Microscale Phenomena: Planetary Boundary Layer

Definition

The Planetary Boundary Layer (PBL) can be defined as that part of the atmosphere that is directly influenced by the presence of the planet surface, and responds to surface forcings with a timescale of about an hour or less. Its structure changes during the course of the day, it being different from daytime to nighttime. Under fair weather conditions (light winds) PBL evolution follows Fig. 2.1.

Figure 2.1: Evolution of the PBL during the course of the day under fair weather conditions. Picture taken from Stull, R. An Introduction to Boundary Layer Meteorology (Springer,1988).

We describe next the PBL formed both during the daytime and the nighttime.

Daytime PBL


During the daytime, the PBL can be divided into three layers, namely: the Surface Layer (SL), the Convective Mixed Layer (CML), and the Entrainment Zone (EZ), as shown in Fig. 2.2. The SL, both day and night, and both under fair weather conditions or not, is defined as the region at the bottom of the PBL where turbulent fluxes and stress vary by less than 10% of their magnitude at surface.

The sharpest variations in meteorological magnitudes take place in this layer, and consequently, the most significant exchanges of momentum, heat, and mass. Under fair weather conditions during the daytime, the SL exhibits a superadiabatic lapse rate (potential temperature decreases with height), becoming an unstable layer from the local static stability point of view. In addition, there exists wind shear, for the wind is stopped by the ground.

Thus, both convection and shear rule its evolution. Its top can be regarded as that height where shear effects become negligible compared to convective effects, and turbulent fluxes vary by more than 10% of their magnitude at surface.

The CML, which only appears under fair weather conditions, is defined as that region of the convective PBL characterized by an intense convectivenature vertical mixing, which tends to leave variables such as potential temperature and humidity nearly constant with height, and even the wind speed and direction . Thus, the CML exhibits an adiabatic lapse rate (potential temperature constant with height). However, it is still unstable from the static stability point of view, for the SL is warmer, and therefore, a plume rising from the SL would have positive buoyant throughout the CML. The convective source acting in the bulk of the CML corresponds to the heat transfer from the warmer surface, whereas the wind shear is negligible in the CML. On Earth, it typically encompasses between 40-70 % of the PBL height. Its top zi (often also defined as h, as in Fig. 2.2) is usually defined as the level of most negative heat flux and it coincides with the middle of the EZ.

 

Figure 2.2: Planetary Boundary Layer structure during the daytime under fair weather conditions.

In general, not the CML but the more generic Mixed Layer (ML) can be formed as a consequence of: (i) convection, (ii) shear, or (iii) due to the simultaneous action of both mechanisms. The EZ, which is just found during the daytime and under faith weather conditions, is defined as that layer which separates the free atmosphere from the CML, and exhibits a subadiabatic lapse rate (potential temperature is locally an increasing function of height). Through this layer there is downward entrainment of free atmosphere air and upward thermal overshooting. It acts as a lid to the thermals rising through the CML, restraining the domain of turbulence.

Nightime PBL

The SL, attending to its definition, remains also at night. Yet at this time, as opposed to the daytime SL, it exhibits a strong static stability (therefore, a subadiabatic lapse rate). At night, the SL is inserted into a more general Stable Boundary Layer (SBL), defined as the region extending from the ground to the height where the subadiabatic lapse rate ends up becoming adiabatic, as shown in Fig. 2.3.

Figure 2.3: Idealized vertical profile of mean potential temperature in the nighttime PBL.


Across the whole SBL, there usually exists a balance between generation of turbulence by shear and damping by stability, being the net turbulence, sporadic and patchy. This causes the upper portion of the SBL to be sometimes decoupled from surface, and thus makes its study much more difficult to be carried.

Above the SBL, we can find the Residual Layer (RL), named this way because it is formed after the decay of the CML. For the radiative cooling is uniform throughout the RL, it holds the adiabatic lapse rate formed during the CML. This fact, together with the subadiabatic lapse rate existing in the SBL, makes the RL become a static stable layer.

Importance

The Mars Exploration Program Analysis Group (MEPAG) is a group chartered by NASA to assist them in planing the scientific exploration of Mars. MEPAG has just published in 2009 a white paper summarizing the most important Martian science goals. They are the next:

  1. Determine if life ever arose on Mars.
  2. Understand the processes of climate.
  3. Understand the evolution of surface and interior.
  4. Prepare for human exploration.

For all of them to be addressed, a comprehensive study of the Martian PBL (MPBL) is needed. In fact, a detailed characterization of the surface and near surface environment is required to determine the possible conditions for life at the Martian surface. In addition, phenomena with different time scales are interrelated as it happens on Earth.

That is, the thermodynamic of the MPBL affects mesoscale and synoptic phenomena, which, in turn, affect climatological phenomena. As a consequence, MPBL processes are important to understand the climate and its evolution. On the other hand, MPBL phenomena, such aeolian ones, extreme daily variations of ground temperature, etc, have a key role modifying the surface. Finally,the MPBL is the place in which current and future robotic and manned missions will operate. Therefore, a deep knowledge of its dynamical and thermal behavior should be gained for the design of both the spacecrafts and sensors.

In short, the MPBL plays an important role to gain insight into the four above mentioned goals expected to be covered by NASA, which supports its importance.