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CUT-OFF LOW

CUT-OFF LOW. CUT-OFF LOW. DEFINITION THE DEFORMATION OF THE MIDDLE & UPPER TROPOSPHERIC FLOW CONNECTED WITH THE DEEPENING OF WAVES IN THE WESTERLIES RESULTS IN THE FORMATION OF CLOSED LOWs EQUATORWARD OF THE MAIN BELT OF ZONALLY AVERAGED WESTERLIES.

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CUT-OFF LOW

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  1. CUT-OFF LOW

  2. CUT-OFF LOW DEFINITION • THE DEFORMATION OF THE MIDDLE & UPPER TROPOSPHERIC FLOW CONNECTED WITH THE DEEPENING OF WAVES IN THE WESTERLIES RESULTS IN THE FORMATION OF CLOSED LOWs EQUATORWARD OF THE MAIN BELT OF ZONALLY AVERAGED WESTERLIES. • SUCH A SYNOPTIC SITUATION IS KNOWN AS CUT-OFF LOW.

  3. BEHAVIOUR OF A COLUMN DISPLACED EQUATORWARDS POTENTIAL VORTICITY : [ ζ + f ] / Δp = CONSTANT WHERE ζ IS RELATIVE VORTICITY AND ΔpIS THE DEPTH OF COLUMN ASSUME THAT THE REL VORTICITY IS INITIALLY (AND REMAINS ) ZERO. HENCE Δp WILL BE PROPORTIONAL TO f WHICH IMPLIES THAT AS THE AIR COLUMN IS DISPLACED SOUTHWARDS, IT SHOULD SHRINK VERTICALLY. THUS IF THE TOP OF A COLD AIR COLUMN WERE INITIALLY AT 300hPaAT LATITUDE 60°, ITS DISPLACEMENT TO LAT 30 WOULD RESULT IN SHRINKAGE IN DEPTH BY 42%. THE TOP AT THE FINAL LAT will be AT 590 hPa

  4. IN THE PROCESS, THE TEMP AT THE TOP OF COLUMN WHICH WAS INITIALLY -58 C , DUE TO THIS ADIABATIC COMPRESSION WOULD HAVE RESULTED IN WARMING TO -12 C. THIIS CORRESPONDS TO MID LAT WARM AIR. • THEREFORE IF THE POLAR AIR IS TO RETAIN ITS IDENTITY ON MOVING TO LOWER LATITUDES, THERE MUST BE SOME LIMIT IMPOSED ON ITS DEGREE OF SINKING . • NOW IF ζIS ALLOWED TO CHANGE, THE DEPTH OF THE AIR COLUMN COULD REMAIN CONST ONLY IF THERE IS AN INCREASE IN REL VORTICITY ζ

  5. CUT-OFF LOW CHANGE IN DEPTH OF AN AIR COLUMN MOVING FROM NORTH TO SOUTH WITH ZERO RELATIVE VORTICITY TEMPERATURE AT 300 hPa = -59ºC TEMPERATURE AT 590 hPa = -12ºC Potential Vorticity [ ζ + f ] / Δp = Constant

  6. When Rel Vor is allowed to change with Δp remaining constant, [ ζ + f ] / Δp = Constant ------1 As per the equation,as , f decreases (Δp = const) , Rel vorticity is to increase. An air parcel moving equator ward without shrinking has a tendency to curve cyclonically (until it reaches a lowest latitude and again starts moving pole wards). Let us examine the ratio of the depth at 30 0 Lat to that at 60 0 using Eq 1 [ ζ30+ f30] / Δp30 [ ζ60 + f60] / Δp60 Assuming the initial Vor as zero ie ζ60 = 0 the ratio becomes Δp30[ ζ30+ f30] = r = Δp60f60 This ratio will be =1 ie no shrinking if ζ30 = f60 – f30 WHICH IS >0 Hence when an air parel moves from lat 60 to lat 30 without shrinking ie retaining its identity then it necessarily attains cyclonic vorticity The reverse is true when it moves poleward. ( Examine the table)

  7. CUT-OFF LOW RATIO OF FINAL TO INITIAL DEPTH OF COLD AIR, FOR AN AIR COLUMN WITH ZERO RELATIVE VORTICITY AT 60º LATITUDE, WHICH ARRIVES AT LATITUDE 30º WITH A PRESCRIBED RELATIVE VORTICITY . [ ζ30 + f30 ] / H30 = [ ζ60 + f60 ] / H60 r = H30 / H60 = [ ζ30 + f30 ] / [ ζ60 + f60 ] r = [ ζ30 + f30 ] / f60 r = RATIO OF THE DEPTH AT LATITUDE 30º TO THAT AT LATITUDE 60º.

