Download What causes foehn warming?

What causes foehn warming?
The Antarctic Peninsula
as a natural laboratory
Andy Elvidge and Ian Renfrew (School of Environmental Sciences, University of East Anglia)
What is foehn warming?
Foehn warming is the warming effect experienced in the lee of mountains
Generally accompanied by foehn winds and a decrease in humidity
Foehn winds
Prevailing
flow

Humidity 
Temperature mountain
Why is leeside air warm and dry?
Four mechanisms identified in the literature
Only two of which have been widely considered
Why is leeside air warm and dry?
Why is leeside air warm and dry?
Why is leeside air warm and dry?
Why is leeside air warm and dry?
Foehn paradigms
Hann (1901) – first thorough examination of foehn theory
“Precipitation on the windward side is not a necessary condition for foehn.”
Ficker (1920):
Latent heating and
Isentropic drawdown
Foehn paradigms
Hann (1901) – first thorough examination of foehn theory
“Precipitation on the windward side is not a necessary condition for foehn.”
Ficker (1920):
Latent heating and
Isentropic drawdown
Ficker and de Rudder (1943) – the paradigm shifts:
Latent heating
In later edited editions of Hann’s famous textbook, the text was changed to mention only the latent heat effect.
Foehn paradigms
Hann (1901) – first thorough examination of foehn theory
“Precipitation on the windward side is not a necessary condition for foehn.”
Ficker (1920):
Latent heating and
Isentropic drawdown
Ficker and de Rudder (1943) – the paradigm shifts:
Latent heating
In later edited editions of Hann’s famous textbook, the text was changed to mention only the latent heat effect.
And then…
Foehn paradigms
Hann (1901) – first thorough examination of foehn theory
“Precipitation on the windward side is not a necessary condition for foehn.”
Ficker (1920):
Latent heating and
Isentropic drawdown
Ficker and de Rudder (1943) – the paradigm shifts:
Latent heating
In later edited editions of Hann’s famous textbook, the text was changed to mention only the latent heat effect.
ALPEX, Seibert (1990), Ólafsson (2005): Effects of latent heating concluded to be insignificant relative to the effect of isentropic drawdown.
Isentropic drawdown
Foehn paradigms
Hann (1901) – first thorough examination of foehn theory
“Precipitation on the windward side is not a necessary condition for foehn.”
Ficker (1920):
Latent heating and
Isentropic drawdown
Ficker and de Rudder (1943) – the paradigm shifts:
Latent heating
In later edited editions of Hann’s famous textbook, the text was changed to mention only the latent heat effect.
ALPEX, Seibert (1990), Ólafsson (2005): Effects of latent heating concluded to be insignificant relative to the effect of isentropic drawdown.
Isentropic drawdown
Seibert et al. (2000), Richner and Hächler (2013): Report foehn cases characterised by upwind ascent from near‐surface level and severe precipitation over the windward slopes
Latent heating and
Isentropic drawdown
Foehn paradigms
Hann (1901) – first thorough examination of foehn theory
“Precipitation on the windward side is not a necessary condition for foehn.”
Ficker (1920):
Foehn warming contributions have never before been In later edited editions of Hann’squantified
famous textbook, the text was changed to mention only the latent heat effect.
Latent heating and
Isentropic drawdown
Ficker and de Rudder (1943) – the paradigm shifts:
Latent heating
ALPEX, Seibert (1990), Ólafsson (2005): Effects of latent heating concluded to be insignificant relative to the effect of isentropic drawdown.
Isentropic drawdown
Seibert et al. (2000), Richner and Hächler (2013): Report foehn cases characterised by upwind ascent from near‐surface level and severe precipitation over the windward slopes
Latent heating and
Isentropic drawdown
The Antarctic Peninsula
A Natural laboratory for the study of foehn:
‐ Continuously high altitude (up to ~2000 m)
s
‐ Elongated, ridge‐like form (~1500 km long)
~1500km
s
‐ Both the westward ocean and the eastward Larsen Ice Shelves are relatively smooth, simple and homogeneous surfaces
Van Lipzig et al. (2008)
Data
•
•
Aircraft observations as part of the OFCAP (Orographic Flows and Climate of the Antarctic Peninsula) field campaign (Jan‐Feb 2011)
UK Met Office Unified Model (MetUM) 1.5km
British Antarctic Survey’s MASIN (instrumented Twin Otter aircraft) during the OFCAP field campaign
MetUM 4 km
MetUM 1.5 km
3 foehn events – foehn winds and warming
Plot height = 300 m
Case A Case B Case C
From MetUM 1.5 km simulation
3 foehn events – foehn winds and warming
Plot height = 300 m
Case A: model validation
Case A
From MetUM 1.5 km simulation
Observations: solid lines
Model: dashed lines
Upwind
Downwind
Further downwind
3 foehn events – back trajectories
Plot height = 300 m
Case A Case B Case C
3 foehn events – back trajectories
Plot height = 300 m
Case A Case B Case C
1) WI = Whirlwind Inlet (JET) 2) N of WI = North of Whirlwind Inlet (WAKE)
3) MOI = Mobil Oil Inlet (JET) 4) N of MOI = North of Mobil Oil Inlet (WAKE)
5) CI = Whirlwind Inlet (JET) 6) S of CI = South of Cabinet Inlet (WAKE)
• Heini Wernli group back trajectory model (Lagranto).
