 - Dennis Roofe
W/WR42x1 - V. Preston
W/PRx1 - M. Sierchio
W/CR42 - L. Krudwig
W/CR3x2 - J. Hughes
W/SR41 - V. Murphy
W/ER1 - M. Brown
W/AR4x1 - N. Marchbanks
PACAF/DOW - Maj Anderson
AFMC/DOW - Maj Elkins
AFSPC/DOW - Maj Schattel
AETC/DOW - MSgt Hill
AMC/DOW - MSgt Siniff
ACC/DOW - TSgt Smith
AFWA/XPPM - Maj Given
AFWA/XONA - Capt Kohn
NWS River Forecast Center HICs
W/OSO1x3 - R. Racer
OSF Senior Staff
APWG members
NSSL - J. Kimpel
NSSL - M. Eilts
NSSL - A. Witt