Transcript ppt Presentation - Department of Meteorology

Mesoscale Group Meeting
25 January 2011
Frequency and distribution of sting jets in
intense winter North-Atlantic cyclones
Oscar Martinez-Alvarado
Sue Gray
Laura Baker
Peter Clark
Department of Meteorology
University of Reading
Shapiro-Keyser model of
cyclogenesis
Browning (2004) after
Shapiro and Keyser (1990)
Sting Jets
• Jet descending from midtroposphere from the tip of the
hooked cloud head
• Located in the frontal fracture
region
• Mesoscale (~100 km) region of
strong surface winds (that can
reach more than 100 km/h)
occurring in rapidly deepening
extratropical cyclones
• Transient (~ few hours),
possibly composed of multiple
circulations
Shapiro-Keyser cyclogenesis
Stage III
Adapted from Clark et al. (2005)
3
Storm Anna:
Sting jet history along trajectories
A
B
Time series along Lagrangian trajectories following the sting jet showing
the ensemble–mean (solid), ensemble-mean plus/minus one standard
deviation (dashed) and instantaneous maxima and minima (dotted) of (A)
pressure and (B) relative humidity.
Figures adapted from Martínez-Alvarado et al.
(2010b)
4
Overview of research path
• Physical mechanisms for sting jets (SJ) (Browning
2004, Clark et al. 2005): Conditional symmetric
instability (CSI) and evaporative cooling.
• Study of CSI and SJ (Gray et al. 2011): Both
updraught and downdraught CSI are present in SJ
storms, DSCAPE is a good diagnostic to evaluate for
SJ.
• CSI in LAM and global models (Martinez-Alvarado et
al. 2011): CSI produced but not realistically released in
low-resolution models, DSCAPE can be used to
evaluate for potential SJ.
• DSCAPE in reanalyses (Martinez-Alvarado et al.
2011): Sting jets under present-climate conditions.
Mg
increasing
Physical mechanisms for SJ - I
qe*
increasing
Unstable slantwise
convective circulations in
an otherwise inertially and
gravitationally stable
atmosphere
Downdraught SCAPE (DSCAPE)
is the potential energy
available to parcel to
descend in slanted
downdraughts
6
Physical mechanisms for SJ - II:
Conditional symmetric instability
C
A
B
Time series along trajectories following the sting jet showing the
ensemble–mean (solid), ensemble-mean plus/minus one standard
deviation (dashed) and instantaneous maxima and minima (dotted) of (A)
moist potential vorticity, (B) moist static stability, and (C) absolute vorticity.
Figures adapted from Martínez-Alvarado et al. (2010b)
7
CSI and SJ:
Downdraught SCAPE
A
B
Downdraught SCAPE (DSCAPE, in J/kg) at (A) 0100 UTC and (B) 0300
UTC on 26 February 2002. The bold dark line represents the edge of the
cloud head; the grey lines are lines of constant wet-bulb potential
temperature (in K). The black circle marks the position of the sting jet at
each time (Gray et al. 2011).
8
CSI in LAM and global models
A
B
Downdraught SCAPE (DSCAPE, in J/kg) at 0100 UTC on 26 February
2002 in (A) the LAM and (B) the global model. The bold dark line
represents the edge of the cloud head; the grey lines are lines of constant
wet-bulb potential temperature (in K). The black circle marks the position
of the sting jet at each time (Martinez-Alvarado et al. 2011).
9
DSCAPE in reanalyses
• ERA-Interim is the new ECMWF reanalysis covering
the period 1989-present
• Resolution: ~0.7°
• This work looks at fields on pressure levels
• Domain limited to 30°N - 70°N, 70°W - 30°E (North
Atlantic and Europe)
• 100 most intense cyclones during the full period in the
reanalysis (1989-1998).
• Only winter months (DJF)
10
Downdraught CSI and
cyclone intensity
(a) Frequency of cyclone occurrence (grey) and downdraught CSI
regions (black) as a function of cyclone intensity (vorticity). (b)
Percentage of downdraught CSI region to number of cyclones.
11
Inter-annual variability of
frequency and intensity
(a) Time series for intensity (vorticity) during cyclone life cycle. (b) Time
series for frequency of the 100 most intense cyclones (gray) and
downdraught CSI regions (black).
12
Location of downdraught CSI
relative to cyclone centres
Mean positions (dots) and overall mean (cross) of
downdraught CSI regions with respect to cyclone
centres (a) and geographical coordinates and (b)
cyclone travel direction
13
DSCAPE and
vertical distribution
Frequency of CSI regions as a function of (a) maximum
DSCAPE and (b) maximum DSCAPE pressure
14
DSCAPE in individual cyclones
ERA-Interim
MetUM - LAM
MetUM - global
Start of
trajectories - LAM
15
Individual analysis of cyclones
Time series along Lagrangian trajectories following
the sting jet showing the ensemble–mean (solid),
ensemble-mean plus/minus one standard deviation
(dashed) and instantaneous maxima and minima (dotted)
of (A) pressure and (B) relative humidity.
16
Contingency tables
•
The contingency table after 15 verified cases
– Including cases where the descent rate was > 0.3 Pa s-1
Obs SJ
Non-obs SJ
0
1
7
8
1
5
2
7
6
9
15
p-value = 0.034 (using Fisher’s exact
test)
17
Summary
•
Mid-tropospheric regions of CSI have been shown to be very well
spatially correlated with descending sting jets in mesoscale
simulations of 3 sting jet storms (and are not present in a storm
without a sting jet).
•
A DSCAPE-based method to detect sting jet precursors has been
developed and has started to give results.
•
This method has being currently applied to the ECMWF reanalysis
ERA-Interim.
•
32 out of 100 most intense cyclones show signs of mid-tropospheric
CSI.
•
A sample of 15 from the 100 most intense cyclones is being
simulated at high resolution to verify the existence or non-existence
of sting jets.
•
These results are to be published in Martínez-Alvarado et al. (2011),
now in preparation.
18
References
1.
Browning KA. 2004. The sting at the end of the tail: Damaging winds
associated with extratropical cyclones. Quart. J. Roy. Meteor. Soc. 130:
375–399.
2.
Clark, P. A., K. A. Browning, and C. Wang, 2005: The sting at the end of the
tail: Model diagnostics of fine-scale three-dimensional structure of the cloud
head. Quart. J. Roy. Meteor. Soc., 131, 2263-2292.
3.
Gray, S. L., O. Martínez-Alvarado, L. H. Baker, and P. A. Clark (2011)
Conditional symmetric instability in sting jet storms. Submitted to Quart. J.
Roy. Meteor. Soc.
4.
Martínez-Alvarado, O., F. Weidle, and S. L. Gray (2010) Sting jets in
simulations of a real cyclone by two mesoscale models. Mon. Wea. Rev,
138, 4054–4075.
5.
Martínez-Alvarado, O., S. L. Gray, L. H. Baker, J. L. Catto, and P. A. Clark,
(2011) Susceptibility of intense North-Atlantic extratropical cyclones to the
production of sting jets. To be submitted to Quart. J. Roy. Meteor. Soc.