QPF

Part 2 of 3

COMAP 99 Wes Junker Monday, 13 September 1999 [email protected]

1

PATTERN RECOGNITION IS VERY, VERY IMPORTANT

 PATTERNS VARY BY – SEASON, GEOGRAPHIC REGION AND SCALE  PATTERNS ARE IDENTIFIED – BY CONVENTIONAL DATA, MODEL OUTPUT, SATELLITE AND RADAR IMAGERY  HEAVY RAINFALL EVENTS SHARE CERTAIN CHARACTERISTICS – EVEN IN WINTER, HEAVY RAIN USUALLY FALLS IN MESOSCALE BANDS  HEAVY RAINFALL EVENTS CAN OFTEN BE IDENTIFIED BY THEIR PATTERNS – BUT YOU NEED TO UNDERSTAND WHAT IT IS ABOUT THE PATTERN THAT FAVORS HEAVY RAINFALL

START BY LOOKING AT SYNOPTIC SCALE (THE BIG PICTURE)

 THERE IS A CLEAR ASSOCIATION BETWEEN SHORT-WAVE TROUGHS AND PRECIPITATION – HOWEVER, THE VERTICAL MOTION ASSOCIATED WITH SYNOPTIC SCALE LIFT DOES NOT TYPICALLY ALLOW PARCELS TO REACH THE LEVEL OF FREE CONVECTION (LFC)  BUT, LARGE SCALE LIFT – STEEPENS LAPSE RATE – PROMOTES MOISTURE TRANSPORT – WEAKENS CAP – AFFECTS VERTICAL SHEAR (more important for severe weather forecasting)

NEXT LOOK FOR MESOSCALE FEATURES

– DO A MESOANALYSIS OF SURFACE AND UPPER AIR DATA IF TIME ALLOWS.

– LOOK AT SATELLITE AND RADAR AND TRY TO IDENTIFY MESOSCALE FEATURES. ALSO TRY TO DETERMINE WHAT IS CAUSING THE CURRENT PRECIPITATION. – IDENTIFY SURFACE BOUNDARIES  (FRONTS, DRY LINES, OUTFLOW BOUNDARIES, SEA BREEZE FRONTS, LAND USE BOUNDARIES, ETC.

USE MODELS TO IDENTIFY SYNOPTIC AND MESOSCALE PATTERNS THAT ARE FAVORABLE TO PRODUCE HEAVY RAINS  CAN USE THE SURFACE, 850- AND 500-MB PATTERNS TO IDENTIFY MADDOX ET AL. OR OTHER TYPES OF HEAVY RAINFALL EVENTS – LOOK CLOSELY AT MODEL FORECASTS OF MOISTURE, MOISTURE TRANSPORT AND INSTABILITY  MODELS PROVIDE DECENT FORECASTS OF LOW LEVEL WIND AND MOISTURE FIELDS – 850 MOISTURE TRANSPORT – PWS  OUTPUT CAN BE USED TO ASSESS FORCING AND TO FORECAST THE LOCATION OF BOUNDARIES • BEWARE OF MODEL BIASES!!

HOW DO YOU APPLY PATTERN RECOGNITION? CAREFULLY. Let’s apply our knowledge to a potential rainfall case. It’s June, what can you say about this pattern from these two fields (surface pressure and pw forecasts)? Does the pattern have potential to produce 3 inches or rain along the east slopes of the mountains in NC and VA

850 wind vectors and best capes

0000Z 1800Z L L L

So far what can you say about the case

?



Deep moisture is present



low level easterly flow is present



positive capes are forecast to develop



is there potential for several inches of rain?

– What other ingredients would you like to see present?

250 isotachs and divergence

700 mb heights (white), relative humidity and vertical motion (red)

VERTICAL MOTION IS PRESENT. THE RELATIVE HUMIDITY IS HIGH. WHAT DOES THE LATTER SUGGEST ABOUT PRECIPITATION EFFICIENCY?

3 3 18 hr v.t. 1800Z 24 hr v.t. 0000Z

500 heights (white) and vorticity (red) at 18Z

NOW WHAT DO YOU THINK?

 Upper level divergence is forecast to be present  The 700 heights and relative humidity are favorable for rain, but the model is only predicting modest vertical motion at best. Do you believe it? Why or why not?

