To get us kicked off, we’ll review several background concepts to build a foundation for everyone to be prepared for the rest of the course. All lecture materials will be recorded and provided electronically here in this section so students can take as much or as little time as needed to familiarize themselves with each topic. This section will wrap up with the first homework assignment of the course!
Overview of Boundary-Layer Processes
All slides now available as PDFs under files ESmith_Slides
This topic has been divided up into 4 sub-topics: wind and flow, turbulent transport, boundary layer structure, and the significance of the boundary layer. Each sub-topic has been treated as an isolated block to break up the content into more easily ingested material of shorter length.
Wind and Flow
Turbulent Transport
Boundary-Layer Structure
Significance of the Boundary Layer
Boundary Layer Thermodynamics
Intro to Boundary Layer Thermodynamics
Thermodynamic Quantities
Static Stability
Data Examples
Basics of Turbulence
Introduction to Turbulence
The Energy Cascade
The Equations of Motion
Note: I made a typo on the Reynold Decomposition slide (4:23): there should be two bars on the first term, not just one big one
Turbulence Closure
Similarity Theory
Additional Content:
Recently, a number of science content creators on YouTube have taken an interest in fluid dynamics, and due to that, there’s some really fascinating videos about turbulence out there now. I embedded a couple of the one’s I’ve enjoyed below. –TB
Basic Instrumentation Concepts
Introduction to Instruments
Circuitry
Extra Resources:
Sparkfun Tutorials & Examples
U Winnipeg Tutorials
U Winnipeg Examples (Links to an external site.)
Characterization & Calibration
Data Collection & Management Principles
Extra Resources:
EOL Readme Recommendations
Demery and Pipkin (2021)
In the Field by ADVANCEgeo & Carleton College
Women in Fieldwork* (AGS Panel Recording) – no captions provided on this recording file; please contact for transcription or other accessibility requests for this media offering
Surface Energy Balance
Radiation Balance
Diurnal Cycle and Deviations From It
This is a bonus material section provided in lieu of the course section we miss for Labor Day. This is all provided as extra, non-mandatory content!
In many of the above topics, the discussed concepts were considered in the context of the idealized quiescent boundary layer. In these conditions, the main drivers of boundary layer evolution are related to radiative flux as the surface of the earth warms and cools. The evolution of the boundary layer under these conditions has been described here as the diurnal cycle. Of course, the real world we live in isn’t always so simple or idealized. There are several conditions, phenomena, and regional impacts that can lead to deviations from the diurnal cycle as it was discussed here. We will briefly introduce some (NOT ALL!) of those here, leaving these topics as optional further reading for the curious reader. Many of this so-called deviation sources are entire fields of study themselves! Complex and evolving, these deviations are driven by physics on scales ranging from the most local (e.g., city-scale) to meso- and synoptic (e.g., hurricanes and mid-latitude patterns)!
Sea breezes
In coastal regions, a breeze commonly blows from the water in over the land (when the sea surface is colder than the adjacent land––or in other words, usually during the day). The opposite occurs at night when the land cools, and the slower-to-cool water’s surface is now warmer than the land. This land-to-sea breeze is commonly called a land breeze. The land-sea breeze circulation can disturb the diurnal cycle in several ways. It slowly penetrates further inland throughout the day, bringing cooler air with it. Additionally, as a circulation, it has an upward leg which can be associated with cloud formation and convection initiation. Cool air protrusion, clouds, and convection can all interrupt the diurnal cycle of the boundary layer.
The Thermally Forced Circulation I: Sea-Breeze module from Comet is available for additional learning. Additional reading from the literature is also provided below. By the way, there is a partially on-going campaign including the OU-BLISS team that focuses on some sea breeze topics in Houston called TRACER ! Delayed by COVID, part of the project is going on right now (CLAMPS1 is deployed!) , while most assets including CLAMPS and some UAS platforms will head down next summer!
Readings:
Augustin et al. (2020)
Li et al. (2020)
Perez and Silva-Dias (2017)
Clouds
As you may recall from introductory courses, radiation classes, or even physics, there is a basic relationship between the surface and the radiative properties of the atmospheric column above it. Of course this would include clouds. As you can imagine, the presence of and change in clouds can dramatically impact the diurnal cycle of the boundary layer.
