A WRF Simulation of the Genesis of
Tropical Storm Eugene (2005)
Associated With the ITCZ Breakdowns
The UMD/NASA-GSFC Users' and Developers' Workshop, September 2007
Chanh Q Kieu and Da-Lin Zhang
Department of Atmospheric and Oceanic Science
University of Maryland
Content
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Introduction
Overview
Model description
Results
Conclusions
Climatological conditions for TC genesis:
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An underlying warm SST of at least 260C;
A finite-amplitude low-level cyclonic disturbance;
Weak vertical wind shear;
A tropical upper tropospheric trough (TUTT);
A moist lower to middle troposphere; and
A location poleward of 50 latitude.
TC genesis may occur from
 Synoptic-scale control, e.g., MJO, AEWs, monsoon
troughs, cold surges
 Mesoscale convective systems or MCVs
 Mixed gravity-Rossby waves
 topography
 ITCZ breakdown
Breakdown due to barotropic instability, so-called Vortex Rollup (Charney
1962; Nieto Ferreira and Schubert 1997)
Breakdown due to the interaction of ITCZ and easterly MCVs (obs, Wang and
Magnusdottir 2005,2006)
Statistical study by WM06 shows that VR-breakdown is less likely to generate
a storm of tropical storm strength compared with MCV-ITCZ interactions
Scientific questions:
 What are the roles of the ITCZ breakdown and
MCVs in tropical cyclogenesis?
 What are the effects of vertical shear on tropical
cyclogenesis?
They will be addressed through a case study of the
processes leading to the genesis of Tropical Storm
Eugene (2005) using the NCEP reanalysis and satellite
data, and 4-day cloud-resolving (WRF) simulations
with the finest grid size of 1.33 km.
Overview: NCEP’s reanalysis
E
V1
V2
Hovmöller diagram of the 850-hPa vertical relative vorticity (unit: 10-5 s-1)
Overview of Eugene: TCSP-CIMSS satellite observations
V2
V1
GOES-10/12 VIS 0000 UTC 17 Jul 2005
Overview of Eugene: TCSP-CIMSS satellite observations
V2
V1
GOES-10/12 VIS 0300 UTC 17 Jul 2005
Overview of Eugene: TCSP-CIMSS satellite observations
V2
V1
GOES-10/12 VIS 0600 UTC 17 Jul 2005
Overview of Eugene: TCSP-CIMSS satellite observations
V2
V1
GOES-10/12 VIS 0900 UTC 17 Jul 2005
Overview of Eugene: TCSP-CIMSS satellite observations
V2
V1
GOES-10/12 VIS 1200 UTC 17 Jul 2005
Overview of Eugene: TCSP-CIMSS satellite observations
V2
V1
GOES-10/12 VIS 1500 UTC 17 Jul 2005
Overview of Eugene: TCSP-CIMSS satellite observations
V2
V1
GOES-10/12 VIS 1800 UTC 17 Jul 2005
Overview of Eugene: TCSP-CIMSS satellite observations
V2
V1
GOES-10/12 VIS 2100 UTC 17 Jul 2005
Overview of Eugene: TCSP-CIMSS satellite observations
V2 V1
GOES-10/12 VIS 0000 UTC 18 Jul 2005
Overview of Eugene: TCSP-CIMSS satellite observations
V2 V1
GOES-10/12 VIS 0300 UTC 18 Jul 2005
Overview of Eugene: TCSP-CIMSS satellite observations
E
GOES-10/12 VIS 0600 UTC 18 Jul 2005
Overview of Eugene: TCSP-CIMSS satellite observations
E
GOES-10/12 VIS 0900 UTC 18 Jul 2005
Overview of Eugene: TCSP-CIMSS satellite observations
E
GOES-10/12 VIS 1200 UTC 18 Jul 2005
Model setup
 NCEP Initialization at 0000Z 17 July 2005
when MCVs V1 and V2 are about 1000 km
apart
 Nested resolutions: 36, 12, 4 and 1.33 km.