  8. CUT-OFF LOW INFERENCES • IF A DEEP POLAR AIRMASS HAS BEEN ADVECTED TO LOW LATITUDE, IT MUST APPEAR THERE AS A MASS WITH STRONG CYCLONIC VORTICITY; IF THE MASS IS CHARACTERIZED BY ANTICYCLONIC VORTICITY, IT MUST BE SHALLOW. • A VERY COLD AIR MASS COULD MOVE FARTHER EQUATORWARD THAN A MODERATELY COLD ONE WITHOUT LOSING ITS CONTRAST OF TEMPERATURE WITH THE SURROUNDING AIR.

  9. CUT-OFF LOW FORMATION • CUT-OFF LOW FORMS FROM A PRE-EXISTING COLD TROUGH THAT IS ULTIMATELY CUT-OFF FROM THE CONNECTIONS WITH THE POLAR SOURCE REGION. • THERE ARE FOUR CHARACTERISTIC TYPE OF DISTURBANCES RESULTING FROM THE EXTREME GROWTH OF UPPER LEVEL WAVES:- (A) DEGENERATION OF AN UPPER LEVEL COLD TROUGH IN TO A SHEAR LINE, WITH HIGH CYCLONIC VORTICITY BETWEEN THE JET STREAM. (B) A SHEAR LINE COMBINED WITH A COLD UPPER CYCLONE TO THE SOUTH. (C) FORMATION OF NEW JET STREAM TO THE NORTH OF COLD UPPER CYCLONE. (D) ‘VIERERDRUCKFELD’ PATTERN WHEREIN COLD LOWS & WARM HIGHS ARE PAIRED MERIDIONALLY, WITH A BAROCLINIC FIELD BETWEEN THE UPSTREAM AREAS AND AN OVER ALL REVERSAL OF THE NORMAL MERIDIONAL ARRANGEMENT OF AIRMASS IN DOWN STREAM PAIR.

  10. CUT-OFF LOW CHARACTERISTICS (A) DEFINITIVE ‘CUTTING-OFF’ PROCESS. (B) POOL OF COLD AIR MASS ROTATING IN CYCLONIC VORTEX. (C) MANIFESTS IN UPPER TROPOSPHERE i.e. 500 - 300 hPa. (D) COLD CORE SYSTEM. (E) PLAYS AN IMPORTANT ROLE IN MAINTENANCE OF EARTH-ATMOSPHERIC HEAT BUDGET. (F) ‘CUT-OFF LOW’ EQUATORWARD & ‘BLOCKING HIGH’ POLEWARD OF ZONAL FLOW. (G) PERSISTS FOR A WEEK AND PROFOUND INFLUENCE ON WEATHER.

  11. CUT-OFF LOW MOVEMENT & DISSIPATION • THE CUT-OFF LOWs GENERALLY HAVE A TENDENCY TO MOVE SLOWLY EASTWARD APPROXIMATELY 500 KM PER DAY. • IN CONNECTION WITH A SLOW SINKING IN THE COLD AIR, THE TEMPERATURE OF SUCH COLD VORTEX GRADUALLY INCREASES AND ULTIMATELY THE LOW WEAKENS AND BECOMES ABSORBED BY THE SURROUNDING WARM AIR. • THE CUT-OFF LOWs NORMALLY LASTS FOR 3 TO 4 DAYS.

  12. CUT-OFF LOW ORGANISATION OF VARIOUS FIELDS (A) THERMAL. COLD CORE, REMARKABLY SYMMETRIC ABOUT CENTRAL AXIS. (B) CONTOUR. CLOSED ‘LOW’ SLIGHTLY TO ‘EAST’ IN MIDDLE & UPPER TROPOSPHERE. (C) WIND. CIRCULAR VORTEX BEST MARKED IN THE MIDDLE & UPPER TROPOSPHERE.

  13. CUT-OFF LOW ORGANISATION OF VARIOUS FIELDS (D) OTHER FEATURES ARE:- * JET MEANDERS AROUND AND OFTEN A NEW JET IS SEEN TO THE NORTH. * PPTN FIELD ASYMMETRIC, MUCH AHEAD, MAXIMA IN EAST OR SOUTHEAST. * GENERALLY MOVES ‘EASTWARD’ (SE/NE) SOMETIMES ‘RETROGRADES’. * DISSIPATES WHEN ‘SINKING’ PROCESS SLOWS DOWN AND TEMPERATURE OF ‘COLD CORE’ INCREASES.

  14. CUT-OFF LOW WEATHER • WEATHER SHOWS A MARKED ASYMMETRIC DISTRIBUTION WITH PPTN ON THE EAST & SOUTHEAST SIDE AND CLEAR SKIES ON THE WEST & SOUTHWEST SIDE WHETHER THE VORTEX IS MOVING EASTWARD OR WEASTWARD. MOVEMENT • ERRATIC, IT CAN TAKE A LOOP. VERY SLOW MOVEMENT.

