Model simulations of inversion buildup and cold-air ...
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manuela.lehner @utah.edu Model simulations of inversion buildup and cold-air outflow in a small Alpine sinkhole Manuela Lehner (1) , C. David Whiteman (1) , and Manfred Dorninger (2) (1) Department of Atmospheric Sciences, University of Utah (2) Department of Meteorology and Geophysics, University of Vienna P1.29 ICAM 2015 Innsbruck Introduction Grünloch basin ◮ Limestone sinkhole in the eastern Alps of Austria ◮ Diameter: ≈1 km, depth: ≈100–200 m ◮ Three major saddles intersect the surrounding ridgeline (Fig. 1): Lechner Saddle (≈55 m above the basin floor), Seekopfalm Saddle (≈130 m), and Ybbstaler Saddle (≈180 m). Model simulation ◮ CM1 (Bryan and Fritsch 2002, MWR, 130, 2917–2928) ◮ Stretched grid: Δx =Δy =30–150 m, Δz =10–400 m ◮ The simulation is initialized with a quiescent and dry atmosphere. ◮ The model topography is a simplified and smoothed representation of the Grünloch topography (Fig. 1): Lechner Saddle (≈50 m above the basin floor) and Seekopfalm Saddle (≈150 m). o E 15.03 15.04 15.05 15.06 o N 47.805 47.81 47.815 47.82 47.825 47.83 47.835 GL LS SAS YTS Distance (km) 3 4 5 6 7 8 Distance (km) 3 4 5 6 7 8 Height (m MSL) 700 800 900 1000 1100 1200 1300 1400 1500 1600 Fig. 1: Gruenloch topography (left) and idealized model topography (right). GL—Grünloch, LS—Lechner Saddle, SAS—Seekopfalm Saddle, YTS—Ybbstaler Saddle. The blue and red lines indicate the locations of the vertical and horizontal cross sections in Figs. 4 and 6, respectively. Temperature inversion buildup ◮ Comparison of the model simulation with tethered-balloon measurements from 20 October 2008: spd (m s -1 ) 0 0.5 1 1.5 2 dir ( o ) 0 90 180 270 360 T ( o C) 0 5 10 16 CET 17 18 19 20 21 22 23 00 01 02 spd (m s -1 ) 0 0.5 1 1.5 2 z (m) 0 50 100 150 200 250 dir ( o ) 0 90 180 270 360 z (m) 0 50 100 150 200 250 T ( o C) 0 5 10 z (m) 0 50 100 150 200 250 20 Oct 2008 Fig. 2: Vertical profiles of temperature, wind speed, and wind direction in the Grünloch from tethersonde observations (left) and the model simulation (right). ◮ An approximately 150-m deep inversion forms, with the highest stability within the lowest 50–100 m. ◮ The strong cooling after sunset is followed by small temperature changes later during the night. T ( o C) -2 0 2 4 6 8 5± 2 m AGL 41± 2 m AGL CM1 TS Time (CET) 1600 1700 1800 1900 2000 2100 2200 2300 0000 0100 0200 d θ /dt (x 10 -3 K s -1 ) -1 -0.5 0 0.5 advection mixing radiation Fig. 3: Time series of simulated and observed temperatures at 5 and 41 m AGL (top) and potential temperature tendency terms at 5 m AGL (bottom). ◮ A jet-like wind profile forms, with maximum wind speeds near the top of the layer with highest stability and near the height of the Lechner Saddle. ◮ The wind turns from southerly to southeasterly at the height of the jet maximum, that is, towards the direction of the Lechner Saddle. Cold-air outflow ◮ Air flows out of the basin through the Lechner Saddle, mostly below the height of the Seekopfalm Saddle. ◮ Above the height of the Seekopfalm Saddle, the flow is mostly directed into the basin along the surrounding topography. Distance (km) 0 1 2 3 4 5 6 7 Height (m) 1300 1350 1400 1450 1500 1550 SAS elevation 0200 CET spd (m s -1 ) -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Fig. 4: Wind speed perpendicular to the surrounding topography. The cross section is along the blue line in Fig 1. ◮ The katabatic flow along the east sidewall separates from the surface near the top of the layer with highest stability. ◮ A jet-like wind structure over the basin connects the katabatic flow on the east sidewall and the outflow through the Lechner Saddle. A B 0200 CET W-E distance (km) 5.2 5.4 5.6 5.8 6 6.2 S-N distance (km) 4.7 4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 5.6 A--B Distance (km) 0 0.2 0.4 0.6 0.8 1 1.2 Height (m) 1250 1300 1350 1400 1450 spd (m s -1 ) 0 0.5 1 1.5 2 2.5 3 3.5 Fig. 5: Horizontal cross section of wind speed and wind arrows approximately 10 m above