| Development |
Conservation of Mass |
The development
of Walker-type circulation cells can be simplified into a two step process,
Step A - A warming atmospheric column and Step B - A cooling atmospheric column.
Step A - A warming atmospheric column (Figure 1)
First, a temperature maximum developes in the lower atmosphere. As the warm, bouyant air rises, it is replaced by cooler air converging into the column at lower levels. This results in a cyclonic heat low, whose secondary circulation is an upward vertical velocity (i.e. more warm air rising). As the warm air rises to higher altitudes, adiabtic cooling of the air parcel occurs, and the parcel's bouyancy is reduced. Upon reaching the tropopasue, which acts as a vertical boundry, the air diverges out of the column, resulting in anticyclonic motion aloft.
Step A - A warming atmospheric column (Figure 1)
First, a temperature maximum developes in the lower atmosphere. As the warm, bouyant air rises, it is replaced by cooler air converging into the column at lower levels. This results in a cyclonic heat low, whose secondary circulation is an upward vertical velocity (i.e. more warm air rising). As the warm air rises to higher altitudes, adiabtic cooling of the air parcel occurs, and the parcel's bouyancy is reduced. Upon reaching the tropopasue, which acts as a vertical boundry, the air diverges out of the column, resulting in anticyclonic motion aloft.
Figure 1. A skematic showing the key processes involved in setting up a warming atmospheric column, and the associated Walker-type dynamics.
Though the temperature maximum is often due to spatial variations in radiational heating, other process may be imporatant as well.
topography - the high altitude of mountains allows for radiational heating higher up in the atmmospheric column (~ 800mb)
land-sea heat gradiant - differences in the heat storage capacities between air and water make for strong thermal gradiants
moist processess- warm moist air that rises will release latent heat, further warming the column
Step B - A
cooling atmospheric column (Figure 2)
The air that diverges aloft out of a warming atmospheric column must have somewhere to go, and if mass is to be conserved there must be a process that cycles that air back to it's origin. Coupling of a warming atmospheric column with a cooling column completes this cycle. A cooling atmospheric column convergences aloft, drawing in warm, dry, air that has diverged aloft from the warming column. Upon converging into the cooling column, the warm air subsides. This subsidence helps to heat the column and stabalize the atmosphere, which will reduce radiational cooling to space. Near the surface, there is diverging flow out of the cooling column. To complete the cycle, the surface divergence out of the cooling column is transported and drawn into the surface convergence of the warming column.
The air that diverges aloft out of a warming atmospheric column must have somewhere to go, and if mass is to be conserved there must be a process that cycles that air back to it's origin. Coupling of a warming atmospheric column with a cooling column completes this cycle. A cooling atmospheric column convergences aloft, drawing in warm, dry, air that has diverged aloft from the warming column. Upon converging into the cooling column, the warm air subsides. This subsidence helps to heat the column and stabalize the atmosphere, which will reduce radiational cooling to space. Near the surface, there is diverging flow out of the cooling column. To complete the cycle, the surface divergence out of the cooling column is transported and drawn into the surface convergence of the warming column.
Figure 2. A
skematic showing the key processes involved in setting up a cooling atmospheric
column, and the associated Walker-type dynamics.
Cooling atmospheric columns are cold relative to warm columns. Often, they are subtropical regions of lesser radiational heating, but again note that the processes mentioned above as being important for setting up a temperature maximum are also important in creating a cooling column.
Example: The Sahara Desert, where moist processes do not exsist.
The Sahara recieves large amounts of radiational heating, but warm air parcels that rise from the surface do not release latent heat, and can only rise so far before adiabatically cooling and losing their bouancy. Thus, near the surface, hot turbulant air parcels are rising and falling, but above this, the atmospheric air column is radiatively cooling to space.
Conservation of Mass (Figure 3)
The closed nature of Walker-type circulations can be by analysed using laws of mass conservation. Following methods developed by Yano et al. 2002, the Walker-type system is closed if the charcteristic length scale, L, and vertical velocity, w, out of the warming column are consistant with that for the cooling column.
Figure 3. A skematic showing the closed nature of Walker-type circulations.
Vertical velocities associated with convection and rising air are much grater than those conected to subsidence. This implies that the length scale associated with warming atmospheric columns will be much grater than the length scale for regions of cooling columns. This also accounts for the difference in the vertical height of the atmospheric columns, requiring that warming columns be relatively tall and thin compared to the realtively short and fat cooling columns.
Walker-type circulations contribute to climate variations through thier distribution of rising and sinking air. While it is clear that local anomalies in the warming and cooling columns may effect local flood and drought conditions, because of the closed nature of Walker-type systems increased convection in one part of the globe can result in increased subsidence in another part of the globe.
Cooling atmospheric columns are cold relative to warm columns. Often, they are subtropical regions of lesser radiational heating, but again note that the processes mentioned above as being important for setting up a temperature maximum are also important in creating a cooling column.
Example: The Sahara Desert, where moist processes do not exsist.
The Sahara recieves large amounts of radiational heating, but warm air parcels that rise from the surface do not release latent heat, and can only rise so far before adiabatically cooling and losing their bouancy. Thus, near the surface, hot turbulant air parcels are rising and falling, but above this, the atmospheric air column is radiatively cooling to space.
Conservation of Mass (Figure 3)
The closed nature of Walker-type circulations can be by analysed using laws of mass conservation. Following methods developed by Yano et al. 2002, the Walker-type system is closed if the charcteristic length scale, L, and vertical velocity, w, out of the warming column are consistant with that for the cooling column.
Figure 3. A skematic showing the closed nature of Walker-type circulations.
Vertical velocities associated with convection and rising air are much grater than those conected to subsidence. This implies that the length scale associated with warming atmospheric columns will be much grater than the length scale for regions of cooling columns. This also accounts for the difference in the vertical height of the atmospheric columns, requiring that warming columns be relatively tall and thin compared to the realtively short and fat cooling columns.
Walker-type circulations contribute to climate variations through thier distribution of rising and sinking air. While it is clear that local anomalies in the warming and cooling columns may effect local flood and drought conditions, because of the closed nature of Walker-type systems increased convection in one part of the globe can result in increased subsidence in another part of the globe.
| Introduction |
Dynamics |
Examples |
Observations |
Summary |
References |