BOUNDARY LAYER CONVECTION

We now know that turbulence can be created convectively or dynamically. Regardless of the generation mechanism, boundary layer turbulence transports heat, moisture and momentum through the boundary layer.
Let's now look at observations of convectively driven turbulence in detail:
BL convection is generated through two common scenarios:

Daytime solar heating of the ground: convective instability is realized as heat is conducted from the hot ground to the air within the microlayer
Advection of cold air over a warm surface: Lake-effect snows cold-air outbreaks

BL convection often produces clouds, but not always.
When clouds are produced, the "structure, or organization" of the convection often becomes apparent:
Check out the example to the right of a cold-air outbreak over the Greenland Ice Sheet ->
Q: What type of convective structures do you see? ANSWER
The "linear" cloud bands are often referred to as Horizontal Convective Rolls (HCRs)

Figure from Houze 1993 Copyright AP

HCRs

HCRs are essentially horizontal helices of air flow usually oriented near parallel to the mean ambient flow direction in the boundary layer

The cloud lines are referred to as "cloud streets"

Figure from Houze 1993 Copyright AP

Here is an aircraft perspective of cloud streets

Figure from Houze 1993 Copyright AP

They can also be seen in "clear air radar data" with the 88-Ds:

HCRs generally form in an environment that is convectively unstable and has some amount of vertical wind shear. We'll discuss this in more detail later in this section.

Open and Closed Cell Mesoscale Cellular Convection

Both open and closed cell convection occur in weaker shear environments than what supports HCRs

The conditions that favor open over closed cell convection and vice versa are not well understood

Typical aspect ratios (the ratio of their horizontal scale [X] to their vertical scale [Z]) for closed and open cell convection are:

Closed Cell: X/Z = 30/1

Open Cell: X/Z = 15/1

Open and Closed Cell Mesoscale Cellular Convection

Here is a satellite perspective:

Q: Is this an example of open or closed-cell convection? ANSWER

Figure from Houze 1993 Copyright AP

Open Cell Mesoscale Cellular Convection

Here is a close-up view of open cell convection from a satellite perspective
Note the hexagonal structure of the convection. Such a structure is reproduced in laboratory experiments of convection

Closed Cell Mesoscale Cellular Convection

Here is a close-up view of closed cell convection from a satellite perspective
Again, note the hexagonal structure of the convection.

Open and Closed Cell Mesoscale Cellular Convection

Here is a radar perspective: What types of convective structures do you see in this imagery?

Open and Closed Cell Mesoscale Cellular Convection

Here is a climatology of where open and closed-cell convection commonly occurs

Figure from Houze 1993 Copyright AP

More on Horizontal Convective Rolls

Typical aspect ratio is 3:1

However, it can vary from 3:1 to 10:1

Theory predicts roll spacing (l) = 2(2)^0.5Z_i where Z_i is the boundary layer depth.

So, for a depth of 1.5 km, l = 4.2 km

Typical updraft strength is 1-3 m/s.

Along-line periodicities are often observed along HCRs...., why??

Figure adopted from Houze 1993 Copyright AP

Examples of periodic cloud development are shown above in a cloud photo and to the right in satellite imagery.

The "periodic" nature of the cloud field is often referred to as "pearls on a string"

Figures from Wakimoto and Atkins 94 MWR Copyright AMS

Causes for the HCR along-line periodicities

While the mechanisms that generate the HCR along-line periodicities are not well understood, there are a few theories out there. Two of these involve waves:

Gravity Waves

Kelvin-Helmholtz Waves

Let's discuss each in detail

Gravity Waves

Gravity waves are essentially a buoyancy oscillation. They are unable to transport mass, though they are effective and very important for transporting energy.

Gravity waves are generated by a number of different mechanisms.

In the context of the boundary layer and their potential role in modulating HCRs, they are generated as the HCR updrafts perturb the stable layer of air within the entrainment layer.