• UM 1.5km as input.
• Trajectory initiation at 950 hPa.
Plot height = 300 m
Case A Case B Case C
Blue dots = ~150 km upwind of Peninsula’s crest
(150 km ≈ Rossby radius of deformation for the Antarctic Peninsula)
Foehn heat budget model
Foehn heat budget model
C
C
C
Foehn heat budget model
B
B
B
Foehn heat budget model
A
A
A
Foehn heat budget model
Foehn heat budget model
Foehn heat budget model
Due to mechanical mixing
Foehn heat
budget model
results
+ = Advective drawdown
+ = zupwind – zdownwind
zupwind
zdownwind
Foehn heat
budget model
results
+ = Advective drawdown
+ = zupwind – zdownwind
zupwind
zdownwind
Foehn heat
budget model
CASE A ‐ Isentropic drawdown dominates
results
Potentially warm, dry air
+ = Advective drawdown
+ = zupwind – zdownwind
Cool,
Moist air
Flow blocking
zupwind
zdownwind
Mountain
Warm, dry air
Foehn heat
budget model
results
+ = Advective drawdown
+ = zupwind – zdownwind
zupwind
zdownwind
Foehn heat
budget model
results
+ = Advective drawdown
+ = zupwind – zdownwind
zupwind
zdownwind
Foehn heat
budget model
CASE C – Latent heating and precipitation dominates
results
Latent heat release
+ = Advective drawdown
+ = zupwind – zdownwind
zupwind
Cool, moist air
zdownwind
Mountain
Warm, dry air
Foehn heat
budget model
results
+ = Advective drawdown
+ = zupwind – zdownwind
zupwind
zdownwind
Foehn heat
budget model
CASE B – no dominant mechanism
results
+ = Advective drawdown
+ = zupwind – zdownwind
zupwind
zdownwind
Foehn heat
budget model
CASE B – Mechanical mixing is significant
results
Potentially warm, dry air
Sensible heat flux
+ = Advective drawdown
+ = zupwind – zdownwind
zupwind
z
downwind
Cool, moist air
Mountain
Warm, dry air
Conclusions
Conclusions
•
First quantitative assessment of the causes of foehn warming, using a novel heat budget model.
Conclusions
•
First quantitative assessment of the causes of foehn warming, using a novel heat budget model.
•
Results show that:
– both established foehn‐warming mechanisms are important.
– a third, hitherto neglected, foehn‐warming mechanism can also be significant; mechanical mixing.
Conclusions
•
First quantitative assessment of the causes of foehn warming, using a novel heat budget model.
•
Results show that:
– both established foehn‐warming mechanisms are important.
– a third, hitherto neglected, foehn‐warming mechanism can also be significant; mechanical mixing.
•
Any of these three mechanisms can dominate, depending on the larger‐scale meteorological conditions and the orographically‐forced flow dynamics.
Conclusions
•
First quantitative assessment of the causes of foehn warming, using a novel heat budget model.
•
Results show that:
– both established foehn‐warming mechanisms are important.
– a third, hitherto neglected, foehn‐warming mechanism can also be significant; mechanical mixing.
•
Any of these three mechanisms can dominate, depending on the larger‐scale meteorological conditions and the orographically‐forced flow dynamics.
•
All 3 mechanisms must be well represented in numerical weather prediction models. Notably a need for accurate representation of turbulent‐mixing in foehn flows (known to be underestimated in current models).
Conclusions
•
First quantitative assessment of the causes of foehn warming, using a novel heat budget model.
•
Results show that:
– both established foehn‐warming mechanisms are important.
– a third, hitherto neglected, foehn‐warming mechanism can also be significant; mechanical mixing.
•
Any of these three mechanisms can dominate, depending on the larger‐scale meteorological conditions and the orographically‐forced flow dynamics.
•
All 3 mechanisms must be well represented in numerical weather prediction models. Notably a need for accurate representation of turbulent‐mixing in foehn flows (known to be underestimated in current models).
•
Improved forecasting and representation of foehn events would have ramifications for associated hazards such as ice shelf melt, wild fires and avalanches.
Californian wildfire, driven by foehn (Santa Ana) winds
Thanks for your attention
Photo from NASA
For more foehn winds over the Antarctic Peninsula, see my second talk at WWOSC at 1pm on Thursday (ISS session):
Foehn jets and their implications for the Larsen C Ice Shelf, Antarctica