NOCN02 CWAO 271930 
GENOT FAX NO. 011

LE GENOT NO. 013 EST LA VERSION FRANCAISE DU GENOT NO. 011.

MODIFICATIONS TO THE CMC REGIONAL FORECAST SYSTEM...
PART 1 OF 2...

ON TUESDAY SEPTEMBER 1 1998, AT 1200 UTC, THE CANADIAN 
METEOROLOGICAL CENTRE WILL IMPLEMENT A NEW VERSION OF THE GLOBAL 
ENVIRONMENTAL MULTISCALE (GEM) FORECAST MODEL RUNNING IN REGIONAL 
CONFIGURATION. THE MAIN CHANGES AFFECT THE HORIZONTAL RESOLUTION 
OF THE MODEL GRID AND THE PARAMETERISATION OF THE PHYSICAL 
PROCESSES.

1.  HORIZONTAL RESOLUTION AND TIME STEP
THE HORIZONTAL RESOLUTION IN THE UNIFORM GRID CORE WILL INCREASE 
FROM .33 DEG (ABOUT 35 KM) TO .22 DEG (ABOUT 24 KM). THE TIME 
STEP WILL BE REDUCED CONSEQUENTLY FROM 1350 SEC TO 720 SEC. THE 
NUMBER OF LEVELS IN THE MODEL REMAINS UNCHANGED AT 28.

2.  GEOPHYSICAL FIELDS
THE PERMANENT GEOPHYSICAL FIELDS SUCH AS TOPOGRAPHY, VEGETATION, 
LAND-SEA MASK AND ROUGHNESS LENGTH WILL NOW BE EXTRACTED FROM A 
DATA BASE AT 1 KM RESOLUTION, RATHER THAN AT 16 KM AS CURRENTLY 
DONE IN THE GEM 35 KM OPERATIONAL MODEL.

3.  DEEP CONVECTION
THE FRITSCH-CHAPPELL (FC) SCHEME REPLACES THE KUO SCHEME IN THE 
UNIFORM GRID AREA. THE FC SCHEME IS BASED ON THE SUPPRESSION OF 
THE CONVECTIVE AVAILABLE POTENTIAL ENERGY (CAPE). THE TRIGGERING 
MECHANISMS ARE SUCH THAT CONVECTION IS TRIGGERED LESS OFTEN, BUT 
WHEN IT IS, THE CONVECTIVE PRECIPITATION IS OFTEN MORE IMPORTANT. 
AN INDIRECT AND POSITIVE CONSEQUENCE OF THE USE OF THE FC SCHEME 
IS A MORE ADEQUATE REDISTRIBUTION OF THE LATENT HEAT IN THE 
VERTICAL, WHICH WILL REDUCE THE TENDENCY EXHIBITED BY THE 
OPERATIONAL GEM 35 KM MODEL TO OVERDEVELOP SOME SYNOPTIC SYSTEMS 
WHEN DEEP CONVECTION IS PRESENT.

THE KUO-SYMMETRIC SCHEME WILL BE USED OUTSIDE THE UNIFORM GRID. 
IT IS THE SAME SCHEME AS THE ONE USED IN THE HIMAP VERSION OF GEM.

4.  STABLE CONDENSATION
MODIFICATIONS HAVE BEEN DONE TO THE SUNDQVIST STABLE CONDENSATION 
SCHEME IN ORDER TO INCREASE THE EVAPORATION OF PRECIPITATION, 
JUDGED INSUFFICIENT. THIS MODIFICATION CONTRIBUTES TO REDUCE THE 
EXTENT OF THE PRECIPITATION ENVELOPE, WHICH IS A NOTICEABLE 
IMPROVEMENT GIVEN THE FACT THAT THE OPERATIONAL GEM 35 KM MODEL 
EXHIBITS A CLEAR TENDENCY TO OVERFORECAST THE EXTENT OF SMALL 
PRECIPITATION AMOUNTS (E.G. PRECIPITATION TRACE).

5.  OTHER CHANGES TO THE PARAMETERISATION

A)  CALL TO THE RADIATION CODE
THE NEW MODEL EXECUTES A COMPLETE CALL EVERY HOUR, INSTEAD EVERY 
2 HOURS IN THE GEM 35 KM. THE OBSERVED CHANGES ARE POSITIVE, 
ESPECIALLY WHEN THE TIME SERIES OF THE SURFACE RADIATION BALANCE 
ARE EXAMINED.

B)  GRAVITY WAVE DRAG
THE GRAVITY WAVE DRAG PARAMETERISATION DEVELOPED FOR RELATIVELY 
COARSE RESOLUTION MODELS IS NOT NECESSARY ANY MORE AT 24KM AND 
WAS DROPPED.

6.  EXPECTED CHANGES
THE MODEL, RUN ON A SET OF TEST CASES, HAS SHOWN AN IMPROVEMENT 
OF THE MSLP FIELD BIAS, AN IMPROVEMENT OF THE LOW LEVEL HUMIDITY 
BIAS AND GENERALLY FEW CHANGES IN THE OTHER DYNAMIC FIELDS. FOR 
PRECIPITATION, SIGNIFICANT IMPROVEMENTS WERE MADE IN THE LOW 
AMOUNTS (0 TO .2 MM OVER 24 HOURS), MORE MODEST GAINS IN THE 
LARGE AMOUNTS (MORE THAN 15 MM OVER 24 HOURS), BUT FEW CHANGES IN 
THE INTERMEDIATE AMOUNTS.

MCBEAN/ADMA/TORONTO