20051219: NCEP NAM

• To: nws-changes@xxxxxxxxxxxxxxxx
• Subject: 20051219: NCEP NAM
• From: Linda Miller <lmiller@xxxxxxxxxxxxxxxx>
• Date: Mon, 19 Dec 2005 11:52:52 -0700

982
NOUS41 KWBC 191757
PNSWSH

TECHNICAL IMPLEMENTATION NOTICE 05-80
NATIONAL WEATHER SERVICE HEADQUARTERS WASHINGTON DC
100 PM EST MON DEC 19 2005

TO:       FAMILY OF SERVICES /FOS/ SUBSCRIBERS...NOAA WEATHER
         WIRE SERVICE /NWWS/ SUBSCRIBERS...EMERGENCY MANAGERS
         WEATHER INFORMATION NETWORK /EMWIN/ SUBSCRIBERS...
         OTHER NATIONAL WEATHER SERVICE /NWS/ CUSTOMERS AND
         PARTNERS...NWS EMPLOYEES

FROM:     PAUL HIRSCHBERG
         CHIEF...SCIENCE PLANS BRANCH
         OFFICE OF SCIENCE AND TECHNOLOGY /OST/

SUBJECT:  NEW CALCULATION OF CAPE IN NCEP NAM MODEL:
 EFFECTIVE FEBRUARY 7 2006 AT 1200 UTC

EFFECTIVE FEBRUARY 7 2006...BEGINNING WITH THE 1200 COORDINATED
UNIVERSAL TIME /UTC/ RUN...THE NATIONAL CENTERS FOR ENVIRONMENTAL
PREDICTION /NCEP/ WILL CHANGE THE COMPUTATION OF THE SURFACE-
BASED CONVECTIVE AVAILABLE POTENTIAL ENERGY /CAPE/ AND CONVECTIVE
INHIBITION /CIN/ PARAMETERS IN THE NORTH AMERICAN MESOSCALE /NAM/
MODEL.  THE CURRENT FORMULATION SEARCHES THE LOWEST 70 MILLIBARS
/MB/ FOR THE LEVEL WITH THE HIGHEST EQUIVALENT POTENTIAL
TEMPERATURE /THETA-E/. THIS PARCEL IS THEN LIFTED TO GENERATE
VALUES FOR CAPE AND CIN.

VARIOUS NWS OFFICES AND THE NCEP STORM PREDICTION CENTER HAVE
SUGGESTED THIS FORMULATION OCCASIONALLY RESULTS IN A PARCEL BEING
LIFTED THAT IS NOT REPRESENTATIVE OF TRUE SURFACE CONDITIONS.
SEVERAL CASES HAVE BEEN IDENTIFIED IN WHICH CONDITIONS AT THE
SURFACE WERE NOT UNSTABLE...HOWEVER THE CALCULATION GENERATED A
HIGH VALUE OF CAPE BY LIFTING A PARCEL WELL ABOVE THE
GROUND. BECAUSE THE NAM OFFERS SEVERAL CAPE/CIN COMPUTATIONS TO
REPRESENT ELEVATED INSTABILITY...NWS DECIDED THE CURRENT
DETERMINATION OF A SURFACE-BASED PARCEL SHOULD BE REVISED TO MORE
ACCURATELY REPRESENT LOW-LEVEL CONDITIONS.

THE REVISED COMPUTATION WILL SEARCH FOR THE HIGHEST THETA-E AT
EACH GRID POINT OVER A DEPTH SMALLER THAN THE 70 MB CURRENTLY
USED. THE PRECISE DEPTH WILL BE BASED ON THE SURFACE PRESSURE TO
MAKE SURE THAT A LEVEL CLOSE TO THE GROUND IS USED AT LOW
ELEVATIONS...WHILE ABLE TO SEARCH OVER A LARGER DEPTH AT HIGHER
ELEVATIONS WHERE THE FIRST MODEL LEVEL ABOVE THE GROUND IS OFTEN
MUCH HIGHER ABOVE THE SURFACE.  THIS FORMULATION LEADS TO A RANGE
OF DEPTHS FROM ABOUT 10 MB AT SEA LEVEL TO AS MUCH AS 50 MB OVER
THE HIGHEST TERRAIN.

PLEASE NOTE THAT THERE ARE CURRENTLY FOUR SETS OF CAPE/CIN
COMPUTATIONS IN THE NAM.

1.  THE SURFACED-BASED FIELDS
2.  BEST CAPE/CIN COMPUTED BY FINDING THE HIGHEST THETA-E
   IN THE SIX MIXED 30 MB DEEP LAYERS CLOSEST TO THE GROUND
3.  MIXED LAYER CAPE/CIN COMPUTED BY TAKING THE AVERAGE
   THERMODYNAMIC PROPERTIES IN THE LOWEST 90 MB
4.  BEST CAPE/CIN COMPUTED BY FINDING THE LEVEL IN THE
   LOWEST 300 MB WITH THE HIGHEST THETA-E

THIS CHANGE WILL AFFECT ONLY THE FIRST ITEM IN THE LIST ABOVE.
DAILY PLOTS OF ALL OF THESE FIELDS CAN BE VIEWED AT /USE LOWER
CASE LETTERS/:

     HTTP://WWW.EMC.NCEP.NOAA.GOV/MMB/NAMSVRFCST

IF YOU HAVE QUESTIONS CONCERNING THIS CHANGE...CONTACT:

DR. GEOFF DIMEGO
CHIEF... NCEP...MESOSCALE MODELING BRANCH
CAMP SPRINGS MARYLAND
PHONE: 301 763 8000 X7221
EMAIL: GEOFF.DIMEGO@xxxxxxxx

OR

GEOFF MANIKIN
NCEP... MESOSCALE MODELING BRANCH
CAMP SPRINGS MARYLAND
PHONE: 301 763 8000 X7263
EMAIL: GEOFFREY.MANIKIN@xxxxxxxx

THIS AND OTHER NWS TECHNICAL IMPLEMENTATION NOTICES ARE AVAILABLE
ONLINE AT /USE LOWER CASE LETTERS/:

HTTP://WWW.NWS.NOAA.GOV/OM/NOTIF.HTM

$$
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