NOCN02 CWAO 161930
GENOT FAX NO. 001

NO. 001 EST LA VERSION FRANCAISE DE GENOT FAX  NO.002

MODIFICATIONS TO THE SCHEME "FORCE-RESTORE" WILL BE IMPLEMENTED
IN THE CODE OF THE REGIONAL GEM MODEL ON WEDNESDAY, JANUARY 21
1998 AT 1200 UTC. THIS SCHEME ALLOWS TO CALCULATE IN THE MODEL
SURFACE TEMPERATURE AND HUMIDITY AND IS USED AS BOUNDARY
CONDITIONS IN THE SOLVING OF THE VERTICAL DIFFUSION EQUATION.
THE MODIFICATIONS WILL INFLUENCE THE DIRECT MODEL OUTPUT OF
SURFACE TEMPERATURES AND DEW POINTS DURING THE WINTER SEASON,
THUS ALLOWING TO SIGNIFICANTLY IMPROVE FORECASTS OF THESE
VARIABLES.

THREE MODIFICATIONS ARE INVOLVED:

1.  THE THERMAL CHARACTERISTICS OF SNOW HAVE BEEN MODIFIED TO
     RENDER IT MORE INSULATING.

2.  THE TREATMENT APPLIED TO ICE COVERED WATER SURFACES HAS
     BEEN MODIFIED; FROM NOW ON, IF AN ICE SURFACE IS COVERED BY AT
     LEAST 50 CM OF SNOW, IT WILL BE TREATED AS A SNOW SURFACE AND
     NOT AN ICE SURFACE; THE THRESHOLD PREVIOUSLY USED WAS 10 CM.
3.  THE CALCULATION OF THE INFRARED FLUXES ORIGINATING FROM
     THE EARTH'S SURFACE HAS BEEN IMPROVED.

THE FIRST MODIFICATION IS THE MOST IMPORTANT; RENDERING SNOW
MORE INSULATING ALLOWS LARGER VARIATIONS OF SOIL TEMPERATURE IN
RESPONSE TO A GIVEN ENERGY BUDGET. THE GOAL BEHIND THE SECOND
CHANGE IS TO OBTAIN WARMER TEMPERATURES THAN PREVIOUSLY FOR
WATER SURFACES THAT ARE ICE COVERED. THE IMPACT OF THE THIRD
CHANGE IS TO ENHANCE RADIATION LOSSES AT THE SURFACE THE
PRESENCE OF VERY DRY ARCTIC AIRMASSES.

PRACTICALLY, UNDER ARCTIC RIDGES, TEMPERATURE FALLS AT NIGHT AS
FORECAST BY THE MODIFIED MODEL MAY EXCEED BY 10 DEGREES THE
FALLS FORECAST BY THE CURRENT MODEL. THIS WILL GREATLY REDUCE
THE WARM BIAS THAT WAS ASSOCIATED TO THE CURRENT OPERATIONAL
MODEL UNDER SUCH CONDITIONS. INVERSELY, IN REGIONS UNDERGOING
WARM AIR ADVECTION, THE TEMPERATURE FORECAST WILL BE HIGHER
THAN THE ONE FORECAST BY THE CURRENT OPERATIONAL MODEL.
HOWEVER, THE NEW MODEL RESPONDING MORE STRONGLY TO A GIVEN
ENERGY BUDGET, THE ERRORS CAN BE LARGER IF THE CLOUD COVER IS
NOT CORRECTLY FORECAST.

THE OBJECTIVE VERIFICATION OF TEMPERATURE FORECASTS FOR 264
CANADIAN STATIONS GENERATED BY THAT MODEL FOR THE DURATION OF
THE PARALLEL RUN HAS SHOWN A SIGNIFICANT IMPROVEMENT OF THE
SURFACE TEMPERATURE FORECASTS. THE WARM BIAS HAS BEEN GREATLY
REDUCED AND THE ROOT MEAN SQUARE ERROR HAS BEEN SUBSTANTIALLY
REDUCED, ESPECIALLY FOR MINIMUM TEMPERATURES (WITH A GAIN OF
OVER 0.5 DEG. C). THE FORECASTS HAVE IMPROVED OVER ALL CANADIAN
REGIONS, EXCEPT IN THE ARCTIC FOR FORECASTS VALID AT 00Z AND IN
BRITISH COLUMBIA FOR THOSE VALID AT BOTH 00Z AND 06Z. HOWEVER,
IN THOSE CASES, THE SLIGHT REDUCTION IN QUALITY WAS SMALL
COMPARED TO THE GAINS OBTAINED FOR THE OTHER VALID TIMES.

SUBJECTIVE EVALUATION CARRIED OUT BY OPERATIONAL METEOROLOGISTS
AT CMC GENERALLY HAS SHOWN THAT FORECAST SURFACE TEMPERATURES
UNDER ARCTIC HIGHS CLOSE TO THE HOURS OF MINIMUM TEMPERATURE
WERE MUCH COLDER AND VERIFIED BETTER AGAINST OBSERVATIONS FOR
FORECAST ISSUED FROM THE MODIFIED MODEL IF THE CLOUD FORECAST
BY THE MODEL FITTED REALITY. IN THOSE CASES WHERE THE MODEL DID
NOT FORECAST CLOUDS AND IN FACT LOW CLOUDS WERE PRESENT,
SURFACE TEMPERATURES FORECAST BY THE MODEL WERE SOMETIMES TOO
COLD; HOWEVER, THOSE SITUATIONS ARE MUCH COMPENSATED BY THE
SUBSTANTIAL IMPROVEMENT NOTED IN THE MAJORITY OF CASES. THE
FORECAST MASS FIELD AS WELL AS PRECIPITATION HAVE SHOWN VERY
LITTLE DIFFERENCE WITH THOSE OF THE OPERATIONAL MODEL.

STATISTICAL TEMPERATURE FORECASTS BASED ON THE PERFECT PROG
SYSTEM DRIVEN BY THE REGIONAL MODEL WILL BE AFFECTED INDIRECTLY
BY THIS MODIFICATION SINCE THE SOME OF THE 1000 HPA FIELDS USED
BY THE SCHEME WILL BE MODIFIED BY CHANGES IN SURFACE
TEMPERATURES. EXPECTED IMPROVEMENTS IN STATISTICAL TEMPERATURE
GUIDANCE WILL BE HOWEVER NOT AS LARGE AS THE ONES ASSOCIATED TO
THE DIRECT MODEL OUTPUTS.

NOTE THAT THIS MODIFICATION DOES NOT AFFECT THE GLOBAL SEF
MODEL.

MCBEAN/ADMA/TORONTO