The Arctic Boundary Layer

The Arctic Boundary Layer

Introductory Remarks

Surface is comprised of:
- Land - Tundra (dominated by mosses and lichen)
- Sea:
  - Winter - ice/snow
  - Summer - sea ice
Diurnal variation is much less pronounced that mid-latitude continental boundary layers
In fact, quasi-steady boundary layers in the arctic are well documented.
Boundary layer depth (during summer) is often much shallower that observed for mid latitude continental regions
The arctic boundary layer is often capped by arctic stratus clouds (ASC).
- Why????
- ASC are quite persistent over the arctic during summer.
- Hence, they play an important role in the arctic boundary layer (ABL) structure and turbulent transport through the ABL.
Turbulent transport processes are inherently different in the ABL than the mid-latitude continental BL.
Finally, the ABL has recently been recognized as an important component of the observed and modeled warming at high latitudes.

Winter ABL

Dominated by large radiation inversion - stably stratified for sure!!
- little/no solar radiation
- snow-covered surface
Observed during fall, winter and spring
Are heat, moisture and momentum transported through this boundary layer?
- sure - but not by convective eddies!!
- There are two important flux transport mechanisms in this boundary layer:
  - Shearing-induced turbulence transporting heat, moisture and momentum up from the ice-air interface and down through the top of the inversion from the free troposphere
  - Penetration of a warm, moist updraft through the surface layer and boundary layer directly into the free troposphere. The updraft is generated by a recently opened lead.

Leads

So, you ask, What is a lead?
A lead is an area where the ice pack has shifted, exposing the warmer sea water to the cold air above.
The resulting air-water temperature difference can huge!!
Leads can be a significant source of heat and moisture that is transported through the arctic boundary layer.
Convective turbulent eddies transport this heat and moisture upward, generating an internal boundary layer that is shallow at the upwind side of the lead and becomes deeper as one progresses across the lead.
The internal boundary layer is a locally-generated boundary layer within the nocturnal boundary layer that exists within the immediate vicinity of the lead and is a result of the lead's existence.
Is this really any different than the Lake Effect phenomenon? Do you suppose that internal boundary layers exist over the Great Lakes?
The amount of heat and moisture that is transported upward is very much a function of the fetch across the lead:
In the figure to the right:
- N is the heat flux Nusselt number (for both sensible and latent heat)
- R is proportional to the fetch distance across the lead
So, what is the interpretation of this figure?

Summer ABL and Arctic Stratus Clouds (ASC)

During the summer season in the arctic, low-level stratiform clouds are a prevalent feature of the central Arctic.
- During May-September, as much as 70% of the Arctic can be covered with persistent stratus.
- This is followed by a rapid decrease during October to a value of less than 20%. They reappear in April/May.
- They can be formed through two processes:
  1. As cold polar air moves over a relatively warmer sea surface, convective instability is realized.
  2. warm moist air moves over a cold surface. Low-levels clouds are produced as the air cools and reaches saturation.
- They occur within the ABL and lower troposphere below 2000 m.
- They are usually 150-500 m thick
- They interact with both short and long wave radiation and hence exert an important influence on the heat balance of the surface ice pack and snow conditions.
So, what is the structure of the cloud-topped arctic boundary layer and what are the important transport processes through it? Let's look at a case study.

Case Study of an ABL topped by ASC

The following aircraft data were collected during the Marginal Ice Zone Experiment (MIZEX).

Mean boundary layer characteristics are shown below:

Figures 6 and 7 show vertical profiles of horizontal winds, mixing ratio, liquid water content, droplet concentration and droplet diameter. Questions:
- Where are the clouds relative to the ABL?
- Is there evidence of turbulence in the ABL?
- What do the microphysical data mean to you?

In Figure 9 variables related to turbulent kinetic energy are shown.

Notice:
- The maximum in TKE at the ground and again near the top of the ABL. What is responsible for these maxima?
- Compare the profiles of u^'2 and w^'2. What does this tell you about the magnitude and therefore importance of convectively driven turbulence in this boundary layer?
- Also notice that the vertical transport of horizontal momentum is negative and is maximum near the ground.

Now, let's look at the vertical profile of heat and moisture fluxes ->

Notice:
- The relatively small heat flux below cloud base. This is due to very weak convectively driven and mechanically produced eddies. The temperature difference between the ice/water and air above it was only about 1 K. So, even if w' is large, q' will be small and therefore, w'q' is small.
- Within the cloud, the heat flux increases to about 15 W m^-2. This is largely due to radiative cloud-top cooling. Hence, radiative cloud top cooling is an important process for the vertical transport of heat through this ABL.
- At the top of the boundary layer, the heat flux is negative. This is due to entrainment of relatively warmer air the the entrainment layer and free troposphere down into the cooler ABL.
- The moisture flux (panel c) is positive throughout the entire boundary layer.

References: