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)
IHOP_2002 Design and Nocturnal
Convection
• Nocturnal precipitation recognized in planning
through evening low-level jet flights designed to
better understand nocturnal systems (Parsons)
• Bores flight mission added (Koch and Geerts)
• Much of bore data collected via ground-based
measurements and fortuitous circumstances (e.g.,
Demoz and Raman lidar calibration, evening LLJ
flights)
Primary Motivation for this Study
Some long known facts…….
• The Southern Great Plains region has a nocturnal maximum in warm
season precipitation.
• Long-wave radiation cooling and an absence of solar heating means
cooler temperatures at night and likely less potential for convection.
• There is no shortage of theories proposed to explain this nocturnal
maximum.
– Some form of propagation (latest proponent—Carbone and colleagues)
– Some form of low-level dynamic forcing (tides, LLJ and regional
circulations, etc – dates back to Pleagle et al, recently Dai)
– Dynamic forcing north of the stationary front and above the stationary front
Hypothesis: If nocturnal convection was forced by
persistent regional forcing (LLJ-heating on sloped
terrain, tides, etc)………..
Then some pronounced signal should appear in the
diurnal signal of CAPE and CIN
CAPE and CIN Diurnal Variations during Nauru99
CIN (J kg-1)
600
700
800
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
900
1000
1100
1200
0500 LST
0800 LST
2300 LST
2000 LST
1100 LST
1700 LST
1400 LST
CAPE (J kg-1)
0200 LST
CAPE/CIN: mean
• Larger CAPE for LLJ
throughout the diurnal
cycle
• Maximum CAPE but
minimum CIN in the
afternoon for LLJ
• The 2nd small
maximum at ~0.5 km
around early morning
Speculation:
If nocturnal low-level forcing occurs
it is weak and perhaps insignificant
in forcing the nocturnal convection.
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?
GOAL:
Attempt to answer the conundrum of why there is a
nocturnal precipitation maximum over the
Southern Great Plains when the convective
stability is less favorable.
METHOD:
Focus on nocturnal precipitation systems. Link
together IHOP_2002 remote sensing data from (at
this point) radar, lidar, and profiler with stability
measurements from radisondes.
•Question #1
How do nocturnal convective systems
“behave”?
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
4 June
S-Pol Bore/Wave Events
27 MAY
11 June
18 June 2002
2 June Bore/Wave Event
12 June Bore/Wave Event
13 June Bore/Wave Event
21 June Bore/Wave Event
25 June Bore/Wave Event
Question #1
How do nocturnal convective systems
“behave”?
Answer #1
They behave badly by the standards of
daytime convection !! They often produce
bores, while daytime convection has long
been known to favor gust fronts.
Question #2
How “common” are these so-called bores and
what are their characteristics?
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
BORES!!!!!
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
Max Surface Wind vs. Bore/Wave Events
16
14
12
10
8
Max Nocturnal
Wind Speed (m/s)
6
4
2
S1
S-Pol and MAPR
bore/wave events
43
41
39
37
35
33
31
29
25
27
IHOP Date
23
21
19
17
15
13
11
9
7
5
3
1
0
~18 bore and 8 wave events were observed in the S-Pol
and MAPR data sets. Bore events are observed in the
later stages of LLJ periods when precipitation occurs.
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
Answer #2: Bore Charateristics
•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)
• A single radars can undercount bore/wave events, since the
lifting can be limited to heights above the PBL. Thus, ~26
events may be an undercount.
•These disturbances are (almost) always initiated by
convection (evidence for both a secondary evening and larger
nocturnal initiation). Bores occur later in the program and we
did not see bore triggering by dry fronts.
Bore Characteristics
• 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.
• The winds just ahead of the bores are typically a strong lowlevel jet.
Question #3: So what or why are
bores important (e.g., Jim Wilson
only found three initiation events)?
20 June (MAPR)
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
Water Vapor: 20 June
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!!!!
IHOP_2002 Sounding Western OK
1730 pm LST
CAPE
CIN
20 June: 3 am Sounding
Dramatic moisture increase
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.
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
S-Pol Bore/Wave Events
27 MAY
11 June
2 June Bore/Wave Event
BORE
Example
From
MAPR
4 June
Pre-bore height
Post height
12 June Bore/Wave Event
13 June Bore/Wave Event
21 June Bore/Wave Event
25 June Bore/Wave Event
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.
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
Time of
Occurrence
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
20 June Event (cont.)
Max Surface Wind vs. Bore/Wave Events
16
14
12
10
8
Max Nocturnal
Wind Speed (m/s)
6
4
2
S1
S-Pol and MAPR
bore/wave events
43
41
39
37
35
33
31
29
25
27
IHOP Date
23
21
19
17
15
13
11
9
7
5
3
1
0
~18 bore and 8 wave events were observed in the S-Pol
and MAPR data sets. Bore events are observed in the
later stages of LLJ periods when precipitation occurs.
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
Composite MAPR hodograph before bore passage
800
30
1100
900
1000
600
700
1199
20
1300
500
10
400
-30
-20
-10
0
0
3800
3700
3600
3500
3400
3300
3000
200
-10
-20
-30
1400
1500
1599
1699
1800
1900
2000
2100
2200
2300
27002400
2800
2500
2600
2900
3200
4000
103100
20
30 m s -1
Example
20
June
Doppler
Velocity
of a Doppler
Nocturnal
Undular
Velocity
Bore
Water Vapor: 20 June