 What can you say about the 500h forecast. Does it favor rainfall maximum of between 1 and 2 inches or a max of more than 3? Why?

Model terrain and 850-300 mean wind forecast valid 18Z

00Z forecast mean winds and model terrain

What do the mean winds imply about cells locking into the terrain ?

 Now do you think there is potential for 3 inch or greater amounts? Why or why not?

 In what direction do you think propagation would try to move the cells?  Would the net result of Cell movement (due to the winds) and propagation tend to keep the convection on the mountains or would the convection move away from the mountains?

 What do you think the area average rainfall was near the mountains (1”?, 2”? Or more?).

1 to 2 inch rainfall amounts fell over the mountains

.



The pattern would have had more rainfall potential if

–

the lowest pressures were west of the mountains

–

the 500 mb trough had a negative tilt. This would allow the mean flow to become more easterly. Cell would move westward allowing new cells to develop on the mountains.

–

If the low level jet were stronger or the mean winds were weaker (but had an easterly component)

You need to know how a models physics may bias its forecast, for example   The eta grid scale precipitation uses a different critical relative humidity for condensation over land and water – this can help produce an artificial boundary near the coast The Betts-Miller convective sheme – looks at the instability in the lower 130 mb and therefore does not handle elevated convection well – does not explicitly predict outflow – uses reference profiles

ANOTHER CASE OF PATTERN RECOGNITION. DO THE 500 AND SURFACE PATTERNS LOOK LIKE ONES YOU HAVE SEEN BEFORE?

IS THERE A SURFACE BOUNDARY TO BE CONCERNED ABOUT?

A STRENGHENING LOW LEVEL JET IS PRESENT TO TAP MOISTURE.

HOW DO THE PWS AND 850 WINDS COMPARE TO THE AVERAGE FOR MADDOX FRONTAL TYPE EVENTS?

A surface boundary is located over Nebraska and a cold pool is located over Iowa

The strongest moisture convergence is located over Nebraska

K INDICES ACROSS EASTERN NEBRASKA WERE FORECAST TO BE IN THE UPPER 30S TO LOW 40S K index at 00Z 17 JULY

STRONG MOSTURE FLUX IS IS PRESENT

WHERE DO YOU THINK CONVECTION MIGHT FORM AT 03Z?

Very strong moisture transport is present. The axis of strongest moisture flux or transport shifts very slowly eastward 06Z forecast 09Z forecast Where do you think the MCS should be located at 06Z? 09Z?

24 HR PRECIPITATION VALID 12 UTC 12 JULY 1996

MESOETA FORECAST ANALYSIS A good model forecast, how did you do?

6” OR MORE 3” OR MORE 1” OR MORE

Investigation of the MCS during the Great Flood of 1993

 MCSs were investigated for June-Sept.

 all 2, 3, 4 and 5 inch areas were measured for each MCS identified  systems were categorized based on the size of the 3” coverage  The largest scale, heaviest events were compared with smaller scale events that produced less rain.

Average size of various precipitation thresholds for each category (km 2 ) during June-Sept. 1993

SIZE CRITERIA OF EACH CATEGORY DURING 1993 STUDY OF MCSS

Cases where lower relative humidity and/or a stronger cap are more likely to have the convection form north of the front.

L

552 558 564

PW=0.80”

THICKNESS VALUES FOR 70% SATURATION THICKNESS P.W.

THICKNESS P.W.

522 528 534 540 546 552 558 561 .22

.27

.35

.43

.55

.70

.80

.90

561 564 567 570 573 576 579 582 .90

1.05

1.15

1.25

1.40

1.55

1.70

1.90

INFLOW 570

=CONVECTIVE AREA L

570 INFLOW 552 558 564

outflow boundary PW=1.15”

THE LARGER SCALE HEAVY RAINS FELL WITH HIGHER RH VALUES. THERE WERE CATEGORIES BASED ON THE AREAL EXTENT OF THE 4 INCH. CAT 1 HAS NO 3 INCH AREA, WHILE CAT 4 HAD 3600 SQ. NAUTICAL MI. OR MORE