the height of the Lechner Saddle (left) and vertical cross section of wind speed along the red line (right). Suggested reading Steinacker, R., and Co-authors, 2007: A sinkhole field experiment in the eastern Alps. Bull. Amer. Meteor. Soc., 88, 701-716. Pospichal, B., S. Eisenbach, C. D. Whiteman, R. Steinacker, and M. Dorninger, 2003: Observations of the Cold Air Outflow from a Basin Cold Pool through a Low Pass. Extended Abstracts, ICAM 2003, 153-156. Acknowledgments This research is supported by the National Science Foundation through grant AGS-1361863. Mass budget Distance (km) 5 5.5 6 6.5 Distance (km) 4.4 4.6 4.8 5 5.2 5.4 5.6 0200 CET w (m s -1 ) -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 Fig. 6: Vertical velocity at the height of the Seekopfalm Saddle within the area indicated by the red line in Fig. 1. ◮ Katabatic flows along the sidewalls result in sinking motions. ◮ Upward motions occur in the layer adjacent to the katabatic-flow layer. ◮ Vertical velocities over the interior of the basin are weak. ◮ Comparison of the outflow through the Lechner Saddle with the inflow through katabatic flows along the sidewalls: Time (CET) 1700 1800 1900 2000 2100 2200 2300 0000 0100 0200 V flux (x 10 5 m 3 s -1 ) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 outflow horiz plane slope-wind layer Fig. 7: Time series of volume flux through the Lechner Saddle below the height of the Seekopfalm Saddle, through a horizontal plane at the height of the Seekopfalm Saddle, and the vertical flux in the slope-wind layer at the height of the Seekopfalm Saddle. ◮ The upward motions adjacent to the slope-wind layer are mostly balanced by the weak downward motions over the basin interior. Tendency terms ◮ Pressure-gradient driven cold-air outflow: -5 0 5 CTR u v du/dt (10 -3 m s -2 ) -5 0 5 z (m) 1250 1300 1350 1400 1450 1500 LS total advection mixing pgf Fig. 8: Time-averaged vertical profiles of u and v tendency terms at the Lechner Saddle (LS) and the center of the Grünloch (CTR) between 2200 and 0400 CET.
Transcript of Model simulations of inversion buildup and cold-air ...
Model simulations of inversion buildup andcold-air outflow in a small Alpine sinkhole
Manuela Lehner(1), C. David Whiteman(1), and Manfred Dorninger(2)
(1) Department of Atmospheric Sciences, University of Utah(2) Department of Meteorology and Geophysics, University of Vienna
P1.29
ICAM2015
Innsbruck
Introduction
Grünloch basin
◮ Limestone sinkhole in the eastern Alps of Austria◮ Diameter: ≈1 km, depth: ≈100–200 m◮ Three major saddles intersect the surrounding ridgeline
(Fig. 1): Lechner Saddle (≈55 m above the basinfloor), Seekopfalm Saddle (≈130 m), and YbbstalerSaddle (≈180 m).
Model simulation
◮ CM1 (Bryan and Fritsch 2002, MWR, 130, 2917–2928)◮ Stretched grid: ∆x = ∆y =30–150 m, ∆z =10–400 m◮ The simulation is initialized with a quiescent and dry
atmosphere.◮ The model topography is a simplified and smoothed
representation of the Grünloch topography (Fig. 1):Lechner Saddle (≈50 m above the basin floor) andSeekopfalm Saddle (≈150 m).
oE
15.03 15.04 15.05 15.06
oN
47.805
47.81
47.815
47.82
47.825
47.83
47.835
GLLS
SAS
YTS
Distance (km)3 4 5 6 7 8
Dis
tanc
e (k
m)
3
4
5
6
7
8
Hei
ght (
m M
SL)
700
800
900
1000
1100
1200
1300
1400
1500
1600
Fig. 1: Gruenloch topography (left) and idealized model topography (right). GL—Grünloch,LS—Lechner Saddle, SAS—Seekopfalm Saddle, YTS—Ybbstaler Saddle. The blue and red linesindicate the locations of the vertical and horizontal cross sections in Figs. 4 and 6, respectively.
Temperature inversion buildup
◮ Comparison of the model simulationwith tethered-balloon measurementsfrom 20 October 2008:
spd (m s-1)0 0.5 1 1.5 2
dir (o)0 90 180 270 360
T (oC)0 5 10
16 CET17181920212223000102
spd (m s-1)0 0.5 1 1.5 2
z (m
)
0
50
100
150
200
250
dir (o)0 90 180 270 360
z (m
)
0
50
100
150
200
250
T (oC)0 5 10
z (m
)
0
50
100
150
200
250
20 Oct 2008
Fig. 2: Vertical profiles of temperature, wind speed, and winddirection in the Grünloch from tethersonde observations (left) andthe model simulation (right).
