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NCEP PTYPE Algorithms Fred H. Glass NOAA/NWS St. Louis LSX Winter Weather Workshop – November 19, 2008

NCEP PTYPE Algorithms

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NCEP PTYPE Algorithms. Fred H. Glass NOAA/NWS St. Louis. LSX Winter Weather Workshop – November 19, 2008 . Why Have Ptype Algorithms?. Forecasting winter weather is a significant challenge A variety of precipitation algorithms have been developed in an effort to address this challenge!. - PowerPoint PPT Presentation

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NCEP PTYPE Algorithms

NCEP PTYPE AlgorithmsFred H. GlassNOAA/NWS St. LouisLSX Winter Weather Workshop November 19, 2008 1Why Have Ptype Algorithms?Forecasting winter weather is a significant challengeA variety of precipitation algorithms have been developed in an effort to address this challenge!

2Algorithms 101Ptype from the algorithms is derived by post-processing of the model outputPtypes are generated when even just a trace of precipitation is generated by the modelNo single algorithm handles all ptypes in a sufficient mannerAlways examine soundings; even when they are accurate the output from the various algorithms may generate conflicting ptypes due to their methodologies and assumptionsAn ensemble approach should be considered by comparing the output of the different algorithms3Algorithms/TechniquesNCEP Baldwin-SchichtelNCEP RevisedRamerBourgouinCzys

4NCEP Baldwin-Schichtel(aka NCEP, Baldwin, or BTC)Developed based on MS research at Univ. of Oklahoma by Schichtel, utilizing ETA model vertical thermodynamic profiles and hourly precipitation reports Utilizes a decision tree approachIdentifies warm and cold layers by calculating the area between the 0C or -4C isotherm and the wet-bulb temperatureCompares magnitude of warm/cold layers (area) with the surface temperature to identify ptype 5NCEP Baldwin-Schichtel(the steps)First identifies the highest saturated layer (considered to be the precipitation generation layer)Next determines the initial state of these hydrometers T < -4C assumed to be ice crystals T -4C assumed to be supercooled water dropsIf supercooled water droplets, then checks the surface temperature (lowest model layer) Tsfc 0C freezing rain Tsfc > 0C rain

6NCEP Baldwin-Schichtel(the steps)If ice crystals, then the magnitude of the area between the -4C isotherm and the wet-bulb temperature profile in the sounding is computed if area 3000 deg m snow if area > 3000 deg m ice crystals melted checks to see if hydrometers re-freeze into ice pellets or if they fall to the surface as rain or freezing rain7NCEP Baldwin-Schichtel(reviewing the process)

8NCEP Baldwin-Schichtel(Strengths and Weaknesses)StrengthsEasily applied and widely usedInitial check for hydrometer stateUtilizes the wet-bulb temperatureForecasting freezing rain and sleetWeaknessesWill forecast freezing or liquid precipitation with deep isothermal layer near the surface with Tw between 0C and -4CIgnores impact of dry layersTendency to over-forecast freezing rain and sleet9NCEP Baldwin-Schichtel(problem sounding)

10NCEP RevisedA modified version of the NCEP Baldwin-SchichtelAttempts to balance the freezing rain and sleet bias of the regular version by having a bias towards snowInstead of the -4C area check, it computes the area in the sounding with a wet-bulb temperature greater than 0C11NCEP Revised(modified step)If ice crystals, then the magnitude of the area in the sounding with a wet-bulb temperature > 0C is computed if area 500 deg m snow if area > 500 deg m ice crystals melted checks to see if hydrometers re-freeze into ice pellets or if they fall to the surface as rain or freezing rain12NCEP Revised(Strengths and Weaknesses)StrengthsEasily appliedInitial check for hydrometer stateUtilizes the wet-bulb temperatureEliminates the near surface isothermal layer problem with the original algorithmRemoves the freezing rain and sleet biasWeaknessesNot readily availableNCEP websitePart of dominant techniqueIgnores impact of dry layers13Developed in the early 1990s utilizing over 2000 cases of collocated surface precipitation observations and upper air soundings Utilizes T, RH, and the Tw at different pressure levels as inputBased on the pressure level data, it identifies layers where precipitation is likely and calculates an ice fractionFollows the idealized precipitation parcel down to the ground from a precipitation generating level, anticipating the state of the hydrometerRamer

14Ramer(the steps)Two preliminary checks are completed before the method performs a full calculation if surface Tw > 2C rain is diagnosed if surface Tw 2C and the Tw < -6.6C at all other levels snow is diagnosedIf these checks fail then a full calculation of the ice fraction of the hydrometer is computed

15Ramer(the steps)Determine the precipitation generating level highest level with RH > 90% level must be located at or below 400 mbDetermine the initial hydrometer state at the generating level if Tw < -6.6C completely frozen (ice fraction=1) if Tw -6.6C completely liquid (ice fraction=0)The algorithm then determines the amount of freezing and melting and the resultant ice fraction of the hydrometer is computed

16Ramer(the steps)The algorithm then determines the amount of freezing and melting and the resultant ice fraction as the hydrometer descends from the generation level Identifying layers warmer/colder that 0C based on the depth of the layer and average Tw Assigning an ice fraction at each levelPtype is determined by the final ice fraction of the hydrometer at the surface > 85% = sleet,