BOUNDARY LAYER CONVECTION
Daytime solar
heating of the ground:


Advection of cold
air over a warm surface:


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 88Ds:
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:

Open and Closed Cell Mesoscale Cellular Convection Here is a satellite perspective: Q: Is this an example of open or closedcell convection? ANSWER

Figure from Houze 1993 Copyright AP 
Open Cell Mesoscale
Cellular Convection


Closed Cell Mesoscale Cellular 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 closedcell 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)^{0.5}Z_{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 13 m/s. Alongline 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 alongline periodicities While the mechanisms that generate the HCR alongline periodicities are not well understood, there are a few theories out there. Two of these involve 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 
KHWaves

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
So you ask, what is the MoninObukhov 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 1200 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:


MID DAY:


MID AFTERNOON:

OK, let's put this all together.....
Observations suggest that:



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