Description of the Global Scale Wave Model

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Major Features

The Global-Scale Wave Model (GSWM) performs numerical experiments on the global influences of solar-forced diurnal and semidiurnal tides, as well as non-tidal planetary waves. The waves are solutions of simultaneous coupled linear differential equations. This approach offers the ability to solve piecewise extensions of the classical tidal solution, adding realistic forcing from ozone (absorption of UV radiation) and water vapor (absorption of IR radiation), a background climatology of actual measured mean zonal winds and ozone concentrations, and parametrizations of energy dissipation phenomena. The linearized equations make experimentation with different combinations of these influences straightforward and illustrative.

The FORTRAN model currently runs on NCAR's Cray J90 computer, and provides an ASCII format output file.

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Background Climatology

A standard model run uses a background wind composed of measurements made by the HRDI instrument on UARS. The values are zonally averaged over 4 years, and do not vary with longitude. Below 12km the winds are those of the Groves/MSISE model. The background temperature and density are taken from the MSISE90 model using a solar flux of 120 and an Ap of 4.

These empirically-based mean background winds introduce significant asymmetries about the equator in both amplitude and phase in the solstice months.

Contour plots of the backgrounds that show the values of zonal mean temperature and zonal mean zonal winds for standard runs are available for each month from this web site. Other wind backgrounds commonly used for experimentation include composite wind fields which use measurements from the UARS HRDI instrument within a range of altitudes, mesospheric winds calculated by Portnyagin and Solov'eva (1992), pure geostrophic winds, and zero background winds.

Background ozone concentrations can also be chosen for each experiment. The standard configuration uses 7 year-averaged measurements from the HALOE instrument on the UARS satellite, enhanced with MLS measurements above about 50km. Other ozone backgrounds available come from the CIRA database, CLAES, and MLS. Plots of HALOE/MLS ozone used for standard model output are available here.

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Tidal Forcing

Forcing Regimes The GSWM uses several heat forcing models. The figure illustrates the location and mechanism of each major source of thermal excitation available in the GSWM.

In the standard configuration, the tropospheric IR heat forcing models (blue raindrops) are based on Groves (1982), and are available for 4 months (January, April, July, October), representing 4 seasons of the year. All standard model output is therefore restricted to these months.

The standard model uses latitude and altitude-dependent ozone concentrations to calculate ozone (UV) forcing (red striped ellipse) in the stratosphere.

Latent heating models (low pink cloud)(Forbes and Groves, 1987; Williams and Avery, 1996; Hong and Wang 1980) that simulate the transport of energy by meterological systems are a more significant source of forcing for non-migrating tides, and are not used in the standard migrating tide model. However, these models may be combined with the ozone (UV) and water vapor (IR) forcing models for migrating tides, as well.

Finally, a simplified Hough (1,1) tropospheric driver may be used for experiments that only approximate combined forcing to the first order.

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Tidal amplitudes generally increase with altitude as the density of the atmosphere decreases, however damping effects also become strong at higher altitudes. Gravity waves deposit momentum in the mesosphere and lower thermosphere as they break, which dominates the the tidal flow and "drags" it to the phase velocity of the gravity waves. This effect cancels the propagation of the diurnal (24 hour) migrating tide at the mesosphere (making it "effervescent", or "trapped"), so that diurnal winds are much weaker in the lower thermosphere.

Rayleigh Coefficients

The GSWM uses an effective Rayleigh friction coefficient in calculations of the diurnal tide to simulate this interaction (Miyahara and Forbes, 1991). The figure shows the coefficients under solstice conditions. The coefficients are reduced to 10% of these values for equinox conditions to represent weaker interation in spring/fall:

The turbulence caused by these breaking and unstable gravity waves further hinders the propagation of the tide by diffusing its momentum. This eddy diffusion is modeled in the GSWM by season-dependent coefficients of diffusivity that vary by latitude and altitude (Garcia and Solomon, 1985).

Eddy G&S

The GSWM also includes parametrizations of damping from heat dissipation (Newtonian Cooling), ion drag from the mechanical acceleration of ions through the Earth's magnetic field, latitude-dependent thermal conductivity, and molecular diffusion of momentum. The figure shows a sample of the Newtonian cooling and thermal conductivity parameters for January investigations.

Cooling and Conductivity

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Description of Model Output

The propagation of the tide is presented in an ASCII output table (both diurnal and semidiurnal) which contains the amplitude and phase of each component of the tide at gridpoints of latitude and altitude. The long (175kbyte )file lists the maximum amplitude and phase of the maximum for 4 tidal fields in this order:

Latitude, U (East Wind)/U phase, V (North Wind)/V phase, W (Upward Wind)/W phase, and T (Temperature)/T phase.

Latitude values range from 87 degrees (North) to -87 degrees (South), every 3 degrees, giving 59 measurements of each tide at each altitude. The values for each of the 4 fields is listed for each latitude at a given altitude, then this pattern is repeated for the next altitude. The output tables resemble a series of output 'blocks', one for each altitude.

The altitude of each block is indicated at the top of the block by the scale height index, X, and the corresponding geographic height, Z(km). Blocks are printed approximately every 4 km of altitude, beginning at 0km and continuing to 124km, making 31 blocks.

For example,

X= 7.6000 Z= 12.201
LAT= 87.0 U=.278E-02/ 8.5 V=.429E-02/ 5.3 W=.107E-04/ 14.2 T=.681E-03/ 11.1
LAT= 63.0 U=.189E-01/ 13.9 V=.188E-01/ 10.9 W=.111E-13/ 12.0 T=.192E-03/ 11.8
LAT= 60.0 U=.199E-01/ 14.0 V=.217E-01/ 11.0 W=.110E-12/ 12.0 T=.397E-03/ 12.0
LAT= 57.0 U=.243E-01/ 13.9 V=.247E-01/ 11.0 W=.435E-13/ 12.0 T=.165E-03/ 8.6
LAT=-87.0 U=.706E-02/ 6.4 V=.702E-02/ 10.4 W=.643E-05/ 6.8 T=.278E-03/ 15.7

X= 8.2000 Z= 16.218
LAT= 87.0 U=.188E-01/ 16.1 V=.120E-01/ 0.6 W=.140E-04/ 14.7 T=.115E-02/ 11.6

The format in pseudocode and FORTRAN to read each block is:

READ * Skip the line before the block
READ X,Z Read the altitude of this block
FOR LOOP 1 to 59 Read a Line for each of the Latitude values
READ(5x,f5.1,5x,e8.3,1x,f5.1,4x,e8.3,1x,f5.1,4x,e8.3,1x,f5.1, 4x,e8.3,1x,f5.1)
READ * Skip the line after the block

The variables read in for each latitude value are: Latitude, U_amplitude, U_phase, V_amplitude, V_phase, W_amp,W_phz, and T_amp,T_phz. The units of amplitude/phase are m/s for the wind, degrees Kelvin for temperature, and hours of phase.

There are 4 lines at the top of the file which identify the run, including the month, the period, and the wave number. A positive wave number in the GSWM convention indicates a westward propagating component.

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Contour Plots of the Background Atmosphere for the Standard Model Configuration

The background climatology is the same for diurnal and semidiurnal tides. These files present the background temperature and winds that are used in the standard configuration of the GSWM, on the GSWM background grid of 8 km altitude increments and 10 degree latitude increments.

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