Distant Effects of Recurving Tropical Cyclones on Rainfall Production in Midlatitude Convective Systems Russ S. Schumacher 1, Thomas J. Galarneau, Jr. 2, and Lance F. Bosart 2 1 National Center for Atmospheric Research, Boulder, CO and Department of Atmospheric Sciences, Texas A&M University, College Station, TX 2 Department 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 ~1000 km Bosart and Carr (1978) conceptual model of antecedent rainfall for 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 PRE Erin WSR-88D base reflectivity mosaic Source: NCAR case-selection archive Mature Weakening Forming Organizing

0000 UTC 16 Aug hPa h (dam) 925 hPa  e (K) 200 hPa wind speed (m s  1 ) 250 hPa h (dam), 700 hPa  (10  5 s  1 ), 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 ] ° GFS analysis

0000 UTC 17 Aug hPa h (dam) 925 hPa  e (K) 200 hPa wind speed (m s  1 ) 250 hPa h (dam), 700 hPa  (10  5 s  1 ), 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 ]

925 hPa h (dam) 925 hPa  e (K) 200 hPa wind speed (m s  1 ) 250 hPa h (dam), 700 hPa  (10  5 s  1 ), 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 ] UTC 18 Aug 2007

925 hPa h (dam) 925 hPa  e (K) 200 hPa wind speed (m s  1 ) 250 hPa h (dam), 700 hPa  (10  5 s  1 ), 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 ] H UTC 19 Aug 2007

925 hPa h (dam) 925 hPa  e (K) 200 hPa wind speed (m s  1 ) 250 hPa h (dam), 700 hPa  (10  5 s  1 ), 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 ] H UTC 19 Aug 2007

925 hPa h (dam) 925 hPa  e (K) 200 hPa wind speed (m s  1 ) 250 hPa h (dam), 700 hPa  (10  5 s  1 ), 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 ] H UTC 19 Aug 2007

WRF simulations Advanced Research WRF model, version 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: 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 PBLMYJ Land surfaceNoah Turbulence2D Smagorinsky Shortwave radiationDudhia Longwave radiationRRTM 6 th order diffusion and positive definite advection

Precipitation Observations (stage IV) 1200 UTC 18 August UTC 19 August Simulation mm Max = 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 water Control run Note other moisture sources Erin moisture X Erin Reduced moisture run 0000 UTC 18 August

Precipitable water Control run Reduced moisture run 1200 UTC 18 August

Precipitable water Control run Reduced moisture run 0000 UTC 19 August

Precipitable water Control run Reduced moisture run 0600 UTC 19 August

Precipitable water Control run Reduced moisture run 1200 UTC 19 August

Convective organization and motion 2200 UTC 18 August Simulated reflectivity, winds on lowest level,  (red contours every 2 K), and frontogenesis (contours, averaged over lowest 5 model levels) ControlModified Almost identical at this time, prior to deep convection initiation

Convective organization and motion 0300 UTC 19 August Simulated reflectivity, winds on lowest level,  (red contours every 2 K), and frontogenesis (contours, averaged over lowest 5 model levels) ControlModified 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 motion 0600 UTC 19 August Simulated reflectivity, winds on lowest level,  (red contours every 2 K), and frontogenesis (contours, averaged over lowest 5 model levels) ControlModified 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 August ControlModified  (alternating green/white), circulation vectors, and CAPE (black contours every 500 J/kg)

Vertical Structure 0400 UTC 19 August ControlModified  (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 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 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