7) Turbulent transfer above the snow
Fluxes of sensible and latent heat above the snow. For a good introductory text, we recommend Terrestrial Hydrometeorology by W. James Shuttleworth. In particular, Chapter 2 on water vapor in the atmosphere, Ch 3 on vertical gradients in the atmosphere, and Chapters 15-19 on turbulence are helpful with this week's lessons. If you don't have access to the text book, you may be interested in the U. Hawaii online text on water vapor and on atmospheric stability and the Penn State online text on the boundary layer.
Background
Click on the photo above to hear Steve Oncley, from NCAR's Earth Observing laboratory, explain how the EC150 sonic anemometers work. This week we will be using data from these sensors to study turbulent transfer or heat, moisture, and momentum.
Michi Haugeneder of SLF in Switzerland is collaborating on our project and has worked on novel ways to visualize turbulence over snow. You can click here to watch a brief video of Michi explaining how we visualize turbulence with an IR camera and a sheet at the Kettle Ponds site.
Lab 7: Turbulence and the mass and energy balance
Download the lab and data files to your computer. Then, upload them to your JupyterHub following the instructions here.
- Lab 7-1: Calculating atmospheric stability, the Richardson Number, and plotting turbulence data
- All of the good stuff is in one lab this time, but Problem 2 will require info from prior labs and homework.
Homework 7
Problem 1: Do Stable Atmospheric Conditions Shut Down Turbulence?
Most of us have seen buoyancy-driven convective plumes when heating from below causes a fluid to rise, but snow is generally colder than the air above. When temperature increases with height, we say the atmosphere is stable. Lab 6-4 presents the concept of atmospheric stability and relevant variables using SAIL sounding data, and Lab 7-1 walks you through how to convert our temperature measurements into virtual potential temperatures, which correct for density adjustments due to changes in pressure with elevation and due to different moisture contents. With these corrections, we can assess the stability of the atmosphere above the snow surface. Pick three different periods of 3-5 days in our dataset. For each, assess the stability, the winds, the Richardson number, and the turbulent fluxes (TKE and sensible and latent heat fluxes, at two or more different heights). How are these related? What are the highest turbulent fluxes you observe during a period considered “strongly stable,” with a Richardson number greater than 0.2? (Hint: Histograms might be helpful here.)
Check out Eli Schwat’s github repo on altair plotting of our datasets. It makes some of the homework plots, particularly from Module 6, much more tidy.
Problem 2: Mass vs Energy Fluxes in Sublimation
Consider again, the major wind event of 22 December 2021 that moved a fair bit of snow around. Over the 24-hour period of measured blowing snow at 1 m (as documented by the FlowCapt sensor), calculate (a) the total mass (in grams/m^2) of snow that horizontally moved past the sensor, (b) the total mass (in grams/m^2) of water vapor that was transported vertically away from the snow over this time interval, (c) the total energy (in Joules, using the latent heat of sublimation) that went into converting snow to the amount of water vapor calculated in b, and (d) the total energy change per m^2 area, in the snowpack, calculated with the specific heats of ice and air, the density of the snow to represent fractional ice content, the depth over which a change was observed, and the observed temperature change (as measured from the thermistors in the snowpack at this time).