  15. SCHEMATIC REPRESENTATION OF PRECIPITATION

  16. CUT-OFF LOW CASE STUDIES

  17. CUT-OFF LOW PK NIGAM(FORMATION DURING WINTER) (A) FORMATION OF CUT-OFF LOW WAS PRECEDED BY DEVELOPMENT OF RIDGE AHEAD OF THE TROUGH. THEN INTENSIFICATION & INCREASE IN AMPLITUDE IS NOTED. SIMULTANEOUSLY THE RIDGE IN THE REAR WAS GROWING. THEN CUT-OFF SITUATION OCCURS. (B) PATIALA, DELHI & JODHPUR SHOW SIGNIFICANT RISE IN TEMPERATURES AT 850, 700, 500 & 300 hPa. RAINFALL OCCURS WITHIN 24 HOURS. (C) CONTOUR TENDENCIES SHOW FALL MUCH AHEAD OF COMMENCEMENT OF RAIN/ T’SH.

  18. CUT-OFF LOW MOHANDAS (1981) STUDIED THE CUT-OFF LOWs IN PRE-MONSOON SEASON AND CONCLUDED THAT:- * THE CUT-OFF LOW IN PRE-MONSOON WHICH MOVE OVER AFGHANISTAN, PAKISTAN & NW INDIA GENERALLY FORM AROUND 35°N TO 38°N AND 55°E TO 65°E. * THE VORTEX OF CUT-OFF LOW IS OF COLD CORE NATURE WITH THERMAL SYMMETRY. * THE LIFE PERIOD OF CUT-OFF LOW IS 3 TO 4 DAYS AND THEY MOVE AT A SPEED OF ABOUT 500 KM PER DAY. * THE RAIN BELT IS TO THE EAST & SE 400 – 700 KM AWAY FROM THE CENTRE OF CUT-OFF LOW. * HEAVILY COVERED AREAS OF CLOUDS ARE SEEN 400 TO 1200 KM FROM THE CENTRE OF CUT-OFF LOW IN THE ENE TO EAST SECTOR.

  19. CUT-OFF LOW MOHANDAS …… * CONTOUR LOWs AT 500 & 300 hPa WITH THERMAL SYMMETRY. * FAIRLY WIDESPREAD SEVERE WX LIKE T’SH/SQUALL OCCUR IN THE ENE SECTOR BETWEEN 700 & 1300 KM AND SCATTERED IN SE SECTOR BETWEEN 900 & 1300 KM FROM THE CENTRE IN THE LATE EVENING & NIGHT. * COOLING AT UPPER LEVELS 5-8°C UPTO 200 hPa. * THE INITIAL DISTURBANCE CAN USUALLY BE IDENTIFIED WITH A TROUGH WHICH IS SITUATED UPWIND FROM THE TROUGH. IN THE CONTOUR THERE IS A CAA IN THE REAR OF THE TROUGH AND WAA FURTHER UPWIND. THESE ADVECTIONS APPEAR TO BE FAVOURABLE FOR GROWTH OF CONTOUR WAVE.

  20. CUT-OFF LOW MOHANDAS ….. * SHWALTER INDEX (SI) FROM MORNING T- GRAMS OF DLH & LKN INDICATE A VALUE – 6 TO – 8 AND INCREASE IN MOIST LAYER. * THERE IS MARKED COOLING AT UPPER LEVELS OF 5 – 8°C UP TO 200 hPa. * REGIONAL ZONAL INDEX DECREASES DURING THE CUT-OFF LOW. * 0° ISOTHERM IS LOWER THAN NORMAL DURING CUT-OFF LOW.

  21. CUT OFF LOW TYAGI (1979)GAVE A MEAN MODEL OF WX PATTERN OVER INDIAN REGION ASSOCIATED WITH CUT-OFF LOW AND BROUGHT OUT THAT OVERCAST SKY OCCURS IN THE SECTORS EAST & SOUTHEAST WITH SEVERE CONVECTIVE ACTIVITY BETWEEN 5° TO 10° LONG AHEAD OF CUT-OFF LOW CENTRE. SEVERE CONVECTIVE ACTIVITY IS FOUND TO BE IN THE REGION OF STEEP TEMPERATURE GRADIENT IN THE MIDDLE & UPPER TROPOSPHERE, WHERE UPWARD VERTICAL MOTION TAKES PLACE AND ATMOSPHERE IS UNSTABLE.

