Post on 18-Dec-2015
Justin MinderYale Geology & GeophysicsDavid KingsmillNOAA-ESRL (HMT)
Mesoscale variations in Sierra Nevada snow lines: Climatology, case study, and mechanisms
Also: Gerard Roe, Dale Durran (UW)
What role to mesoscale processes play in modulating the rain-snow transition over mountains?
Mountain snow and climate change
The partitioning between rain & snow has big impacts on both hydrological resources & hazards
z
x
wind
ZS
Z0CδS
δδ0C
The mountainside snow line& mesoscale controls
ZS: Snow lineZ0C : Zero-degree line
ZBB: Bright band elevation
ZBB
Snowline observations: Case studies
Distance (km)
Altit
ude
(km
)
Lowering of ZBB & Z0C by up to 1km over:• N. Sierra Nevada• Oregon Cascades• Italian Alps
Marwitz (1987)
Medina et al. (2005)
N. Sierra: Isotherms (oC)
Italian Alps: Reflectivity(dBZe)
Z0C
ZBB
• Lowering of the snowline is a climatological feature• ΔZS ~100m’s, big enough for major impacts on snowpack and flooding
2005/6 -2007/8 storm-based climatology
ZBB drops
ZBB rises
Snowline observations: N. Sierra Climatology
What causes ΔZS (ΔZBB) and its variability?
~50km
Z 0C
Why does the snowline dip downwards towards the terrain?Possible mechanisms
Pseudo-adiabatic cooling from lifting
Z 0C
Cooling by melting of orographically enhanced snowfall
Z 0C
Z S
Increased melting distance for orographically enhanced snowfall
Z 0C
Along-barrier transport of cool/dry air
2-D semi-idealized numerical simulations
w-damping Layer (Klemp 2009)
stratosphere
tropospherehm
,a
RH = 20% N
d = 0.01 s-1
U
WRF-ARW (v3.0.1) Δx=2km 201 vertical eta-levels
(Δz=20-400m) Open BCs in x Periodic BCs in y Free slip bottom boundary Thompson et al. bulk microphysics
(6 phases, incl. Graupel) f-plane
( f = 10-4 s-1)
Initial conditions:Prescribe: T
s : temp. at z=0
Nm
: moist stability
RH : relative humidity U : cross-mountain windsTerrain:
cos4 ridge, with prescribed:h
m : height=1.5km
a
: half-width=40kmRun to steady state
RH = 95%N
m = 0.005 s-1
U = 15 m s-1
2D semi-idealized WRF simulations:
Mechanisms
Minder, JR, DR Durran, GH Roe, 2011: Mesoscale Controls on the Mountainside Snow-line. Journal of the Atmospheric Sciences, 68, 2107-2127.
wind
~20%
~45%
~30%
Minder et al. (2011)
Ts=7oCT
s=3oC
• Z0C
upwind rises 742 m with warming, ZS on the mountainside rises only 530 m.
• Mesoscale processes “buffer” effects of warming on the snow-line by ~30%.
2D semi-idealized WRF simulations:
Sensitivity to Temperature
Feb 8-12, 2007
A case study of Northern Sierra Nevada snowline behavior
GOES IR (shading)&
Sea Level pressure (contours)
• Do idealized results carry-over?• Do 3D dynamics, PBL fluxes, radiation, etc. change the answer?
CFCATA
BBDBLU
SHS profiler & balloons
profilers sfc. met.
Mesoscale ObservationsNOAA HMT-West
White et al. (2002) algorithm for ZBB detection
• 72 hr simulation
Feb 8, 12 UTC – Feb 11, 12 UTC
• 4 nested domains
(27km; 9km ; 3km; 1km)
• IC’s and BC’s from NARR
Nudge doms. 1-2 towards NARR
• 118 vertical levels
Δz = 30 m from 1-3km
• Parameterizations:
WSM6 microphysics
Kain-Fritsch convection (d1-3)
MYJ boundary layer
Mesoscale Modeling
OLR (shading) Sea Level Pressure (contours)
Surface (Precip. & Temp.)
