Fundamentals of Atmospheric Science

11.4 Frozen: The Taylor Hypothesis

We would like to be able to take snapshots of the eddies in three dimensions and measure all their sizes each instant. Unfortunately, we do not have a good way to do this. Instead, we can simply measure the fluctuations of a variable such as wind speed or specific humidity with a sensor at one location for a period of time. In this way, we watch the eddies drift by the sensor. But the eddies could be changing size and shape as they drift by. Let’s put this physical concept in the context of the total derivative.

Take a variable like temperature, T. We know that the change in T with time at any location is the sum of the local change within the air parcel (such as by solar heating) and the change due to advection of warmer or colder air over the location.

T t = DT Dt v · T MathType@MTEF@5@5@+=faaagCart1ev2aqaKnaaaaWenf2ys9wBH5garuavP1wzZbItLDhis9wBH5garmWu51MyVXgaruWqVvNCPvMCG4uz3bqee0evGueE0jxyaibaieYlNi=xH8yiVC0xbbL8FesqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbbG8FasPYRqj0=yi0dXdbba9pGe9xq=JbbG8A8frFve9Fve9Ff0dc9Gqpi0dmeaabaqaciGacaGaaeqabaWaaeaaeaaakeaaqaaaaaaaaaWdbmaalaaapaqaa8qacqGHciITieWacaWFubaapaqaa8qacqGHciITcaWF0baaaiabg2da9maalaaapaqaa8qacaWFebGaa8hvaaWdaeaapeGaa8hraiaa=rhaaaGaeyOeI0Iab8NDa8aagaWca8qacqWIpM+zcuGHhis0paGbaSaapeGaa8hvaaaa@4389@

Taylor’s hypothesis says that we can assume that the turbulent eddies are frozen as they advect past the sensor and thus the local change within each eddy is negligible:

DT Dt ~0 MathType@MTEF@5@5@+=faaagCart1ev2aqaKnaaaaWenf2ys9wBH5garuavP1wzZbItLDhis9wBH5garmWu51MyVXgaruWqVvNCPvMCG4uz3bqee0evGueE0jxyaibaieYlNi=xH8yiVC0xbbL8FesqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbbG8FasPYRqj0=yi0dXdbba9pGe9xq=JbbG8A8frFve9Fve9Ff0dc9Gqpi0dmeaabaqaciGacaGaaeqabaWaaeaaeaaakeaaqaaaaaaaaaWdbmaalaaapaqaaGqad8qacaWFebGaa8hvaaWdaeaapeGaa8hraiaa=rhaaaGaaiOFaiaaicdaaaa@384A@

so that:

T t = v · T MathType@MTEF@5@5@+=faaagCart1ev2aqaKnaaaaWenf2ys9wBH5garuavP1wzZbItLDhis9wBH5garmWu51MyVXgaruWqVvNCPvMCG4uz3bqee0evGueE0jxyaibaieYlNi=xH8yiVC0xbbL8FesqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbbG8FasPYRqj0=yi0dXdbba9pGe9xq=JbbG8A8frFve9Fve9Ff0dc9Gqpi0dmeaabaqaciGacaGaaeqabaWaaeaaeaaakeaaqaaaaaaaaaWdbmaalaaapaqaa8qacqGHciITieWacaWFubaapaqaa8qacqGHciITcaWF0baaaiabg2da9iabgkHiTiqa=zhapaGbaSaapeGaeS4JPFMafy4bIe9dayaalaWdbiaa=rfaaaa@3FE7@

Local temperature gradients, which might be present from one side of an eddy to another, are advected across the sensor by the mean wind without the eddy changing.

When is this condition valid? Experiments suggest that this hypothesis is valid when the variation of the wind speed due to turbulence is less than ½ of the mean wind speed.

We start this study with methods to separate wind motion driven by larger scale processes, such as gradient flow or geostrophic flow, from turbulence.

two men setting up anemometers in the field
Anemometers. A cup anemometer and wind vane are on the left. A sonic anemometer, which uses sound to measure vertical as well as horizontal winds more than ten times a second, is on the right. You may have seen sonic anemometers at state weather stations along highways. Sonic anemometers are so fast that they are great for measuring turbulence and turbulent transport.
Credit: Department of Energy Atmospheric Radiation Measurement Carbon Program

Quiz 11-1: Boundary layer behavior.

  1. Please note: there is no practice quiz for Quiz 11-1.
  2. When you feel you are ready, take Quiz 11-1 in Canvas. You will be allowed to take this quiz only once. Good luck!