3 INCH CATEGORY IN SQUARE DEG. LAT.

595 590 LINE REPRESENTS 70% MEAN RH 585 580 575 570 565 560 0.8 0 0 0 0 0 1 1 1 0 1 1 1 1 0 1 1 0 0 0 0 1 3 3 2 0 2 1 0 3 0 0 2 4 2 2 1 0 2 1 1 4 3 4 0 3 1 1 0 4 4 1 2 4 0 3 2 0 1 0 1 1.2 1.4 1.6 1.8 2 2.2 PRECIPITABLE WATER (INCHES) 4 2.4 The fact that few larger scale heavy rainfall events occurred to the of the line may be the reason preferred thickness appears to work

The maximum observed rainfall at a point versus the size of the 2” area 12 10 8 6 4 2 0 2 4 6 8 10 SIZE 2" IN SQ. DEG. LAT.

12 14

AVERAGE SIZE OF THE 3” FOR THE VARIOUS CATEGORIES, NOTE THE SMALL SCALE OF THE MOST INTENSE RAINFALL. THE BOTTOM RIGHT FIGURE IS THE LARGEST 3” DURING THE STUDY CAT 1 CAT 2 CAT 3 CAT 4 LARGEST 3” IT IS VERY HARD TO CORRECTLY FORECAST THE CORE OF HEAVIEST RAINS

ALL THE CATEGORY EVENTS OCCURRED WITH PWS AT OR ABOVE 1.40”. IN GENERAL THE SHEAR WAS WEAK 50 40 30 20 10 0 0 0 0 0 2 2 0 1 1 0 1 1 1 0 0 0 1 3 1 0 2 3 3 4 4 0 1 1 2 1 1 0 2 0 0 0 1 2 3 1 4 1 4 3 4 2 2 2 0 1 4 4 1 0 1 3 0 4 1 1 4 4 4 4 4 1 2 4 0 2 1 0 2 3 0 1 0 4 0 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 PRECIPITABLE WATER (INCHES)

3 INCH CATEGORIES IN SQUARE DEG. LAT.

CAT 0=NONE OBS.

CAT 1=.01-.25

CAT 2=.26-.50

CAT 3=.51-1.0

CAT 4>1.0

700 mb temperatures above 12 o C appear to limit the size of any convective system that forms.

CATEGORIES OF 3 INCH BASED ON SQUARE DEG. LAT

16 14 12 10 8 6 4 2 0 0 0 1 1 0 1 1 1 1 0 1 2 1 0 0 3 1 3 2 2 0 1 0 3 0 4 2 0 1 1 3 0 4 2 1 2 0 2 1 1 0 4 1 0 3 4 0 2 0 2 1 4 0 1 0 4 0 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 PRECIPITABLE WATER (INCHES) CAT 0 = NON OBS.

CAT 1 = .01-.25

CAT 2 = .26-.50

CAT 3 = .51-1.0

CAT 4 > 1.0

The HPC rule of thumb that 700 mb temperatures above 12 o C will provide an effective cap is a decent first guess BUT you should also look at the negative area on the sounding

COMPOSITES OF THE 12 LARGEST SCALE HEAVY RAINFALL EVENTS DURING THE STUDY

 WERE CENTERED ON THE RAINFALL MAXIMUM  USED RDAS GRIDDED FIELDS AND INTERPROLATED THE DATA TO THE 2 DEGREE LATITUDE GRID

COMPOSITE OF 12 LARGEST SCALE CATEGORY 4 EVENTS.

THE CENTER 0F HEAVIEST RAIN (BLACK DOT) OCCURS AT THE NOSE OF THE LOW-LEVEL JET IN/OR NEAR THE STRONGEST WARM ADVECTION IN/OR JUST NORTHEAST OF A THETA-E RIDGE IN AN 100 94 AREA OF THETA-E ADVECTION 88 100 94 88 46

1.0

.5

42 38

5 10 15 20 20 15 10 5 850 ISOTACHS (DASHED RED, EVERY 5 KTS), WIND DIRECTION (ARROWS) AND WARM ADVECTION (BLUE LINES, STRONGEST VALUES ARE SHADED BLUE)

46 42

325 330 335 340 3.0

2.0

. 1 0 1.0

38

345 345 340 850 MB EQUIVALENT POTENTIAL TEMPERATURE (RED LINES), ADVECTION OF EQUIVALENT POTENTIAL TEMPERATURE (DASHED BLUE), WIND SPEED GREATER THAN 20 KTS (YELLOW SHADING), WIND DIRECTION (ARROWS)

The moisture transport (flux or qV) and moisture convergence are dependent on the low-level jet.

