Analyzing the entirety of the atmosphere is vital in studying weather, understanding synoptic systems, and ultimately, providing forecasts. However, when studying smaller mesoscale phenomena, the atmospheric boundary layer (ABL) is the way to go. The ABL is the layer of the atmosphere which is directly influenced by the surface of the earth through exchanges in energy, moisture, and turbulence. To give an idea of its size, the troposphere (where almost all weather occurs) in mid-latitude regions is on average 12 miles high, but the ABL is only about a tenth of that. Two subsets of the ABL, each with their own distinct processes and functions, will be investigated in depth to understand its importance in the weather world.
The Land Boundary Layer
The boundary layer over land, also known as the land boundary layer (LBL) is one that almost everyone sees and interacts with every day. This boundary layer forms by growth with the diurnal cycle. The sun heats up the ground which in turn destabilizes the lower part of the atmosphere, generating dry convection. This convection mixes the air vertically elevating the boundary layer as it does so. In addition to heightening the LBL, this creates low level turbulence.
The height of the boundary layer depends on many different weather processes that occur daily, not just convection on a sunny day. For example, fronts, deep moist convection, precipitation, temperature and moisture advection, amongst others can significantly impact the height of the LBL. For a typical summer sunny day, with no significant weather features in the area, heights of the LBL extend up about 1-2 km with extreme cases such as in desert environments up to 4-5 km.
How do we know where to spot the boundary layer on a typical day? There are a few ways to go about this. One visual way is to look at the sky. Fair weather cumulus clouds, if present, will typically give an estimate of the LBL’s elevation. Below these clouds lies the mixing of the air from convection and hence a turbulent environment of varying intensities. Above these clouds exists an inversion, essentially the best way to pick out the LBL depth.
Another method utilizes a vertical profile of the atmosphere. In the sounding below from North Platte Regional Airport on the 27th of June 2018 at 00Z, the boundary layer is shown to be around 1.2 km above ground level (AGL) (the surface elevation is about 850 m, meaning the boundary layer is from the surface to 1.2 km above this). The temperature around this height shows a distinct increase for the next few hundred meters while the moisture significantly drops off above this increase in temperature. What this physically signifies is the almost uniform air below the inversion because of the mixing from thermals, shown by the unvarying moisture throughout the LBL. The widely varying moisture above this level is a testament to the mixing not being present and hence the boundary layer ceasing to exist at and above, in this case, 1.2 km AGL.
The LBL and its processes are very important to the Plains and Midwest United States, especially during the nighttime hours. Essentially a nocturnal increase in winds develops around 850mb after the sun goes down and lasts into much of the overnight providing moisture, instability, and wind shear to support thunderstorms during the nighttime hours. More on this process and the physics behind it will be provided in a future article to further explain the LBL’s importance in the weather world.
The Marine Boundary Layer
Switching gears, the marine boundary layer (MBL) chiefly differs that of the land boundary layer in that it has direct contact with the ocean instead of the land, allowing for large amounts of heat and moisture to be exchanged. However, the MBL is harder to study than the LBL for a few reasons. First off, there is a lack of observations due to the difficulty of studying the atmosphere over the ocean. Even on an island, local influences from the land can affect soundings. Consequently, little is known about the MBL.
Whereas cumulus clouds can usually be seen above the LBL, stratocumulus clouds are more likely to be seen over the MBL. These flat, low-lying clouds are very important to the earth’s radiation budget, since these clouds essentially act as a white blanket over large swaths of the ocean. This large blanket has a very high albedo (high reflectivity), meaning that these clouds will globally cool the earth’s surface. See below for a satellite image of a swath of these clouds.
The stratocumulus clouds in this layer are formed top-down. An air parcel on the top of the cloud will become negatively buoyant and stable by the cooling of the stratocumulus cloud. This will cause the parcel to sink; therefore causing surrounding parcels to rise (imagine dropping a rock in a cup of water, the water level will rise to make room for the rock as it sinks to the bottom). This process in the MBL creates a well-mixed layer with lots of turbulent air (see here for a great article on turbulence). This turbulent air allows water vapor and heat to be transported up to the cloud, which the cloud needs to stay alive.
Note the difference in boundary layer formation from the LBL. To recap, the MBL develops via stability and sinking air (top-down process) whereas the LBL develops in pretty much the exact opposite way; destabilization and convection (bottom-up process) matures the LBL.
Another important phenomenon that occurs in the MBL is decoupling. Essentially, decoupling occurs when the air that sinks below the cloud base does not reach the ocean surface; this is usually due to intense sunlight. The warming of the sun allows the stable air to retain some buoyancy, and therefore does not completely sink to the bottom of the ocean surface. This will cease the turbulence halfway up the cloud layer, meaning there is little to no air movement. As a result, this disallows the warm water vapor at the ocean surface to reach the cloud surface. Due to this lack of water vapor reaching the cloud, the cloud therefore thins and evaporates. Therefore, this diurnal process of decoupling essentially dissipates the cloud. See below for a comparison of a well-mixed and decoupled MBL.
As a whole, the planetary boundary layer is one of the most important aspects of the climate system. It is in direct contact with either the land or ocean surface, making boundary layer research essential to more accurate forecasting. Many important processes occur in the boundary layer, such as convection, turbulence, and decoupling, making it a vital part of our atmosphere to study and understand.
To learn more about the boundary layer, both over land or water, stay tuned in to GWCC and be sure to click here!
©2018 Meteorologist Joseph DeLizio(Land Boundary Layer)
©2018 Meteorologist Joseph Fogarty (Marine Boundary Layer)