BRETTS-MILLER-JANJIC SCHEME

INTRODUCTION Betts and Miller (1986) proposed a convective adjustment scheme that includes both deep and shallow convection. The Betts–Miller scheme has been used for simulation/prediction of tropical cyclones and for the simulation of Orographic–convective rainfall over the Western Ghats of India and also for the prediction of monsoon rain over Indian region.

INTRODUCTION Alapaty et al (1994b) in their study with the Betts–Miller scheme on monsoon rainfall prediction found that this scheme is unable to shift the rainfall over land and the rainfall remains mainly over the oceanic region. They suggested that proper selection of adjustment parameters is required for good forecasts of rainfall over the Indian landmass.

INTRODUCTION Janjic (1994) found that Betts–Miller scheme produced spurious rainfall normally over warm water and light rainfall over large oceanic regions. With some complex modifications, the spurious rainfall over the oceanic regions is suppressed, which brought out overall improvement in the rainfall forecast.

BMJ ‘CP’ SCHEME DESCRIPTION. BETTS-MILLER-JANJIC(BMJ) SCHEME IS SLIGHTLY MORE COMPLEX THAN KUO SCHEME. IT ADJUSTS SOUNDING TOWARD A PRE- DETERMINED, POST-CONVECTIVE REFERENCE PROFILE DERIVED FROM CLIMATOLOGY. MODELS. USED IN OPERATIONAL NCEP NAM MODEL AND SOME MEMBERS IN NCEP SHORT RANGE ENSEMBLE FORECAST SYSTEM.

BMJ ‘CP’ SCHEME CONVECTIVE PROCESS TRIGGER : THREE CONDITIONS TO TRIGGER CONVECTION :- AT LEAST SOME CAPE; CONVECTIVE CLOUD DEPTH EXCEEDING A THRESHOLD VALUE; MOIST SOUNDINGS TO ACTIVATE.

BMJ ‘CP’ SCHEME COMPARED TO INITIAL SOUNDING, REFERENCE SOUNDING HAS A DIFFERENT AMOUNT OF PRECIPITABLE WATER AND SOME NET HEATING OR COOLING.

BMJ ‘CP’ SCHEME CONVECTIVE CHANGES : A REFERENCE PROFILE IS A CLIMATOLOGICALLY DEFINED POST-CONVECTIVE STATE, DEFINED BY POINTS AT CLOUD BASE, CLOUD TOP, AND FREEZING LEVEL.

BMJ ‘CP’ SCHEME

BMJ ‘CP’ SCHEME SCHEME MODIFIES REFERENCE PROFILE, SO LATENT HEATING EQUALS NET HEATING IN SOUNDING. RAIN IS PRODUCED FROM A REDUCTION IN PRECIPITABLE WATER GOING FROM ORIGINAL SOUNDING TO REFERENCE SOUNDING. LATENT HEATING PRODUCED BY SQUEEZING WATER OUT OF AIR MUST BE CONSISTENT WITH NET WARMING IN TEMPERATURE PROFILE. REFERENCE TEMPERATURE AND DEWPOINT PROFILES SLIDE IN TANDEM LEFT OR RIGHT ON SOUNDING, UNTIL A POSITION IS FOUND WHERE LATENT HEATING PRODUCED BY SCHEME PRECIPITATION IS CONSISTENT WITH SENSIBLE HEATING CHANGES TO SOUNDING.

BMJ ‘CP’ SCHEME LINK TO LARGE-SCALE FORCING:- NO DIRECT LINK : SCHEME RELIEVES INSTABILITY EVERYWHERE IT IS PRESENT, GIVEN SUFFICIENT MOISTURE. IS LINKED INDIRECTLY : BECAUSE LARGE-SCALE FLOW GOVERNS MOISTURE DEPTH AND INSTABILITY THROUGH BOTH HORIZONTAL AND VERTICAL ADVECTION. FINAL STATE : EVOLVES TO REFERENCE PROFILE. SUB-CLOUD TEMPERATURE AND MOISTURE PROFILES ARE NOT CHANGED BY CP SCHEME. SCHEME HAS NO DOWNDRAFT COOLING; HOWEVER, OTHER NON-CP PROCESSES (e.g., A REDUCTION IN INCOMING SOLAR RADIATION, EVAPORATIVE COOLING OF MODEL PRECIPITATION) MAY ACT TO COOL LOW LEVELS.

BMJ ‘CP’ SCHEME : STRENGTHS OFTEN WORKS WELL IN MOIST ENVIRONMENTS WITH LITTLE CAP. TREATS ELEVATED CONVECTION BETTER THAN OTHER CP SCHEMES (ALTHOUGH STILL NOT WELL). MOST EFFECTIVE CP SCHEME AT PREVENTING MICRO-PHYSICS SCHEME FROM TRYING TO CREATE CONVECTION. IMPLICITLY INCLUDES EFFECTS ON CLOUD LAYERS OF DOWNDRAFTS, LATENT HEAT OF FUSION FROM FREEZING IN UPDRAFTS, MELTING OF FALLING PRECIPITATION, etc. RUNS QUICKLY; DOES NOT REQUIRE MUCH COMPUTING RESOURCES.

BMJ ‘CP’ SCHEME : LIMITATIONS REFERENCE PROFILES ARE FIXED BASED ON CLIMATOLOGICAL OBSERVATIONS RATHER THAN BEING FLEXIBLE FOR EVERY FORECAST SITUATION; AS A RESULT, IT MAY ELIMINATE IMPORTANT VERTICAL STRUCTURE. IS ONLY TRIGGERED FOR SOUNDINGS WITH DEEP MOISTURE. WHEN TRIGGERED, SCHEME OFTEN RAINS OUT TOO MUCH WATER, EITHER BECAUSE REFERENCE PROFILE IS TOO DRY FOR FORECAST SITUATION, OR TRANSITION TO REFERENCE PROFILE IS TOO RAPID. THIS LEAVES TOO LITTLE WATER VAPOR BEHIND FOR PRECIPITATION OCCURRING LATER OR DOWNSTREAM.

BMJ ‘CP’ SCHEME : LIMITATIONS DOES NOT ACCOUNT FOR STRENGTH OF CAP- INHIBITING CONVECTIVE DEVELOPMENT. DOES NOT ACCOUNT FOR ANY CHANGES BELOW CLOUD BASE:- MAKES NO ATTEMPT TO SIMULATE GUST FRONTS AND THEIR ASSOCIATED MESO-HIGHS. ONLY AFFECTS SURFACE CONDITIONS INDIRECTLY, SUCH AS THROUGH EVAPORATION OF PRECIPITATION AND REDUCTION OF SOLAR HEATING FROM CLOUD COVER.

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