Mars Regional or Global Ray Tracer (MARGRAT)

margrat_thumb The Mars Regional or Global Ray Tracer (MARGRAT) is a highly successful model for simulating non-hydrostatic gravity waves (GWs) propagating through a neutral Martian atmosphere. Although these GWs have been observed extensively in the Martian atmosphere, adequate theoretical explanations do not yet exist to explain their occurrence, and calls attention to the need to improve models of the Mars atmosphere, and specifically the GW parameterization in atmospheric general circulation models (GCMs). In an effort to remedy this situation, CPI scientists at the Naval Research Laboratory (NRL) are working closely with NRL researchers to develop and build MARGRAT to model gravity wave propagation within the Mars atmosphere. MARGRAT is born from the well-tested Gravity Wave Regional or Global Ray Tracer (GROGRAT), a model developed for GWs in the Earth's atmosphere.

Being 95% CO2 by mixing ratio, the thermal balance of the martian atmosphere is driven to a large degree by infrared (IR) radiative transfer (RT) among CO2 bands, particularly 15 μm cooling and heating by absorption of solar radiation in near-IR bands. The advent of satellite remote sensing of the martian atmosphere has spurred development of detailed atmospheric radiative-transfer (RT) codes, to improve both the retrieval of atmospheric parameters from passive IR remote sensors, as well as the parameterizations of radiative heating and cooling rates embedded within martian general circulation models. These models have highlighted the importance of departures from local thermodynamic equilibrium (LTE) in CO2 IR RT that significantly modify net heating and cooling rates above ~80 km altitude. An entirely new radiative damping for gravity waves has therefore been parameterized and implemented in MARGRAT to account for these processes. Wave amplitudes are calculated by solving a wave-action equation in ray-tracing form. Wave breaking is specified using a linear saturation criterion on either vertical or slanting dynamic and static GW-induced instabilities. Scale-dependent radiative damping is applied using climatological profiles of the CO2 mixing ratio and the temperature distribution. The turbulent viscous damping is implemented based on a mean fourth-order Runge-Kutta algorithm with an adaptive (internally determined) time step based on prescribed levels of numerical accuracy. MARGRAT contains comprehensive, accurate physical parameterizations for modeling GW evolution globally from the surface up to 150 km altitude.

The vertical profiles used for the martian atmosphere are shown in Figure 1. Figures 2 through 4 show the solutions for a wave of c = 12.9 m s-1, which yields a source-level vertical wavelength λz = 10 km. This wave grows in amplitude with height and attains a breaking amplitude at ~60 km, whereupon it continues breaking as a saturated wave up to ~100 km before being rapidly dissipated by molecular viscosity thereafter. All three damping mechanisms (IR radiative cooling, molecular viscosity, and wave breaking) play a role in progressively depositing this wave's momentum flux throughout the atmospheric column. To assess the relative contributions, the red curve in Figure 2 plots the cumulative percentage of the wave's original momentum flux that has been deposited into the background flow for the simulation in which all three damping processes were activated (denoted Ptot). It shows that essentially 100% of the wave's momentum flux has been dissipated by the time the wave has propagated to z ~100 km.

Figure 1. Vertical profiles of (a) pressure (Pa), (b) temperature (K), (c) CO2 volume mixing ratio, (d) density (kg m-3), (e) O3P volume mixing ratio, and (f) O3P/CO2 volume mixing ratio. Thick curves are the means of individual profiles at 40°N from the Mars Climate Database (MCD) for average levels of dust loading and solar activity. Thin curves are minimum and maximum profiles, and gray shading in panels b, c, e and f depicts the distribution density of the individual MCD profiles.

Figure 2. Profiles of peak vertical displacement amplitude for a gravity wave of c = 12.9 m s-1.

Figure 3. Profiles of vertical flux of horizontal momentum for a gravity wave of c = 12.9 m s-1.

Figure 4. Profiles of the cumulative deposition of momentum flux Ptot, expressed as a percentage of the original source-level flux, for the simulation with all three dissipation processes activated (red curves in Figures 2 and 3). Remaining curves show the contributions to this cumulative flux deposition percentage from breaking/saturation (Ps, gold solid curve), IR radiative damping (Pr, black dashed curve), molecular viscous damping (Pv, blue dotted curve), and combined molecular viscous and IR radiative damping (Pv + Pr, cyan solid curve).

References

Eckermann, S. D., J. Ma and X. Zhu (2010), Scale-dependent infrared radiative damping rates on Mars and their role in the deposition of gravity-wave momentum flux, Icarus, 211(1), January 2011, 429-442, doi:10.1016/j.icarus.2010.10.029.