Distant Effects of Recurving Tropical Cyclones on Rainfall Production in Midlatitude Convective Systems

Russ S. Schumacher1, Thomas J. Galarneau, Jr.2, and Lance F. Bosart2

1 National Center for Atmospheric Research, Boulder, CO and Department of Atmospheric Sciences, Texas A&M University, College Station, TX

2Department of Atmospheric and Environmental Sciences, University at Albany/SUNY, Albany, NY

13th Conference on Mesoscale Processes, Salt Lake City, UT

17 August 2009

Purpose• Predecessor rain events (PREs),

defined by Cote (2007), occur poleward of recurving tropical cyclones and are high-impact weather events that frequently result in significant inland flooding

• Extremely heavy rainfall ( >350 mm in 24 h) and historic flooding occurred in Minnesota and Wisconsin on 19 August 2007, ahead of Tropical Storm Erin

• What effect did the transport of moisture by Erin have on the rainfall amounts in the Midwest?

Photo from Minneapolis Star Tribune

Predecessor Rain Events (PREs)• Coherent area of rain displaced poleward of TC• Moisture transport from TC toward PRE• Event duration ~ 12 h• Maximum rainfall rates typically ≥ 100 mm (24 h)1

• Time lag between PRE and TC passage ~ 36 h

~100

0 km

Bosart and Carr (1978) conceptualmodel of antecedent rainfallfor TC Agnes (1972)

Detailed study of PREs in Cote (2007)and in paper to be submitted to MWR by Galarneau et al.

18Z/18

06Z/19

00Z/19

12Z/19

PREPRE

PRE

Erin

Erin Erin

Erin

WSR-88D base reflectivity mosaic

Source: NCAR case-selection archive

Mature Weakening

Forming Organizing

0000 UTC 16 Aug 2007

925 hPa h (dam)925 hPa e (K)

200 hPa wind speed (m s1)

250 hPa h (dam), 700 hPa (105 s1),precipitable water (mm)

850–500 hPa mean wind (kt)

850 hPa h (dam), (K), wind (kt)900–800 hPa frontogenesis

[K (100 km)1 (3 h)1]

99 66

0.5° GFS analysis

0000 UTC 17 Aug 2007

925 hPa h (dam)925 hPa e (K)

200 hPa wind speed (m s1)

250 hPa h (dam), 700 hPa (105 s1),precipitable water (mm)

850–500 hPa mean wind (kt)

850 hPa h (dam), (K), wind (kt)900–800 hPa frontogenesis

[K (100 km)1 (3 h)1]

6 699

925 hPa h (dam)925 hPa e (K)

200 hPa wind speed (m s1)

250 hPa h (dam), 700 hPa (105 s1),precipitable water (mm)

850–500 hPa mean wind (kt)

850 hPa h (dam), (K), wind (kt)900–800 hPa frontogenesis

[K (100 km)1 (3 h)1]

99 66

0000 UTC 18 Aug 2007

925 hPa h (dam)925 hPa e (K)

200 hPa wind speed (m s1)

250 hPa h (dam), 700 hPa (105 s1),precipitable water (mm)

850–500 hPa mean wind (kt)

850 hPa h (dam), (K), wind (kt)900–800 hPa frontogenesis

[K (100 km)1 (3 h)1]

H99 66

0000 UTC 19 Aug 2007

925 hPa h (dam)925 hPa e (K)

200 hPa wind speed (m s1)

250 hPa h (dam), 700 hPa (105 s1),precipitable water (mm)

850–500 hPa mean wind (kt)

850 hPa h (dam), (K), wind (kt)900–800 hPa frontogenesis

[K (100 km)1 (3 h)1]

H99 66

0600 UTC 19 Aug 2007

925 hPa h (dam)925 hPa e (K)

200 hPa wind speed (m s1)

250 hPa h (dam), 700 hPa (105 s1),precipitable water (mm)

850–500 hPa mean wind (kt)

850 hPa h (dam), (K), wind (kt)900–800 hPa frontogenesis

[K (100 km)1 (3 h)1]

