Coastally Trapped Wind Reversals

Module Goal and Objectives

Module Goal

The goal of this training module is to help you increase your understanding of how and why coastally trapped wind reversals (CTWRs) occur. Such understanding, in turn, can help you more efficiently and accurately evaluate the synoptic and mesoscale conditions that might result in a CTWR.

Performance Objectives

At the end of the module you should be able to do the following things:

With regard to characteristics of CTWRs

  • Describe how pressure, temperature, and wind change with passage of a coastally trapped wind reversal (CTWR)
  • Recall how quickly CTWRs propagate up the U.S. West Coast
  • Recall why SLP rises after passage of a CTWR
  • Locate areas likely to experience CTWRs on a physical map of the world
  • Recall the frequency of CTWRs along the California coast
  • Explain why CTWRs are best explained as a Kelvin wave, rather than a gravity wave

With regard to the structure of CTWRs

  • Describe how the MBL changes with passage of a CTWR
  • Recognize how a cross-coast profile of the MBL changes during a CTWR
  • Recognize a CTWR on a wind profiler record
  • Recall the height at which wind first reverses direction as a CTWR propagates
  • Recall the association of stratus formation with CTWRs

With regard to the synoptic evolution of CTWRs

  • Describe how MSLP, 850mb heights, and 500mb heights depart from climatologic norms during a CTWR
  • Describe how changes in MSLP and 850mb pressure force low-level offshore winds, and how this affects sensible weather along the coast
  • Describe how variations in MSLP affect along-shore pressure gradients

With regard to the mesoscale evolution of CTWRs

  • Recall how the synoptic setup forces the mesoscale offshore low
  • Recall how the offshore low moves during a CTWR
  • Describe how coastal mountains force ageostrophic flow
  • Recall how coastal mountains contribute to warming of offshore winds
  • Describe how and why a mesoscale high forms along the coast
  • Recall the factors that cause northward propagation of the CTWR

With regard to forecasting CTWRs

  • Recall the 3 best synoptic clues for forecasting a CTWR
  • Recall where the offshore low forms with respect to the low-level offshore flow
  • Recall where the stratus surge initiates with respect to the offshore low
  • Describe the use and limitations of mesoscale NWP models in predicting CTWRs