  22. CUT-OFF LOW TYAGI (1979) STUDIED THE CUT-OFF LOWs OVER AREA (40°E – 120°E FOR 300 hPa & 30°N – 45°N EVERY 10° LONG) AND CONCLUDED THAT:- (A) CUT-OFF LOW TENDS TO FORM OVER AFGHANISTAN, PAKISTAN & INDIAN REGIONS. (B) CUT-OFF LOW SITUATIONS OBSERVED IN WIND, TEMPERATURE & CONTOUR FIELDS. (C) THERMAL SYMMETRY AROUND CUT-OFF LOW CENTRE AT 500 hPa. (D) VORTEX OF CUT-OFF LOW IS COLD CORE WITH MAXIMUM INTENSITY AT MID & UPPER TROPOSPHERE. (E) LIFE CYCLE (NO CONCLUSION).

  23. CUT-OFF LOW TYAGI (1979) (F) MEAN STRUCTURE:- (I) CLOUD COVER OF > 4 OCTA IS FROM NE TO SOUTH (THROUGH EAST) WITH OVERCAST SKY IN EAST & SE SECTORS. (II) R/F OF 1–2 CM IN EAST BETWEEN 600 & 1100 KM AND IN SE BETWEEN 1100 & 1700 KM. RAIN MAXIMA 2.5 CM. R/F < 0.5 CM IS SEEN UPTO 400 KM FROM CENTRE. (III) TEMPERATURES BELOW NORMAL UPTO 12 KM MAXIMUM DEPARTURE OF 5–7°C BELOW NORMAL AT 300 hPa. (IV) BELOW NORMAL CONTOURS AT 500 hPa & 300 hPa (500 hPa – 85 GPM & 300 hPa – 195 GPM). (V) REGIONAL ZONAL INDEX – BELOW NORMAL ON THE DAYS OF CUT-OFF LOW SITUATION.

  24. 2 0 4 8 6 8 6 6 0 2 4 SPATIAL DISTRIBUTION OF CLOUDS (OKTA)

  25. CUT-OFF LOW CUMULONIMBUS ACTIVITY. SPATIAL DISTRIBUTION OF CB ACTIVITY IS SHOWN IN FIGURE. IT SHOWS THAT CB ACTIVITY TAKES PLACE IN THE QUADRANT IV AND QUADRANTS II & III ARE RELATIVELY FREE. THE MAXIMUM CB ACTIVITY WITH ISOLATED HAILSTORM OCCURS BETWEEN 5° &10° LONGITUDINAL BELT TO THE EAST OF THE CENTRE. IT IS OBSERVED THAT ITS ASSOCIATION WITH A CUT-OFF LOW SITUATED OVER NORTHERN DIVISIONS OF PAKISTAN, THE BELT OF T’STORM EXTEND AS FAR SOUTH AS MP & ADJOINING MAHARASHTRA. CAA AT HIGHER LEVELS STEEPENS LAPSE RATE AND IT IS ONE OF THE FAVOURABLE SITUATIONS FOR THE OCCURRENCE OF SEVER CONVECTIVE WX. IT CAUSES MARKED CAA IN THE HIGHER LEVELS OVER NW INDIA AS SHOWN BY 6-8°C BELOW NORMAL TEMPERATURE OF DLH T- Ø GRAM.

  26. SPATIAL DISTRIBUTION OF CUMULONIMBUS CLOUDS

  27. CUT-OFF LOW COMPARISON BETWEEN CUT-OFF LOW & WESTERN DEPN

  28. CUT-OFF LOW COMPARISON OF WINTER & PREMONSOON CUT-OFF LOW

  29. CUT-OFF LOW CONCLUSIONS. THE STUDY SUGGESTS THE OCCURRENCE OF CUT-OFF LOW OVER AFGHANISTAN, PAKISTAN & ADJOINING NORTH INDIAN REGION. THE CUT-OFF SITUATION IS CLEARLY OBSERVED IN THE WIND, TEMPERATURE AND CONTOUR FIELDS. THERE IS A MARKED THERMAL SYMMETRY AROUND CUT-OFF LOW CENTRE AT 500 hPa. THE VORTEX OF CUT-OFF LOW IS OBSERVED TO BE OF COLD CORE NATURE WITH MAXIMUM INTENSITY IN MIDDLE & UPPER TROPOSPHERE. HOWEVER, THE LIFE CYCLE OF CUT-OFF-LOW OVER INDIAN REGION REQUIRES FURTHER DETAILED STUDY.

  30. CUT-OFF LOW CONCLUSION * CUT-OFF LOWs HAVE A WELL ESTABLISHED BEARING ON WX. * CUT-OFF LOWs WHICH FORM OVER AFGHANISTAN AFFECT THE WX OF INDIAN REGION. * CUT-OFF LOWs AFFECT INDIAN REGION BOTH IN WINTER & PRE-MONSOON. * THE CUT-OFF LOWs GIVE WX ON THE EAST & SE SECTOR BETWEEN 5° TO 10° LATITUDE FROM THE CENTRE.

  31. THANK YOU

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