Figure 7.2.1 (p. 202)Precipitation formation. Water droplets in clouds are formed by nucleation of vapor on aerosols, then go through many condensation-evaporation cycles as they circulate in the cloud, until they aggregate into large enough drops to fall through the cloud base (from Marsh (1987)).

Horizontal Convergence

• Large-scale air mass convergence (typically occurs in Tropics [Inter Tropical Convergence Zone (ITCZ)]-- see atmos. circ. schematic

• Also occur in tropical cyclones (low pressure systems):

• High pressure systems have the opposite circulation (anti- cyclones) and downwelling motion

• Subtropical cyclones also form at higher latitudes

LPlan: Cross-section:

Frontal Convergence

• Convergence of air masses with different temperatures (temperature fronts); generally occur in midlatitudes

• “Front” is region of strong thermal contrast

• Cold front: colder air approaching warmer; labeled with pointed arrow indicating flow direction

• Warm front: warmer air approaching colder; labeled with rounded arrow indicating flow direction

• Fronts collide and deflect warmer air rises

Figure 7.2.2 (p. 203)Cyclonic storms in mid-latitude (from Masch (1984)).

How to Read a Surface MapSurface maps depict the large-scale elements of the weather. These elements include high and low pressure systems, cold and warm fronts, and precipitation areas.A high pressure system is an area of relative pressure maximum that has diverging winds and a rotation opposite to the earth's rotation. Fair weather is typically associated with high pressure. A low pressure system is an area of relative pressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Stormy weather is often associated with low pressure systems. A cold front is the leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path. Generally, with the passage of a cold front, the temperature and humidity decrease, the pressure rises, and the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere). Precipitation is generally at and/or behind the front, and with a fast-moving system, a squall line may develop ahead of the front. A warm front is the leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. Generally, with the passage of a warm front, the temperature and humidity increase, the pressure rises, and although the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere), it is not as pronounced as with a cold frontal passage. Precipitation, in the form of rain, snow, or drizzle, is generally found ahead of the surface front, as well as convective showers and thunderstorms. Fog is common in the cold air ahead of the front. Although clearing usually occurs after passage, some conditions may produce fog in the warm air. Current surface maps are updated every hour. Forecast surface maps are updated once each day

Orographic Lifting

• Air mass forced to move over obstacle (topography)

• If air is moist enough and/or topography high enough can cause cloud formation and potentially precipitation

• For large mountain ranges (Hawaii, Rockies, Andes) causes sharp contrast:

humid on windward side

dry “rain shadow” on leeward side

Figure 7.2.3 (p. 203)Orographic storm (from Masch (1984)).

Hawaii: Average Annual Precip. (mm)

Tradewinds

Hawaii

Windward side Leeward side

Thermal Convection

• Caused by differential surface heating (land surface absorbs more radiation than overlying atmosphere)

• Warm air beneath cooler air causes instability

• Mechanism principally responsible for convective/ thunderstorms of short duration and high intensity

have diurnal cycle (late afternoon)

uplift velocities high due to instability

causes tall (cumulus) clouds, sometimes to the top of the troposphere

Figure 7.2.4 (p. 204)Convective storm (from Masch (1984)).

Precipitation Climatology (annual)

Precipitation Climatology (annual)

Figure 7.2.7 (p. 206)Average annual precipitation for the world's land areas, excepting Antarctica (from Marsh (1987)).

Figure 7.2.5 (p. 205)Mean annual precipitation for the United States in centimeters and inches (from Marsh (1987)).

Precipitation Extremes

Figure 7.2.14a (p. 215)(a) 2-year 5-minute precipitation (in) (from Frederick, Meyers, and Auciello (1977)).

0.35 0.40

0.45

0.50

0.55

Figure 7.2.14b (p. 216)(b) 100-year 5-minute precipitation (in) (from Frederick, Meyers, and Auciello (1977)).

0.80

0.85

0.90

0.80

0.75 0.70

Mean Areal Precipitation Estimation: Thiessen Polygon Method

Polygons can be constructed graphically to determine areal weights…

Thiessen Polygon Method for Mean Areal Precip. Estimation

Or for larger problems determined via automated method (i.e. nearest-neighbor algorithm in Matlab)…

P̂ = wgpgg=1

G

∑

Figure 7.2.11b (p. 211)(b) Computation of areal average rainfall by the Thiessen method for 24-hour storm. This method assumes that at any point in the watershed the rainfall is the same as that at the nearest gauge, so the depth recorded at a given gauge is applied out to a distance walkway to the next station in any direction. The relative weights for each gauge are determined from the corresponding areas of application in a Thiessen polygon network., the boundaries of the polygons being formed by the perpendicular bisectors of the lines joining adjacent gauges for J gauges, the area within the watershed assigned to each is AJ and Pj is the rainfall recorded at the jth gauge, the areal average precipitation for the watershed is where the watershed area (after Roberson et al.(1998). (Detail on next slide.)

NEXRAD Stations

Minot AFB Base Reflectivity

Satellite-based Estimates