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Rapid Refresh (RAP) and High-Resolution Rapid Refresh (HRRR) Forum

new helicity

Posted By: Geoff Manikin
Date: 5-May-03 2206Z

 This is a follow-up to Stan's earlier message about the upcoming changes to the RUC. Stan noted that a storm-relative helicity computed in the lowest kilometer will be available to complement the existing value computed over the lowest 3 km. While the 0-3 km helicity is a useful parameter in determining the threat of storm rotation and possible tornado formation, the 0-1 km value is an important parameter for assessing the threat of strong or violent tornadoes.

 Anyhow, for those attempting to unpack the new field, there is some significant potential for confusion that I will attempt to head off. The 0-3 km value is put out as a surface field (octet 10 of the PDS has a value of 0 in http://www.nco.ncep.noaa.gov/pmb/docs/on388/table3.html); octets 11 and 12 are given values of 3000 and 0 in an attempt to label the field as being computed in the lowest 3000 meters, although many decoders will ignore those values once they see the surface designator in octet 10. The new 0-1 km value will have an octet 10 value of 106 to indicate a layer specified by two height values. Octets 11 and 12 will designate these height levels (in meters) as 1000 and 0.

 Confusion will likely arise from the 0-1 and 0-3 km values having different level designators. Due to the 0-3 km field being sent to AWIPS, we cannot change the designator for the 0-3 km value without a lengthy paperwork process. This will not be completed before the implementation, so there will be *no changes* to the PDS of the 0-3 km helicity at the time of the implementation. We do hope to change the PDS of that field during the summer to match the designation we give to the 0-1 km value. (The 0-1 km value must be given the correct designation at this time, or else decoders will be confused by both helicity fields being labeled as a "surface" field.) We will likely, at that later time, also change the storm motion vectors to be properly labeled as 0-6 km values instead of surface. We will of course notify the forum when such changes are about to take place.

 Please contact me if you have questions.

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