SHW Ch. 7 - The Development of High and Low Pressure Systems
CONVERGENCE: always associated with increasing surface pressure, since the mass per unit area (i.e., weight) of the air column is increasing with time.
DIVERGENCE: always associated with decreasing surface pressure, since the mass per unit area (i.e., weight) of the air column is decreasing with time.
CONFLUENCE ZONE: places where something is coming together (e.g., height contours on a constant pressure chart); mass increase or decrease not a factor
DIFLUENCE ZONE: places where something is moving apart (e.g., height contours on a constant pressure chart); mass increase or decrease not a factor
Note: on a constant pressure chart, one can have confluence (difluence), without having convergence (divergence), and vice versa; one does not imply the other. (EZ-Pass Toll Booth analogy.)
Curved Wind Flow
GEOSTROPHIC FLOW: air flows parallel to height contours on an upper-level chart
GRADIENT WIND BALANCE: actually an "imbalance" between the pressure gradient force (PGF) and the Coriolis Force (CF); as air moves CCW into a curved flow pattern around a low pressure center, it will accelerate around the low at speeds less than the geostrophic wind flow (GWF); as air moves CW into a curved flow pattern around a high pressure center, it will accelerate around the high at speeds greater than the geostrophic wind flow (GWF). Figure 7.3 & 7.4
On a trough/ridge upper-level flow pattern in which the height contours are uniform (i.e., the magnitude of the PGF and GWF remain constant), the wind will be sub-geostrophic going through the base of the trough and super-geostrophic going through the crest of the ridge. Figure 7.5
The acceleration of air from the base of the trough to the crest of the ridge creates an area of upper-level divergence in-between; likewise, the deceleration of air from the crest of the ridge to the base of the trough creates an area of upper-level convergence in-between.
Surface low pressure will be found directly beneath the area of maximum upper-level divergence
Surface high pressure will be found directly beneath the area of maximum upper-level convergence
Jetstreaks
JETSTREAK: a core of extremely high winds within the jet stream caused by locally strong pressure gradients; usually found on the upper-level constant pressure charts (300 mb, 250 mb, 200mb).
As a parcel of air enters a jetstreak, it accelerates and for a moment the PGF turns the parcel northward toward the low pressure across the height contours, until the (slower acting) CF eventually regains a balance; this creates CONVERGENCE at the LEFT ENTRANCE REGION of the jetstreak and DIVERGENCE at the RIGHT ENTRANCE REGION of the jetstreak; Figure 7.6
As a parcel of air exits a jetstreak, it decelerates and for a moment the (slower acting) CF turns the parcel southward away from the low pressure back across the height contours, until the CF eventually regains a balance; this creates DIVERGENCE at the LEFT EXIT REGION of the jetstreak and CONVERGENCE at the RIGHT EXIT REGION of the jetstreak; Figure 7.6
(SHW Ch. 7 - The Development of High and Low Pressure Systems Continued)
Combined Effects of Curvature & Jetstreaks
When a jetsreak moves through the base of a trough, the MAXIMUM CONVERGENCE occurs in the LEFT ENTRANCE REGION due to the curvature and jetstreaks effects superimposing, north of the jet stream axis. Figure 7.7
When a jetsreak moves through the base of a trough, the MAXIMUM DIVERGENCE occurs in the LEFT EXIT REGION due to the curvature and jetstreaks effects superimposing, again north of the jet stream axis. Figure 7.7
The effects due to curvature and jetstreaks are opposing each other in the RIGHT ENTRANCE and LEFT EXIT REGIONS, south of the jet stream axis and tend to "cancel" out.
The strongest UPPER-LEVEL DIVERGENCE occurs on the NORTHEAST SIDE of the trough in the LEFT EXIT REGION of the jetstreak; strong surface cyclones develop under this region. Figure 7.8
The strongest UPPER-LEVEL CONVERGENCE occurs on the NORTHWEST SIDE of the trough in the LEFT ENTRANCE REGION of the jetstreak. high pressure is generally found under this region. Figure 7.8
MULTIPLE JETSTREAKS: when jetstreaks from the polar jetstream (PJ) and sub-tropical jetstream (SJ) interact, the divergent areas of both jetstreaks can superimpose (right entrance region on the northern PJ jetstreak and the left exit region of the southern SJ jetstreak); this can develop a strong low pressure at the surface between the two jetstreaks. Focus 7.1
Friction Layer
Friction leads to a deflection to the LEFT of the GEOSTROPHIC wind direction.
Friction turns the wind, such that the flow has a component from higher pressure toward lower pressure, turning inward and spiraling into the center of the low. Figure 7.9 A & B
Over land, the frictional turning ranges from 20 to 40 degrees of wind direction.
Over water, the friction turning is less, usually from 10 to 20 degrees of wind direction.
Friction ALWAYS contributes to the weakening of both surface high and low pressure systems.
Effects of Heating and Cooling
ADIABATIC: a process with no exchange of heat with parcel and environment.
DIABATIC: a process that does involve a transfer of heat energy.
Applying HEAT to the center of a column of air (e.g., latent heat release due to condensation) will cause the pressure surfaces to rise (i.e., increasing the THICKNESS of the column); this creates high pressure aloft and low pressure at the surface. Figure 7.11 A,B,C
COOLING the center of a column of air (e.g., nocturnal radiational cooling) will cause the pressure surfaces to lower (i.e., decreasing the THICKNESS of the column); this creates low pressure aloft and high pressure at the surface. Figure 7.11 A,D,E
Development of High and Low Pressure Centers
DYNAMIC: curvature and jetstreak effects associated with force imbalances
THERMODYNAMIC: heating and cooling effects
UPPER-LEVEL DIVERGENCE: associated with changes in the curved flow & jesters (favored on the east side of a trough, north of the jetstream axis) and heating through latent heat release to the NE of low center. Figure 7.12 & 7.13