Observations WRF
T(°C
)Pr
ecip
(mm
)
Time (UTC; dd.hh) Time (UTC; dd.hh)
Obs. WRF
Z0C
Z0C
ZBB
ZBB
Observations
WRF
ZBB & Z0C : upwind vs. mountain
ΔBB
= -110 m
ΔBB
= -140 m
ΔBB
= -70m
ΔBB
= -210 m
• Full drop in Z0C & ZS is more like 400-600m (underestimated by profilers)
WRF cross – section
Z0C
1hr rainfall (shading)500m winds (barbs)
wind
x(km)
y(km
) Frozen precip.mix. ratio (shading)
~20%Causes?• ~100 m melting distance
Trajectory diagnostics (1):Pseudo-adiabatic mechanisms
• Calculate 3-hr back-trajectories from model output, ending at Z0C on transect.
• Try to use simple pseudo-adiabatic parcel model to predict Z0C for each traj.
Total ΔZ0C
Pseu
do-
adia
batic
“oth
er”
~40%
• ~200 m pseudo-adiabatic (3D)
Trajectory diagnostics (2):Cooling by melting mechanism
Δ
7 18• Construct a θe budget for two representative trajectories
• Δθe is almost entirely due to cooling by melting snow
TotalCooling by melting
TotalCooling by melting
fhrfhr ~30%• ~150 m cooling from melting
~30%
~40%
~20%
Case study summary & further work
Still working on:• Sensitivity to microphysics• Sensitivity to temperature
Future plans:• Examine other cases
(low snowline w/ more blocking)• More climatological analysis with
HMT data• LES of melting layer turbulence• Examination of other settings
CONCLUSIONSA mesoscale lowering of the snowline over terrain appears to be a common feature of mid-latitude mountain climate
A range of pseudo-adiabatic, diabatic, & microphysical processes may explain this behavior
These processes may be simulated and diagnosed in mesoscale numerical models, which suggest important roles for several mechanisms
The dependence of these mechanisms on climate may result in modulations of large-scale climate impacts
Climate-connections are being further investigated using multi-year observations and regional climate models
z
zero-degree line
T
z
Z0C
0ºC
ZBB
ZS
Z0C
The atmospheric snowline
The elevation in the atmosphere where falling snow melts into rain
q s,g
z
Z0C
(q s,g
)o
(q s,g
)o/2
ZS
Melting layer
snow line
dbZ
z
Z0C
ZBB
Bright band
bright-band height
(δ)Dmelt
What physical processes determine ZS?
Role of: Melting Distance (Dmelt
)
(δ)Dmelt
= (Z0C
)mtn.
- (ZS)
mtn.
(δ)Dmelt
= 125 m
qc (shading), Isotherms ( contours every 1oC)
What physical processes determine ZS?
Role of: Latent Cooling from Melting (Qmelt
)
(δ)Qmelt
ZS and Z
0C from runs:
with Qmelt
(solid)
without Qmelt
(dashed)(δ)
Qmelt = (δ
0C) - (δ
0C)
no-Qmelt
(δ)Qmelt
= 61 m
Advection through melting layer is too fast for substantial Q
melt
qc (shading)
What physical processes determine ZS?
Role of: pseudo-adiabatic Cooling (Qad.
)
0°C T
Z ΓΓm
δ0C
Ts
dT/dz=Γ
Ts
T(z) Z0C
Γd
parcel
environment
Consider flow over a mountain that is:SteadyInviscidStably stratifiedPseudo-adiabatic
Moist thermodynamics following air parcel determines δ
0C
Captured by simple air parcel model (δ
0C)
parcel depends on: T
s, Γ (N
m), RH
(δ)Qad. = 81 m(δ
0C)
parcel = 107 m
c.f. WRF :
(δ0C
)no-Qmelt
= 81 m
What physical processes determine ZS?
Summary
Z0Cδ
x
z ZS
(δ)Qad.
(δ)Qmelt
(δ)Dmelt
δ = (δ)Dmelt
+ (δ)Qmelt
+ (δ)Qad.
267m = 125 m + 61 m + 81 m
…but how general is this result ?