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