Gravity Wave Diagnosis Support

Rusty Pfost, SOO, NWSFO Jackson, MS, July, 1998

Gravity Wave Diagnosis Support

There are some excellent papers now in the literature concerning operational diagnosis and forecasting of large amplitude gravity waves. Much of this readme is taken from Dr. Steven Koch's article in Weather and Forecasting, June 1997, which I encourage everyone to read through.

The operational meteorological community often considers gravity waves too rare, too weak, or too difficult to forecast and diagnose. Therefore, we almost never makes an effort to look for conditions in which gravity waves can really be a factor in the forecast! The Gravity Wave notebook is made up expressly to change our thinking and to help us look for this particular animal.

Gravity waves are created by the restoring force of gravity acting on air parcels that are displaced vertically in a statically stable atmosphere. They are excited by a wide range of naturally occurring phenomena, including convection, density impulses, geostrophic adjustment associated with unbalanced jet streaks and strong frontal systems, topography, and vertical shear instability.

I like to think of gravity waves like throwing a pebble into a lake... the circular waves propagating outward are like atmospheric gravity waves. Most atmospheric gravity waves, if they do not propagate into a favorable environment, dissipate rapidly. Only when they encounter the proper conditions of wind, wind direction, and ducting will they become large enough to really affect our weather. Note also that gravity waves don't just propagate horizontally... they also propagate vertically and sideways and just about every which way! We have them all the time, we just don't know it and they aren't really important... unless conditions are just right.

FORECASTING CONSIDERATIONS

  • Requires fairly deep stable layer (>1 km), not necessarily surface based.
  • Requires conditionally unstable layer above it.
  • Large amplitude gravity wave favored by critical level in the conditionally unstable layer, where wind velocity and direction approximately equal the wave velocity with respect to the ground.
  • In the initiation region, small amplitude waves will move in all directions and most will dissipate. The large amplitude wave will be the one that moves in the direction of the winds in the conditionally unstable layer.
  • Velocity of the wave is the vector sum of the mean wind in the stable layer and the phase speed (in the preferred downstream direction), as determined by the depth and stability of the stable layer.
  • No wind at any level of the stable layer should match wave velocity with respect to the ground.
  • Presence of mesoscale precipitation or convective bands moving with the same velocity as the wave enhances the wave... wave/band relationship becomes synergistic (feeding off each other).
  • Surface winds and surface pressure fields are dramatically affected.
  • Can initiate convection.
  • Can intensify precipitation.
  • Can cause a changeover of precipitation (cooling of melting level via ascent).
  • Can dessicate cloud decks.

    The pressure fall type gravity wave is most common and apparently of largest amplitude. Large pressure falls typically occur just ahead of and/or just to the rear of a convective cloud and precipitation band. The convection is typically in an elevated unstable layer sitting atop the gravity wave duct.

    Operationally, how can we improve our "weather watch" for gravity wave situations? We can start by becoming familiar with synoptic patterns known to be favorable for the genesis of gravity waves:

    As the flowchart above suggests, Step One is to look for a diffluent trough upstream from the JAN CWA, a jet streak near the inflection axis, and a frontal boundary. A synoptic schematic is show below:

    If this synoptic pattern is diagnosed, Step Two is to mentally (or orally) declare a Wave Alert. Then, analysis of the mesoscale fields using the Lagrangian Rossby Number, the Nonlinear Balance Equation, model soundings, model wind forecasts, and the Duct Factor, will enable us to mark out a smaller area in which to watch for gravity waves. To make this easier, I have written several scripts that can be used with the meso-eta (or regular eta, if the meso is not available) in real time:



    STEPS TO USE EZ SCRIPTS TO DIAGNOSE FAVORABLE REGIONS FOR GRAVITY WAVES

  • Run the ezset script to set the time and date, and the model desired (mso for meso-eta, eta for regular eta).
  • To diagnose the Lagrangian Rossby Number, use ezrol and look for numbers greater than 0.5. Numbers greater than 0.5 show where ageostrophic considerations are strong enough to create unbalanced conditions. Here is a summertime example:

  • To diagnose the Nonlinear Balance Equation, use eznbe and look for numbers greater than 3 (x 10^-8 sec^-2). Numbers greater than 3 again will show where a strong ageostrophic component of the wind is creating unbalanced conditions favorable for gravity wave generation. Here is a summertime example:

  • To diagnose conditions favorable for ducting, use ezduct and look for numbers greater than about 15C. These numbers show where a fairly deep stable layer is overlaid by a conditionally unstable layer above. Here is a summertime example:

  • Also check model soundings and wind forecasts to see if the conditionally unstable layer contains a 'critical level', that is, a level where the speed and direction of the gravity wave matches the speed and direction of the wind in the conditionally unstable layer. Model soundings can be compared to the following idealized sounding favorable for ducting:



    Once an area has been outlined as favorable for gravity wave production, careful watch on the mesoscale pressure fields using ADAP or the RSAS data should be maintained. Watch for any depressions on the order of 1.5 to 3.0 mb over and above the normal diurnal variations. Shortly a pressure ridge will begin to develop ahead of the depression and amplify over time. Convection may develop, and may result in the creation of a succession of ridges and troughs propagating across the area.

    The meso-eta has the capability to forecast such gravity waves. A comparison of what is seen in the mesoscale surface analyses and the meso-eta forecasts can help give the forecaster a tremendous advantage in forecasting possible effects of the gravity wave on CWA weather.

    This next cool season, please remember to keep gravity waves in mind. The yellow gravity wave notebook will be on the forecaster's bookshelf, ready for use when needed.

    RLP