Gravity waves are also observed in satellite imagery as cloud bands

The gravity wave frequency is expressed as:

As the static stability of the entrainment layer increases, the gravity wave frequency ANSWER and the associated gravity wave wavelength ANSWER

Thus, where the HCR and gravity wave updrafts are both positive is where clouds will form.

image adopted from Christian 87

KH-Waves

KH waves have also been thought to create the along roll periodicities.
Recall the sufficient condition for KH instability:
Ri < 0.25 where,
The waves are likely to be generated at the top of the mixed layer where the vertical wind shear is larger.
Similar to gravity waves, cloud development will occur where the KH wave updrafts are superimposed on the HCR updrafts.

image from Houze 93 Copyright AP

Diurnal Evolution of Boundary Layer Convective Structures

As you can imagine, boundary layer convection, whether it's in the form of HCRs, open cells, closed cells, or some other kind of disorganized convection, occurs relatively frequently in the planetary boundary layer somewhere in the country on any given day.

Q: Is there a "typical" evolution of the boundary layer convective mode during a "typical" day? For example, should one expect HCRs in the morning, open cells in the afternoon, etc????

Data collected in Florida, Illinois and Kansas suggests that there is a preferred evolution of boundary layer convective structures. Let's explore this in more detail.

Diurnal Evolution of Boundary Layer Convective Structures

Here are two examples.

Based on this data, how would you characterize the evolution of boundary layer convective structures for each of these two days????

Some background Information

It is generally accepted that HCRs form in environments with some amount of convective instability (vertical heat flux at the surface) and wind shear (in the boundary layer).
- The vertical heat flux generates the convectively driven turbulence
- The wind shear organizes the convection into linear bands
However, one is not guaranteed HCRs in an environment that contains both convective instability and wind shear.
For example, open and closed cell convection tend to occur in environments that are convectively unstable and have weak wind shear.
There are two parameters that appear to be important for the formation and subsequent evolution of HCRs. They are:
- the magnitude of the sensible heat flux from the surface
- the ratio of -z_i/L where L is the Monin-Obukhov Length

So you ask, what is the Monin-Obukhov Length???? Mathematically, it is defined as:

where k is the von Karman constant, g is the acceleration due to gravity. All other variables have been previously defined.

OK, so what does this equation physically mean?

Let's tackle this in parts.

First, what is the physical interpretation of the numerator? ANSWER
Now how about the denominator? ANSWER

Physically, L represents the height above which convectively driven turbulence dominates over mechanically driven turbulence. It generally varies from 1-200 m (notice that this is in the surface layer).
L becomes smaller as the vertical heat flux becomes larger during the day. Hence L is indirectly a measure of the convective instability generated by the vertical heat flux through the surface layer.
Thus, L is only meaningful in daytime convectively driven boundary layers, specifically within the surface layer.

OK, so let's now look at the daytime evolution of the two quantities:

vertical heat flux

-Z_i/L

MID MORNING:

Vertical heat flux is small
Monin Obukhov Length is large
Z_i is small
Therefore -Z_i/L is small

MID DAY:

Vertical heat flux is moderate
Monin Obukhov Length is moderately large
Z_i is moderately large
Therefore -Z_i/L is moderate

MID AFTERNOON:

Vertical heat flux is large
Monin Obukhov Length is small
Z_i is large
Therefore -Z_i/L is large

OK, let's put this all together.....

Observations suggest that:

As surface heating strengthens during the mid morning hours, HCRs are the first form of boundary layer convection that is observed, and only when the vertical heat flux reaches a critical value of 35-50 Wm^-2. The value of -Z_i/L increases during the early-mid morning hours

Close to mid day, the HCRs loose their identity and evolve into open-cellular convection. This occurs when the value of -Z_i/L exceeds a value of about 25.
Open-cellular or disorganized convection dominates for the remainder of the afternoon as -Z_i/L continues to become larger.

Questions about Boundary Layer Convective Structures and their daytime evolution??????