 The 1.33 km domain is activated at 0000Z 18
July and moved manually, following the storm
center
 Lateral boundaries updated every 6-h
 Integrate 4 days to capture the life cycle of the
storm
Model configuration
DN
B
A
C
D1
Tracks and Intensities
38 m s-1
31 m s-1
(a)
989 hPa
987 hPa
(b)
Hovmöller diagram of the 850-hPa vertical relative vorticity (unit: 10-5 s-1) for the period of 0000
UTC 17 - 0000 UTC 21 July 2005 and the longitude interval of 1150 – 950W.
E
V2
V1
6-h accumulated rainfall
Comparison of the
simulated 6-h accumulated
rainfall shaded, mm) over a
subdomain of C to the
corresponding 6-h TRMM
satellite-estimated
(contoured).
3-D flows during the merging period
T = 0600UTC18JUL
T = 1200UTC18JUL
T = 1800UTC18JUL
T = 0600UTC19JUL
B
B
A
V2
V1
B
V1
B
A
E
E
V2
A
A
Vertical cross sections of tangential flow and PV
(shadings) during the merging period
V2
V2
V1
V2
V1
V1
E
Vertical cross sections of the normal component of horizontal winds (at 2 m s -1 intervals), PV (shaded at intervals of 0.5
PVU), superimposed by the system-relative in-plane flow vectors along the centers of V1 and V2
Radar reflectivity and vertical shear
700
500
200
700
900
900
200
500
19/18-66
500
700
900
200
200
700
900
500
20/06-78
700
500
700
900
200
900
200
500
Horizontal distribution of the radar reflectivity (shaded at
5-dBz intervals). Hodographs with VWS (solid) between
900 and 200 hPa (800 km  800 km) is also sketched.
Upper right panels are e 352 K-isosurface
Vertical shear and moist downdrafts
A
(b)
(a)
B
(c)
a) Horizontal distribution of e (2 K), and w (shaded at intervals 0.1 m s-1 for descending and 0.3 m s-1 for ascending) at 700 hPa
at 19/12-60; (b) as in (a) but for vertical cross section through the storm center of e (at intervals of 2 K) and deviation potential
temperature (’, shaded); and (c) as in (b) but for 19/18-66.
Conclusions
 The ITCZ breakdown is important, but merging MCVs are
critical in the genesis of Eugene
 Intensity and track are in mutual influence, especially in
shear environment. Too strong storm will impact the
movement of simulated storm
 Confirms previous findings: Interactions of VWS, ambient
dry air intrusion result in a strong asymmetry of rainfall
patterns
 The interaction between VWS and ambient environment
are not limited just to dry intrusion but to an elevated
dust layer. This also has some implications to the high
frequency cyclogenesis events in East Pacific.
Appendix 1
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Parameterizations: (a) Kain-Fritsch (1990) cumulus parameterization
scheme for the 36- and 12-km resolution domains; (b) the Yonsei
University planetary boundary layer (PBL) parameterization; (c) the
Monin-Obukhov surface layer scheme. Note that no cumulus
parameterization is used in the 4- and 1.33-km resolution domains.
Radiation: the Rapid Radiative Transfer Model (RRTM) scheme for both
longwave and shortwave radiations (Mlawer et al. 1997)
Microphysics schemes: Lin et al. (1983) cloud microphysics scheme
containing six classes of hydrometeors
The four nested-grid domains have the (x, y) dimensions of 251  201
(A), 252 ´ 252 (B), 388 ´ 382 (C), and 451 ´ 451 (D) with the grid size
of 36, 12, 4, and 1.33 km, respectively
38 s levels: 1.000, 0.993, 0.980, 0.966, 0.950, 0.933, 0.913, 0.892,
0.869, 0.844, 0.816, 0.786, 0.753, 0.718, 0.680, 0.643, 0.607, 0.572,
0.538, 0.505, 0.473, 0.441, 0.409, 0.378, 0.348, 0.318, 0.289, 0.260,
0.232, 0.204, 0.176, 0.149, 0.122, 0.095, 0.068, 0.042, 0.018, and
0.000.
The model top is defined at 30 hPa.