H99 66

1200 UTC 19 Aug 2007

WRF simulations• Advanced Research WRF model,

version 3.0.1.1• Initial and boundary conditions: 1º

GFS analyses• Two-way nesting, 3 grids: 27, 9

and 3 km grid spacing• KF convection on outer 2 grids,

explicit convection on inner grid• New Thompson microphysics: 2-

moment prediction of rain• Initialized at 0000 UTC 18 August:

24-36 hours before the event– At this time, Erin moisture was

easy to distinguish from other sources

– Wanted to capture the full transport and evolution of the moisture field and resulting convection

27 km

9 km

3 km

PBL MYJ

Land surface Noah

Turbulence 2D Smagorinsky

Shortwave radiation Dudhia

Longwave radiation RRTM

6th order diffusion and positive definite advection

PrecipitationObservations (stage IV)1200 UTC 18 August--1200 UTC 19 August

Simulation

mmMax = 384 mm

Max = 366 mm

• Overall distribution of precip agrees very well with observations• Maximum accumulation in the simulation displaced slightly north and west of the observed maximum

TS Erin

Extreme-rain-producing MCS• MCS structure in simulation similar to many past studies of

elevated MCSs from observations and modeling

Trier and Parsons (1993) Laing and Fritsch (2000)

Maddox et al. (1979) Schumacher and Johnson (2005)

Sensitivity simulation• How much additional rain fell in the Upper

Midwest because of the tropical moisture brought poleward by Erin?

• Method:– Modify the water vapor field at the initial time:

0000 UTC 18 August; ~24 hours before the initiation of deep convection in Minnesota

– Within the region of increased moisture owing to Erin, anywhere that RH exceeds 55%, reduce it to 55%

Precipitable waterControl run

Note other moisture sources

Erin moisture

X Erin

Reduced moisture run0000 UTC 18 August

Precipitable waterControl run

Reduced moisture run1200 UTC 18 August

Precipitable waterControl run

Reduced moisture run0000 UTC 19 August

Precipitable waterControl run

Reduced moisture run0600 UTC 19 August

Precipitable waterControl run

Reduced moisture run1200 UTC 19 August

Convective organization and motion2200 UTC 18 August

Simulated reflectivity, winds on lowest level, (red contours every 2 K), and frontogenesis (contours, averaged over lowest 5 model levels)

Control Modified

• Almost identical at this time, prior to deep convection initiation

Convective organization and motion0300 UTC 19 August

Simulated reflectivity, winds on lowest level, (red contours every 2 K), and frontogenesis (contours, averaged over lowest 5 model levels)

Control Modified

• Convection initiates at about the same time, organizes into similar training line of cells; displaced slightly north of the control run

• Convection is somewhat less intense and less “wet”

Convective organization and motion0600 UTC 19 August

Simulated reflectivity, winds on lowest level, (red contours every 2 K), and frontogenesis (contours, averaged over lowest 5 model levels)

Control Modified

• New convection still developing upstream in control run, not in modified run

Vertical Structure

• Both have similar frontal structure and plenty of elevated (surface-based) CAPE north (south) of the surface boundary initially

0000 UTC 19 AugustControl Modified(alternating green/white), circulation vectors,and CAPE (black contours every 500 J/kg)

Vertical Structure0400 UTC 19 AugustControl Modified

(alternating green/white), circulation vectors,and CAPE (black contours every 500 J/kg)

• Both have similar frontal structure and plenty of elevated (surface-based) CAPE north (south) of the surface boundary initially

• Control run has unlimited “feed” of CAPE from south, which dry run has CAPE locally which is quickly “used up”

Comparison of rainfall amounts

• As a point of reference, 188 mm in 24 hours is over a 100-year rain event for Minnesota• The actual storm is estimated as a 2000+ year event (MN State Climate Office)• Thus, the tropical moisture from Erin took a notable heavy rain event and turned it into an unprecedented event with major impacts

Conclusions• WRF provides a simulation of the 18-19 August 2007 PRE

that is quite accurate– Moisture transport, convective-system-scale processes, and rainfall

totals all compare favorably with observations

• Structure, organization, and motion of the MCS is similar to past studies of elevated convection on the cool side of a boundary

• Trajectory analysis (not shown) and sensitivity simulation underscore the importance of the tropical moisture brought poleward by Erin

• The effects of the Erin-related moisture on precipitation were quantified: almost a doubling of the maximum precip amount, and a 31% increase overall