Winds

Gradients in pressure and temperature resulting from solar heating produce winds in an attempt to reduce these gradients. Atmospheric pressure and temperature gradients result from three major factors: seasonal changes, dust storms, and diurnal variations. The seasonal changes result from condensation and sublimation of CO2 and H2O from the polar caps. As noted above, dust storms can enhance the temperature gradient leading to stronger winds. The diurnal variations result from the temperature differences between the day and night sides of the planet as well as the presence of passing storm systems. Diurnal wind directions often change throughout the day (Figure 6.8). Examples of the winds triggered by these processes are Hadley circulation, thermal tides, and condensation flows.

If a planet's rotation axis lies perpendicular to the ecliptic plane, the equator receives more solar energy than other latitudes. Convection in the atmosphere causes warmer air to rise and flow toward regions with lower temperature and pressure. Thus the warm air rises over the equator, cools, and sinks back to the surface near the poles. The flow then returns to the equator along the surface. For slowly rotating or

Pressure Thermal Tides Mars Pathfinder

Viking 1 Pathfinder

Figure 6.8 Wind direction varied considerably throughout a martian sol at both the Mars Pathfinder and Viking 1 landing sites. Wind direction is the azimuth angle, measured from north (0°). Wind direction at the Pathfinder site typically changed from south-southwest winds in the morning to north-northeast in the afternoon. (MPF image SS009, NASA/JPL.)

Viking 1 Pathfinder

Figure 6.8 Wind direction varied considerably throughout a martian sol at both the Mars Pathfinder and Viking 1 landing sites. Wind direction is the azimuth angle, measured from north (0°). Wind direction at the Pathfinder site typically changed from south-southwest winds in the morning to north-northeast in the afternoon. (MPF image SS009, NASA/JPL.)

Equator

West

East

Figure 6.9 Differences in solar insolation with latitude combined with a planet's rotation produce Hadley cells which drive atmospheric circulation. Hadley cell circulation produces easterly winds (winds from east to west) within the equatorial zone and westerlies in the 30° to 60° latitude zone in each hemisphere. Arrows on the left show the vertical circulation within each latitude zone, with warm air rising over the equator and cooler air sinking near ±30° latitude.

West

Equator

East

Figure 6.9 Differences in solar insolation with latitude combined with a planet's rotation produce Hadley cells which drive atmospheric circulation. Hadley cell circulation produces easterly winds (winds from east to west) within the equatorial zone and westerlies in the 30° to 60° latitude zone in each hemisphere. Arrows on the left show the vertical circulation within each latitude zone, with warm air rising over the equator and cooler air sinking near ±30° latitude.

non-rotating planets, there are two convection cells, one in the northern hemisphere and one in the south. These convection cells are called Hadley cells.

Venus is an example of a planet with one Hadley cell per hemisphere. The faster rotation of Mars causes the north-south winds (meridional winds) to be deflected and the Hadley cells split into three Hadley cells per hemisphere due to conservation of angular momentum (Figure 6.9). This results in east-west (zonal) winds occurring along the surface, with easterlies (winds from east toward west) dominating near the equator and westerlies (winds from west toward east) occurring in the mid-latitude zones. The velocity gradient of these winds can be estimated from the thermal wind equation:

where H is the pressure scale height (Eq. 6.7) at altitude z, u is the zonal wind velocity, P is the pressure and R is the planet's radius. The angular velocity (O) of the atmosphere in the rotating frame of the planet depends on the latitude (f) of the atmospheric parcel because of the Coriolis force. The potential temperature, h, is given by:

Equation (6.36) predicts zonal wind velocities near 85 m s 1 near the top of the Hadley circulation and ~10 m s-1 near the surface (Read and Lewis, 2004).

The pattern displayed in Figure 6.9 only results when the planet's rotation axis is perpendicular to its orbital plane. Any tilt of the rotation axis causes the Hadley cell circulation to be displaced from the equator and produces seasonally changing weather patterns. The large eccentricity of Mars' orbit also leads to large time-averaged differences between the polar regions. Thus, while Mars' wind patterns are dominated by Hadley cell circulation, complications to this simplified view occur because of its rotation rate, obliquity, and orbital eccentricity as well as the presence of polar caps and thermal continents.

Mars' thin atmosphere is not very efficient at retaining the daytime heating, leading to dramatic temperature decreases at night. The large temperature difference between day and night hemispheres causes air flow from the warmer day side to the cooler night side. Winds produced by this temperature gradient are called thermal tides. Thermal tides are centered in the equatorial regions but extend to mid-latitudes. The thermal gradient and thus the efficiency of thermal tides can be estimated by comparing the solar heat input, Fin, with the heat capacity of the atmosphere. Solar heat input per day is related to the surface area exposed to solar radiation, the amount of the solar radiation absorbed, and the length of the day (td):

where R is the radius of the planet, A0 is the geometric albedo, Fsolar is the solar constant, and rAU is the distance of the planet from the Sun in astronomical units. The amount of heat, Q, necessary to raise the atmospheric temperature by some temperature change, AT, is related to the heat capacity of the atmosphere per unit area and the total area of the atmosphere:

where P0/g is the mass of the atmosphere per unit area of the surface. If all of the solar heat absorbed by the atmosphere is used to raise the atmospheric temperature by AT, then Fin = Q. The fractional increase in the temperature becomes

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