Meteo 465/565 -- The Middle Atmosphere
Winds and Density
1. Tropospheric Winds.
At different latitudes,
What are the winds in the lower troposphere?
What
are the winds in the upper troposphere?
2. Zonal Mean Winds in the midlatitude stratosphere.
Winter hemisphere. The zonal mean winds in the midlatitude winter hemisphere are westerly, with a maximum velocity of 80 m/s at 65 km altitude and 40 degrees latitude.
Summer hemisphere. The zonal mean winds in the midlatitude summer hemisphere are easterly, with a maximum velocity of about 50 m/s at 65 km altitude and 40 degrees latitude.
In the Northern Hemisphere, zonal mean winds switch from westerly to easterly in May, starting at the highest latitudes and altitudes and moving downward toward the tropics. Winds switch from easterly to westerly in September, once again starting from high latitudes and altitudes.
Of course, just as in the troposphere, the actual winds have significant meridional components. Air parcels in midlatitudes typically traverse the globe in one to two weeks, depending on the location and the circumstances.
3. Zonal mean winds in the equatorial stratosphere.
The direction and speed of the zonal mean winds in the tropics is dominated by the Quasi Biennial Oscillation (QBO). The QBO, and thus the equatorial region, extends from approximately 10-15 degrees N and S, although the effects of the QBO are felt in the subtropics to 30 degrees latitude.
In the QBO, equatorial winds switch from easterly to westerly at high altitudes above 30 km, with the easterly and westerly phases descending downward in altitude, so that the easterly winds will at one point be above the westerly winds and the westerly winds will at one point be above the easterly winds.
The period of oscillation is abour 28 months, although the QBO period varies from about 20 months to 30 months.
The strongest easterly winds are about 30 m/s, while the strongest westerly winds are typically 20 m/s.
Westerlies
typically begin descent in June through August, although this is not a hard and
fast rule.
4. Mean meridional winds.
The meridional winds are
much weaker than the zonal winds. The typical transit time for air
parcels to travel from the tropics to the pole is many months in the lower
stratosphere, and longer at higher altitudes. Because tropospheric air
enters the stratosphere through the tropical tropopause and leaves the
stratosphere at high latitudes near and above 60 degrees, the average speed can
be determined from the mean age of the stratospheric air. For air
traveling through the lower stratosphere, the mean age is 3 to 4 years.
For air traveling to and through the upper stratosphere, the mean age is closer
to 6 years.
What
are the mean zonal stratospheric winds, roughly? 20 m s-1 == 1700 km/day == 10 days to circle
globe
What
are the mean meridional stratospheric winds? 3 years to go 6000 km == 0.1 m s-1.
What
are the mean vertical stratospheric winds? lower stratosphere: 5 km yr-1 == 2 x 10-4
m s-1
5. Atmospheric density.
Atmospheric density can be determined from the ideal gas law. We can use mass density, as you are accustomed to for meteorology, or we can use number density, which is just a counting of molecules in a unit volume. An important quantity for you to learn is Loschmit's number, which is the number of molecules in a cubic centimeter of air at standard temperature and pressure: 2.69 x 1019 molecules cm-3. Thus, the number density of molecules can be determined at any location if the pressure and temperature are known. Know this relationship will be very important when we get to chemistry.