The Arctic boundary layer: Characteristics and properties

1. The Arctic boundary layer: Characteristics and properties Steven Cavallo June 1, 2006 Boundary layer meteorology

2. Overview of the Arctic boundary layer (ABL) • An annual overview of observational characteristics from SHEBA • The wintertime ABL: • Characteristics of the near-surface inversion • Leads • Arctic haze • The summertime ABL: • Measurements from AOE 2001 • Arctic stratus clouds

3. Observations from SHEBA • SHEBA (Surface Heat Budget of the Arctic Ocean Experiment) was a year-long field campaign from October 1997- October 1998. • Measurements were taken from a site on the ice with a 20-m tower, drifting with the ice flow more than 1400 km over the year. • Temperature and humidity profiles taken at the surface, and 5 other various heights ranging from 2-18 m, with sampling rates of 5 s. • Winds, friction velocity, and sensible and latent heat fluxes measured at the same levels above with a 10 Hz sampling rate.

4. Observations from SHEBA • 10-m temperatures (solid black line) were at times at much as 5C warmer than surface temperatures (dotted black line) in winter • “Summer melt season” began May 29 • Relative humidity always near saturation, but lowest in summer • Compared to “climatology,” SHEBA results were similar, except for a pronounced period of “above normal” temperatures in the early spring (Perrson et al. 2002)

5. SHEBA Surface Energy Budget 1-month average values • Surface gains heat from April-September • Average longwave flux always negative • Shortwave maximum occurred when albedo was a minimum • Melting  albedo decreases  more shortwave reaches surface • Bowen ratio (Hs/Hl) large during winter Persson et al. 2002

6. SHEBA Surface Energy Budget Daily mean values • Positive net radiation in winter during cloudy periods • Longwave smallest under clear skies when radiation can escape into space • “Spikes” in sensible heat flux during winter from leads (large open cracks in the ice) Persson et al. 2002

7. The wintertime ABL

8. The wintertime ABL • During the winter, there is little to no solar radiation. • Snow and ice covered surface emits longwave radiation upwards faster than the atmosphere, allowing a near-surface temperature inversion to develop. • Stable, shallow BL during the winter Shaw 1995 • Inversion very shallow to the ground in winter, sometimes as strong as 5C+ in lowest 18 m from surface. Persson et al. 2002

9. Leads • An internal boundary layer is created due to convective eddies transporting heat and moisture upward over and downwind of the lead  Heat fluxes can be predicted from the fetch over a lead Andreas 1980

10. The summertime ABL Arctic Ocean Experiment (AOE) August 2001 • Measurements from an ice breaker ship called “Oden,” moored to ice near the NP • Temperatures quite variable in free troposphere, but rather homogeneous near surface • Temperature inversion base most frequently ~ 200 m • Inversion thickness most frequently ~200-500 m • Inversion strength 4-6C most frequently, but sometimes 18C+ Figures from Tjernström et al. 2004

11. Arctic Stratus Clouds (ASC) • Three main types of summer Arctic boundary layer structure observed (Curry et al. 1988): 1) Cloud-topped mixed layer from surface to base of inversion 2) Stable BL with several layers of thin, patchy clouds 3) Stable, foggy BL with a cloud-topped mixed layer above • Three main ideas as to why there is a layering: • Cloud absorption by solar radiation (Herman and Goody 1976) • Weak ascent and entrainment form upper layer, lower layer an advective fog (Tsay and Jayaweera 1984) • Weak rising vertical motion is most conducive for layering (McInnes and Curry 1995)

12. Arctic Stratus Clouds (ASC) McInnes and Curry 1995 Initialized with observations from the Arctic Stratus Experiment (ASE) in 1980 over the Beaufort Sea, a high resolution 1-D model with 2nd order turbulence closure was used to simulate the evolution of an Arctic BL. Mean initial conditions (solid) and after 2 hours of model integration (dashed)

13. Arctic Stratus Clouds (ASC) 3) McInnes and Curry 1995 (cont’d) W = 0 cm/s Control, w = 0.2 cm/s No radiation W = 1 cm/s No drizzle • Weak, rising motion produces most favorable conditions for layered clouds • Radiation enhances condensation from cloud-top cooling in upper layer • Sensible heat loss to underlying sea-ice produces a stable fog/low cloud layer W = -1 cm/s No radiation or drizzle

14. Summary • Temperature inversion is characteristic all year due to ice and snow covered surface; Wintertime it is shallow and based at the surface, while in the summertime it is most frequently ~200 m above surface. • Temperatures do not exceed much beyond 0C in summer near the surface due to energy being used for latent heat release. • Sensible heat fluxes generally much larger than latent heat fluxes, especially in the winter, and is generally upward except at times during the summer. • Leads can can cause significant fluctuations in sensible and latent heat fluxes; These fluctuations can be predicted using by knowing the near-surface wind speeds and “fetch.” • The summertime ABL often consists of layered stratus clouds, for reasons not clearly understood, but related mostly to vertical velocity and radiative transfer.

16. Arctic Stratus Clouds (ASC) 1) Herman and Goody 1976 Cloud optical thickness Thick clouds will absorb enough radiation to cause evaporation in the middle Cloud depth 2) Tsay and Jayaweera 1984 Temperature profile close to saturated lapse rate inside cloud Warm, moist air aloft Cold surface temperatures