Bores During IHOP_2002 and
Speculation on Nocturnal
Or
Convection
Things that go Bump in the Night
David B. Parsons, Crystal Pettet and June Wang
NCAR/ATD
Acknowledgements to Tammy Weckwerth,
Ed Browell et al., Cyrille Flamant et al.,
and Steve Koch and the bore working group
Primary Motivation for this Study
Some long known facts…….
• The Southern Great Plains region has a
nocturnal maximum in warm season
precipitation.
Diurnal Cycle of Rainfall
Diurnal variation of hourly thunderstorm frequency over the United States. Normalized amplitude of the
diurnal cycle is given by the length of the arrows in relation to the scale at bottom left. (Amplitudes are
normalized by dividing by the mean hourly thunderstorm frequency averaged over the 24 hr of the day
at each station.) Phase (time of maximum thunderstorm frequency) is indicated by the orientation of
the arrows. Arrows directed from north to south denote a midnight maximum, arrows directed from
east to west denote a 6 a.m. maximum, those from south to north denote a midday maximum, etc.
[Based on data in Mon. Wea. Rev., 103, 409 (1975).]
(From J.M. Wallace & P.V. Hobbs, “Atmospheric Science An Introductory
Survey”, Academic Press, New York, NY, 1977, pp.43)
Sounding-based Schematic of
Nocturnal Convection Initiation
Cases of this type were
few during IHOP_2002 and
not yet analyzed.
Future
talk.
From Trier and Parsons 1993
US Warm Season
Precipitation
• Eastward propagation of
mountain-generated systems
from the previous afternoon
(Riley et al. 1987, Carbone et
al. 2002)
Speculation: Since
there are no strong
signals in the mean
CAPEs and CINS,
perhaps convection
itself may hold the
key to propagation.
How do nocturnal
convective systems
behave?
•Question #1
How do nocturnal convective systems
“behave”?
20 June Case
• Undular-bore like structure present in radar and
profiler data (actually 3 events were present)
• Net effect of the bore is a (~200 hPa) deepening of
moisture and a reduction in convective inhibition
• Now examining additional cases
• Caveat: Additional changes present, low-level
moisture content increases with SE flow
Nocturnal MCS 20 June
20 June
An example of
a nocturnal
undular bore
20 June – Surface Data
No corresponding
temperature
change
Arrival of wave train in pressure field
Example
20
June
Doppler
Velocity
of a Doppler
Nocturnal
Undular
Velocity
Bore
20 June (MAPR)
Water Vapor: 20 June
20 June Event (cont.)
20 June Case
• Undular-bore like structure present in radar and
profiler data (actually 3 events were present)
• Net effect of the bore is a (~200 hPa) deepening of
moisture and a reduction in convective inhibition
• Now examining additional cases
• Caveat: Additional changes present, low-level
moisture content increases with SE flow
4 June
S-Pol Bore/Wave Events
27 MAY
11 June
18 June 2002
21 June Bore/Wave Event
2 June Bore/Wave Event
12 June Bore/Wave Event
13 June Bore/Wave Event
25 June Bore/Wave Event
BORE
Example
From
MAPR
4 June
Pre-bore height
Post height
CST (h)
7:
3
6:
3
5:
3
4:
3
3:
3
0
0
0
0
0
0
0
2
2:
3
1:
3
0
30
0:
3
23
:
30
30
30
30
1
22
:
21
:
20
:
19
:
30
CST (h)
# of Events
# of Events
7:
3
6:
3
5:
3
4:
3
3:
3
2:
3
1:
3
0
0
0
0
0
0
0
0
30
30
30
30
30
30
0:
3
23
:
22
:
21
:
20
:
19
:
18
:
# of Wave/Bore Events
BORE
STATS
18
:
30
30
30
30
30
30
0:
30
1:
30
2:
30
3:
30
4:
30
5:
30
6:
30
7:
30
23
:
22
:
21
:
20
:
19
:
18
:
Time of Generation (S-Pol)
5
4
3
2
1
Bore/Wave Passage at MAPR
0
3
Local Time (h)
End Time of Bore/waves Event
6
0
5
4
3
2
1
0
Approximate Spatial Dimension of S-Pol Bore/Wave Events
10
9
8
# of Events
7
6
5
4
3
2
1
0
50
150
250
350
Along line length (km)
450
550
Pre-bore Winds: Composite
Composite MAPR hodograph before bore passage
800 m
1000 m
30
20
1300 m
10
-30
-20
-10
0
0
10
20
30 m s -1
2700 km
-10
-20
-30
Bore Height Displacements
•
4.5
Motivated by Belay Demoz’s excellent (yet unpublished case study)
4
3.5
3
Scattering
Layer
Height
(km)
Reference slope of .5 m/s
2.5
Reference slope of .5 m/s
2
1.5
1
0.5
0
0
5
10
15
20
25
30
Time (mins)
35
40
45
50
60
65
75
IHOP_2002 Sounding Western OK
1730 pm LST
CAPE
CIN
20 June: 3 am Sounding
Dramatic moisture increase
Post-bore: Elevated convection is
preferred (high CAPE, low CIN)
Day-time: Surface-based
convection is preferred but
high CIN
“Surface”-based Parcel
20TH June
CAPE vs. CIN
0
Unstable, capped env.
-100
CIN (J kg-1)
-200
Dramatic stabilization,
-300
-400
1730 pm
expected due to radiational cooling !
-500
0301 am
-600
Very stable
-700
-800
0
500
1000
1500
CAPE (J kg-1)
2000
2500
“Surface” and Inversion Parcels
CAPE vs. Convective Inhibition
0
0301 am
-100
CIN (J kg-1)
-200
-300
1730 pm
1730 pm
-400
-500
-600
0301 am
-700
Opposite trends
-800
0
500
1000
1500
2000
2500
CAPE (J kg-1)
In fact the parcels are easier to convect than
Instability increases during the night
during the day!!!!
Question #3: Why are bores
important?
• Bores provide extremely strong lifting that leaves an
environment in their wake that can be unstable to
convective lifting aloft.
• Since this wake air feeds nocturnal convection, bores are a
possible mechanism for maintaining deep convection in
the presence of unstable surface conditions.
• Large stability and moisture variations are found during
the subsequent day. SPC forecaster feel bores likely
explain these variations.
Findings
•Bore/wave disturbances are ubiquitous over this region at night when
convection is present. ~26 event. Most events occur at the end of LLJ
moisture return periods (when convection is present)
•These disturbances can promote intense lifting with net displacements
of up to ~1-2 km. They creating a deeper moist inflow and favorably
impact stability. Peak vertical motions are >1-2 m/s.
• Surface radars undercount bore/wave events (at a fixed location), since the
lifting can be limited to heights above the PBL. Thus, ~26 events is
likely a severe undercount!
•These disturbances are (almost) always initiated by convection (slight
evidence for both a secondary evening and larger nocturnal
initiation). Later in the program and initiation is not by dry fronts.
• Typical spacings of waves ~10-14 km, surface evidence (pressure
disturbances (.25 – 1.5 hpa) with some closed circulations, typical
duration is ~3-6 hrs with mesoscale to synoptic coverage areas.