Depending on the cloud type and cloud coverage, clouds can act to warm or cool the boundary layer. They can prevent or delay the morning transition or they can maintain a warm enough boundary layer to sustain some amount of buoyant mixing. This is because clouds can reflect incoming solar (shortwave radiation) and trap outgoing surface (longwave) radiation. A very dramatic example of clouds impacting the boundary layer is fog, which is simply cloud at the surface. These effects of clouds, and their impacts on the surface and radiative balance, are a huge source of uncertainly in global climate models today.
Research on anvil Shading impacting storm environments
Fronts & Airmass Change
Underlying many of the assumptions that go into the idealized boundary layer that best supports the diurnal cycle we’ve primarily discussed is horizontal homogeneity. In ‘real-world’ terms we can in some ways achieve this assumption by not considering the effects of advection or transport of atmospheric characteristic overtime. Of course we know that isn’t true. Any one of us that has every fielded questions about being a meteorologist has surely been asked about being on TV and those darn fronts. They are, after all, one of the most ubiquitous symbols and features of meteorology. Fronts come along and bring with them drastic airmass changes at all times of the day or night. These can be obviously very disruptive to the diurnal cycle.
This meteorgram shows a dramatic cold front example from 2003 in Mangum, OK. Imagine what that would do to your diurnal cycle! You can read the relevant mesonet ticker here .
Advection across any boundary that brings change to the airmass can modify the diurnal cycle from the ideal examples we’ve discussed previously.
Reading:
Pal et al. (2021)
Storms
When impacted by storms, the boundary layer takes on new characteristics. Precipitation, clouds, local circulation, re-distribution of pressure balance, new boundaries, and more can modify the boundary layer so much that it has nearly no characteristics of its former self.
On large scales, storms can become the driving force of the boundary layer all together. For example, the hurricane boundary layer is a specific type of boundary layer that is an active sub-field of study in meteorology. Understanding how the lowest portions of the hurricane-adjacent atmosphere interacts with the ocean surface is critical to understanding (and thus modeling) hurricane processes and evolution!
Check out Ch. 6 of Comet’s Tropical Meteorology Textbook
Research Reading: Smith and Vogl (2008)
On local scales, storms can introduce effects on a large range on magnitudes. Here, I’ll show an example of CLAMPS2 Doppler lidar vertical velocity observations from 6-7 July 2021 at Kessler Farm (near Washington, OK). These data were collected in clear air on a day and into an evening which would be expected to be ‘textbook’ diurnal. They were meant to sample a simple early evening transition period (see, for example, Bonin et al. 2013 ). However, at about 20 UTC, small, slow-moving thunderstorms initiated to the southeast of the observation point. The storms were fairly short lived and did not directly impact the observation location.
After observing the expected convectively mixed CBL with fair weather cumulus overhead, the early evening transition period began. Prior to sunset, turbulence begins to decay as the angle of solar incidence reduces the total solar heating received at the surface. There is still discussion in the literature about the processes that precisely contribute during this period, and it remains a poorly understood period (see Bonin et al. 2013 cited above). At about 2330 UTC, a weak outflow from a distant dissipating convective cell reached the observation location. This outflow interrupted the diurnal cycle and the ongoing evening transition, and ascent was observed instead. Reinvigorated PBL turbulent mixing was observed for over an hour before the nocturnal boundary layer really formed. Even then, ongoing periodic wave-like features were observed in the upper PBL. Intermittent turbulence activity appeared near the surface overnight as well. All of these disruptions were related to a relatively weak outflow from a small, dissipating storm. (P.S. These data and other cases like them are available online! You can find CLAMPS data on the THREDDS data server that NSSL maintains at https://data.nssl.noaa.gov/thredds/catalog/FRDD/CLAMPS/campaigns/catalog.html. This case was from the BLISSFUL campaign!)
After mastering the concepts covered in this section, you should be ready to tackle homework 1! This first homework assignment will be the lightest in terms of coding out of the assignments given in this course. The purpose here is to really make sure you are on a solid basic boundary-layer foundation before we jump into observation topics. These topics are provided in on-demand formats so you can go at your own pace to make sure you fully understand the ideas and concepts covered in these sections!