8 6 4 7 6 8 2 0 -2 -4 -6 -8 -8 -6 19 18 17 10 9 -4 -2 0 2 4 6 8  850 mb moisture flux (left) and moisture flux divergence (right). Note that the heaviest rain occurred southeast of the strongest 850 mb moisture convergence. The red dot is the center of heaviest rainfall.

IN SUMMARY

 The scale of precipitation associated with an MCS during the study appears to be related to – the relative humidity – the orientation of the moisture convergence band with respect to the mean flow – the width of the axis of stronger moisture convergence

In summary (continued)

 Most of the MCSs formed to the north or northeast of the strongest 850 mb winds and moisture flux.

 Most occurred in an area of 850 mb warm and theta-e advection  most occurred on the southern edge of the 250 mb divergence

RULES OF THUMB FOR PREDICTING HEAVY RAIN

 THE MAXIMUM RAINFALL USUALLY OCCURS WHERE THE CENTER OF THE STRONGEST INFLOW INTERSECTS A BOUNDARY  THE RAINFALL MAXIMUM USUALLY OCCURS JUST NORTHEAST OF THE THETAE RIDGE  IN SUMMER, THE HEAVIEST RAINFALL OFTEN OCCURS ALONG OUTFLOW BOUNDARIES SOUTH OF THE WARM FRONT

RULES OF THUMB CONTINUED

 INVERTED ISOBARS ALONG A FRONT CAN SIGNAL HEAVY RAINFALL POTENTIAL  HEAVY RAIN OFTEN FALLS IN AN AREA OF THICKNESS DIFLUENCE  BEWARE OF THICKNESS LINES WHICH HOLD STEADY OR SINK SOUTHWARD IN LOW LEVEL SOUTHERLY FLOW  HEAVY RAINFALL SOMETIMES FALLS IN A PREFERRED THICKNESS CHANNEL

RULES OF THUMB CONTINUED

 MCSs TRACK ALONG OF SLIGHTLY TO THE RIGHT OF THE 1000-500 THICKNESS LINES  LOOK FOR CONVECTION ALONG THE SOUTHERN EDGE OF THE WESTERLIES  MCCs OFTEN FORM NEAR THE UPPER LEVEL RIDGE AXIS WHERE THERE IS WEAK INERTIAL STABILITY  WATCH FOR HEAVY CONVECTION BEHIND A VORTICITY MAXIMUM OR NEAR A VORTICITY MINIMUM WHEN STRONG THERMAL AND MOISTURE ADVECTION IS PRESENT

MORE RULES OF THUMB

 A FAVORABLE JET STRUCTURE CAN ENHANCE THE HEAVY RAIN POTENTIAL  K INDICES ARE A GOOD MEASURE OF DEEP MOISTURE, BEWARE OF K INDICES IN THE UPPER 30S  THE MAXIMUM RAINFALL IS USUALLY WITH THE TROPICAL CORE OF A TROPICAL SYSTEM AT NIGHT, RATHER THAN THE DAYTIME PERIPHERAL ACTIVITY  BEWARD OF TROPICAL CONNECTIONS AS OBSERVED FROM WATER VAPOR IMAGERY

MORE RULES OF THUMB

    BEWARE OF SLOW MOVING SYNOPTIC CIRCULATION (SHARS) EVENTS, THEY OFTEN HAVE WARM CLOUD TOPS STRONG HEIGHT FALLS AND/OR FAST MOVING SYSTEMS USUALLY PRECLUDE VERY HEAVY RAINFALL, INSTEAD THEY PRODUCE A LARGE AREA OF MORE MODEST RAINFALL (AN INCH OR TWO) NUMERICAL MODELS USUALLY DON’T PREDICT THE AXIS OF HEAVIEST RAINFALL FAR ENOUGH SOUTH (OUTFLOW BOUNDARIES) THE NGM RARELY PREDICTS OVER 3 INCHES OF RAIN