2011 - Cooperative Institute for Research in Environmental Sciences ...

feedbacks, on the SSTs in the models, whereas differences
in the representation of the tropical oceanic wave dynamics
are likely less important.
Product: Shin, SI, PD Sardeshmukh, and K Pegion (2010),
Realism of local and remote feedbacks on tropical sea surface
temperatures in climate models, J. Geophys. Res., 115,
D21110, doi:10.1029/2010JD013927.
PSD-15SurfaceProcesses
FEDERAL LEAD: CHRIS FAIRALL
CIRES LEAD: OLA PERRSON
NOAA Goal 2: Climate
Project Goal: Combine state-of-the-art observations of
surface fluxes, boundary-layer structure and mesoscale
features with high-resolution numerical modeling. Current
work focuses on boundary layers over ice/snow surfaces and
the effects of terrain on coastal precipitation. High quality
observations of surface turbulent, radiative and precipitation
fluxes are combined with radar measurements and mesoscale
model simulations. Parameterizations are being developed
for stable surface-layer flux-profile relationships and linking
terrain slopes and wind-vector profiles to spatial patterns of
precipitation accumulations.
Milestone 1. Analyze data from Surface Heat Budget of the
Arctic (SHEBA); Arctic Summer Cloud Ocean Study (ASCOS);
Arctic Mechanisms of Interaction between Surface and Atmosphere
(AMISA); and other field programs to understand
the links between the clouds, atmospheric boundary layer,
and surface processes over sea ice. Evaluate their respective
contributions to the net surface energy fluxes
Analyses of the soundings, surface energy budget and
surface temperatures at SHEBA and ASCOS and analyses
of soundings and surface temperatures from Soviet drifting
stations have provided insights into the processes producing
the onset and end of melt over sea ice. A paper submitted for
publication on describing the SHEBA melt transitions links the
surface energy budget to free-troposphere synoptic variables,
clouds, precipitation and in-ice temperatures.
The key results are 1) SHEBA melt-season transitions are
associated with atmospheric synoptic events; 2) onset of melt
clearly occurs on May 28, while the end of melt is produced
by a sequence of three atmospheric storm events over a 28day
period producing step-like reductions in the net surface
energy flux—the last one occurs on Aug. 22; 3) Melt onset is
primarily due to large increases in the downwelling longwave
radiation and modest decreases in the surface albedo; 4)
decreases in the downwelling longwave radiation occur for all
end-of-melt transition steps, while increases in surface albedo
occur for the first two; 5) changes in downwelling shortwave
radiation contributes only to the first end-of-melt transition
step; 6) springtime free-tropospheric warming preconditions
the atmosphere-ice system for the subsequent melt onset; and
7) melt-season transitions also mark transitions in system responses
to radiative energy flux changes because of invariant
melt-season surface temperatures. (See Figure 1.)
The last result is the reason that the length of the summer
melt season has a large impact on the mass balance of the
sea ice. Significant energy flux excess, and hence ice loss, can
only occur when the surface temperature is fixed and unable
to radiatively compensate for large positive energy fluxes.
The extensive SHEBA observations enable an understanding
118 CIRES Annual Report 2011
Figure 1: Temperature in the a) atmosphere and c) snow and ice from
April 30 (YD485) to June 20 (YD536), 1998 at SHEBA. Panel b) shows the
daily mean net energy fluxes and the time of melt onset (vertical black
bar). In a) and c), the 0°C isotherm is shown in bold red and the height of
the maximum RHw for RHw > 95% is shown in bold black in a). In b), the
times of springtime synoptic events discussed are shaded but unlabeled,
while P1 and P2 are discussed in Table 3. In c), the snow surface is shown
by the green line, the snow-ice interface by the blue line, and the ice bottom
by the thick black line. Temperatures near the top of the snow may
be biased by solar radiation
Figure 2: Cloud water, vapor, ice and total water tendencies, in units of g
kg-1 day-1. Gray dash lines denote boundaries of cloud top entrainment
zone, mixed layer, lower entrainment zone. Positive (negative) indicates
water gained (lost) by the layer.