HEAVY RAIN EVENTS PRECEDING THE ARRIVAL OF TROPICAL CYCLONES

HEAVY RAIN EVENTS PRECEDING THE ARRIVAL OF TROPICAL CYCLONES

Matthew R. Cote, Lance F. Bosart, and Daniel KeyserDepartment of Earth and Atmospheric Sciences

University at Albany/SUNY, Albany, NY

Michael L. Jurewicz, Sr.National Weather Service Forecast Office

Binghamton, NY

CSTAR II Grant NA04NWS4680005

8th Northeast Regional Operational Workshop 2 November 2006

• Observations

- Predecessor rain events (PREs) form with some regularity well in advance of tropical cyclones (TCs)

- Rainfall can be at least as significant as that directly associated with the TC

• Forecasting Issues- Most operational focus on track and intensity of the TC itself when PREs occur

- Heavy rain in advance of the TC may increase flooding risks

- Flooding may occur where it was otherwise not expected

OVERVIEW AND MOTIVATION

• Demonstrate the PRE identification process

• Present preliminary climatological results

• Follow the evolution of the PREs associated with Katrina (29-30 August 2005)

• Compare the synoptic setup surrounding Katrina with that of the null case of Cindy (6-7 July 2005)

GOALS

IDENTIFYING PREs

• Coherent area of rain displaced downstream from the parent TC

- NCDC and WSI NOWRAD radar imagery - NHC best-track data

• Normalized rainfall greater than 100 mm/day- NPVU QPE archive - NWS text products

• Diagnosis of important synoptic-scale features- NCEP/NARR gridded datasets

PRE

EXAMPLES OF PREs

1800 UTC 05829 WSI NOWRAD 2 km US Radar Mosaic

EXAMPLES OF PREs

PREs

0900 UTC 05830 WSI NOWRAD 2 km US Radar Mosaic

0

1

2

3

4

5

6

21-3

0 A

pr

1-10

May

11-2

0 M

ay

21-3

1 M

ay

1-10

Jun

e

11-2

0 Ju

ne

21-3

0 Ju

ne

1-10

Jul

y

11-2

0 Ju

ly

21-3

1 Ju

ly

1-10

Aug

11-2

0 A

ug

21-3

1 A

ug

1-10

Sep

t

11-2

0 S

ept

21-3

0 S

ept

1-10

Oct

11-2

0 O

ct

21-3

1 O

ct

1-10

Nov

11-2

0 N

ov

21-3

0 N

ov

1-10

Dec

11-2

0 D

ec

21-3

1 D

ec

1-10

Jan

Date

Freq

uenc

y of

TC

s w

ith P

REs

0

10

20

30

40

50

60

70

80

TC F

requ

ency

TCs with PREsTCs

CLIMATOLOGY

Frequency Comparison of TC Occurrence with TCs Producing PREs 1979-2005

0

2

4

6

8

10

12

14

16

18

20

PRE Left of TC Track PRE Along TC Track PRE Right of TC Track

Relative Locations

Freq

uenc

y

CLIMATOLOGY

TC Track vs. PRE Location 32 Cases 2004-2005

• Composition of Data Catalog- PRE 3 h positions based on rainfall centroid - TC positions and intensities during PREs - Approximate rainfall amounts isolated for each PRE

• Major PRE Characteristics- 12 TCs produced 32 PREs in 2004 and 2005

- Median Separation Distance: 987 km - Median Time Lag: 30 h - Median Event Duration: 12 h - Slow-moving TCs more likely to produce multiple PREs compared to those moving more quickly

CLIMATOLOGY

Heavy rainfall developed in association with:

• Mid- and upper-level jet-entrance region confluence zone

• Differential CVA caused by a weak short wave

• Moisture tongue extending through low-level ridge line

Bosart and Carr (1978) conceptual model of antecedent rainfall

PAST RESEARCH ON PRE WITH AGNES (1972)

CASE STUDY: KATRINA (29-30 AUG 2005)

1200 UTC 050829 925 hPa Ht (dam), Theta-e (K), and Winds (kts)

1200 UTC 050829 200 hPa Ht (dam) and Winds (kts) 1200 UTC 050829 700 hPa Ht (dam), AVor (10-5 s-1), and 1000-500 hPa Thck (dam)

1200 UTC 050829 WSI NOWRAD 2 KM US Radar Mosaic
















NULL CASE: CINDY

6-7 July 2005
















CONCLUSIONS• Katrina vs. Cindy Comparison

- Bosart and Carr (1978) schematic model, coupled with separation distance, time lag, and TC speed statistics, may aid in predicting if, where, and how many PREs will develop

- Longitude and orientation of trough axes appear important in determining potential PRE development

• Katrina Case Study- Weak forcing for ascent may be sufficient to trigger coherent areas of heavy rain downstream of approaching TCs

- Positive θe advection and divergent jet entrance and exit regions promote favorable conditions for heavy rain formation

• Expand PRE climatological data further back in time

• Provide adequate physical explanations for the differences between left-of-track, along-track, and right-of-track PREs, and the apparent left-of-track preference

• Examine the role mesoscale features may have played in the development of recent PREs

• Classify and subdivide the different modes of predecessor rain events

FUTURE RESEARCH

QUESTIONS?

COMMENTS?

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Documents

HEAVY RAIN EVENTS PRECEDING THE ARRIVAL OF TROPICAL CYCLONES