WHY ?
• GCMs indicate Arctic highly responsive to increasing greenhouse gases (e.g. IPCC)
• Clouds strongly influence the arctic surface andatmosphere, primarily through radiative interactions
• Factors controlling arctic cloudiness not well known
Observational evidence may support predictions: (Serreze et al. 2000)
Characterization and Radiative Impact of a Springtime Arctic Mixed-Phase Cloudy Boundary Layer observed during SHEBA
Paquita Zuidema
University of Colorado/NOAA Environmental Technology Laboratory, Boulder, CO
annual
IncreasedSpringAndSummerCloudiness
1982-1999AVHRR data(Wang&Key, 2003)
Persistent springtime cloud cover may advance snowmelt onset date (e.g., modeling study of Zhang 1996)
spring
summer
Surface-based Instrumentation: May 1-8 time series
35 GHz cloud radarice cloud properties
depolarization lidar-determined liquid cloud base
Microwave radiometer-derived liquid water paths
4X daily soundings. Near-surface T ~ -20 C, inversion T ~-10 C
-5-45 -20
1 2 3 4 5 6 7 8day
z
-30C
41 8
2
4
6
8
km
100g/m^2
day
-10C
lidar cloud base
Most common ice particle habit: aggregate
(below liquid cloud base)
number area mass
spheresaggregates, small&big
May 6. Intrieri et al., 2002
Depolarization ratioice
water
Liquid/ice discriminationbased on:• depolarization ratio value• backscattered intensity gradient
Monthly-averaged percentages ofVertical columns containing liquid (grey bars)
Nov Aug
LIQUID FIRST
Aircraft path
Lidar cloud base
Temperature inversion
Cloud radar reflectivity
time
Hei
gh
t (k
m)
1
2
dBZ0-50-50
May 4
24:0022:00 23:00UTC
Liquid Characterization
How do clouds impact the surface ?
noon = 60o
Clouds decrease surface SW by 55 W m-2 ,increase LW by 49 W m-2
Surface albedo=0.86; most SW reflected backClouds warm the surface, relative to clear skies with same T& T & RH, by time-mean 41 W m-2* (little impact at TOA)
• Can warm 1m of ice by 1.8 K/day, or melt 1 cm of 0C ice per day, barring any other mechanisms !
Local ice production more evident when boundary layer is deeper and LWPs are higher
May 3 counter-example – variable aerosol entrainment ?
Quick replenishment of liquid: longer-time-scale variability in cloud optical depth related to boundary layer depth changes
How do clouds impact the surface ?
noon = 60o
Clouds decrease surface SW by 55 W m-2 ,increase LW by 49 W m-2
Surface albedo=0.86; most SW reflected backClouds warm the surface, relative to clear skies with same T& T & RH, by time-mean 41 W m-2* (little impact at TOA)
• Can warm 1m of ice by 1.8 K/day, or melt 1 cm of 0C ice per day, barring any other mechanisms !
Great websites with real-time data, historical fotos:
http://www.arctic.noaa.gov/
http://nsidc.orghttp://nsidc.org/cryosphere/glancehttp://nsidc.org/data/seaice-index/
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