◮ An approximately 150-m deep inversionforms, with the highest stability withinthe lowest 50–100 m.
◮ The strong cooling after sunset isfollowed by small temperature changeslater during the night.
T (
oC
)
-2
0
2
4
6
8
5± 2 m AGL
41± 2 m AGL
CM1TS
Time (CET)1600 1700 1800 1900 2000 2100 2200 2300 0000 0100 0200
dθ/d
t (x
10-3
K s
-1)
-1
-0.5
0
0.5
advectionmixingradiation
Fig. 3: Time series of simulated and observed temperatures at 5and 41 m AGL (top) and potential temperature tendency terms at5 m AGL (bottom).
◮ A jet-like wind profile forms, withmaximum wind speeds near the top ofthe layer with highest stability and nearthe height of the Lechner Saddle.
◮ The wind turns from southerly tosoutheasterly at the height of the jetmaximum, that is, towards the directionof the Lechner Saddle.
Cold-air outflow
◮ Air flows out of the basin through the Lechner Saddle, mostly belowthe height of the Seekopfalm Saddle.
◮ Above the height of the Seekopfalm Saddle, the flow is mostly directedinto the basin along the surrounding topography.
Distance (km)0 1 2 3 4 5 6 7
Hei
ght (
m)
1300
1350
1400
1450
1500
1550
SAS elevation
0200 CET
spd
(m s
-1)
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Fig. 4: Wind speed perpendicular to the surrounding topography. The cross section is along the blue line in Fig 1.
◮ The katabatic flow along the east sidewall separates from the surfacenear the top of the layer with highest stability.
◮ A jet-like wind structure over the basin connects the katabatic flow onthe east sidewall and the outflow through the Lechner Saddle.
A
B
0200 CET
W-E distance (km)5.2 5.4 5.6 5.8 6 6.2
S-N
dis
tanc
e (k
m)
4.7
4.8
4.9
5
5.1
5.2
5.3
5.4
5.5
5.6A--B
Distance (km)0 0.2 0.4 0.6 0.8 1 1.2
Hei
ght (
m)
1250
1300
1350
1400
1450
spd
(m s
-1)
0
0.5
1
1.5
2
2.5
3
3.5
Fig. 5: Horizontal cross section of wind speed and wind arrows approximately 10 m above the height of the LechnerSaddle (left) and vertical cross section of wind speed along the red line (right).
Suggested reading
Steinacker, R., and Co-authors, 2007: A sinkhole field experiment in the eastern Alps. Bull. Amer. Meteor.
Soc., 88, 701-716.
Pospichal, B., S. Eisenbach, C. D. Whiteman, R. Steinacker, and M. Dorninger, 2003: Observations of the
Cold Air Outflow from a Basin Cold Pool through a Low Pass. Extended Abstracts, ICAM 2003, 153-156.
Acknowledgments
This research is supported by the National Science Foundation through grant AGS-1361863.
Mass budget
Distance (km)5 5.5 6 6.5
Dis
tanc
e (k
m)
4.4
4.6
4.8
5
5.2
5.4
5.6
0200 CETw
(m
s-1
)
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
Fig. 6: Vertical velocity at the height of the Seekopfalm Saddle withinthe area indicated by the red line in Fig. 1.
◮ Katabatic flowsalong the sidewallsresult in sinkingmotions.
◮ Upward motionsoccur in the layeradjacent to thekatabatic-flowlayer.
◮ Vertical velocitiesover the interior ofthe basin are weak.
◮ Comparison of the outflow throughthe Lechner Saddle with the inflowthrough katabatic flows along thesidewalls:
Time (CET)1700 1800 1900 2000 2100 2200 2300 0000 0100 0200
V fl
ux (
x 10
5 m
3 s
-1)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
outflowhoriz planeslope-wind layer
Fig. 7: Time series of volume flux through the Lechner Saddlebelow the height of the Seekopfalm Saddle, through ahorizontal plane at the height of the Seekopfalm Saddle, andthe vertical flux in the slope-wind layer at the height of theSeekopfalm Saddle.
◮ The upward motions adjacent tothe slope-wind layer are mostlybalanced by the weak downwardmotions over the basin interior.
Tendency terms
◮ Pressure-gradient drivencold-air outflow:
-5 0 5
CTR
uv
du/dt (10 -3 m s -2 )
-5 0 5
z (m
)
1250
1300
1350
1400
1450
1500LS
totaladvectionmixingpgf
Fig. 8: Time-averaged vertical profiles of u andv tendency terms at the Lechner Saddle (LS)and the center of the Grünloch (CTR) between2200 and 0400 CET.
