ERTH 365, Spring 2020 Class Notes for April 16,2020

Adrian Lopez

Assignments

• Study for the midterm. • Pielke and Pielke; Chapter 3 and 5 • Simons and Sutter: Chapter 6 –Sections 6.4 , 6.5 and 6.6. • Williams : Chapter 4, 8 , and 9 ; Hurricanes • Zebrowski : Chapter 5 pp. 131-135 and Chapter 8 251-262

Housekeeping

• Homework #2 due 6pm • Mid-Term 2 review

Fronts Fronts are narrow transition zone between air masses of differing densities. Density differences usually arise from temperature differences. Examples of Frontal symbols:

Cold Fronts displace warmer air(less dense) with colder(more dense) airmass. Larger temperature differences are usually associated with cold fronts. Average speed of a cold front is 30 mph. Precipitation is located on either side of the front. Example of a cold front:

Warm Fronts occur when colder air retreats and is replaced by warmer (less dense) air. Warm fronts tend to have weaker temperature gradients. The average speed of a cold front is 16 mph. Warm fronts temperature tends to rise slowly . Warm fronts lift warm air that produces widespread clouds and precipitation well in advance of boundary.

Occluded Front are cold fronts that typically move faster than warm fronts. Cold fronts can catch up and “overtake ” a warm front. There are two types of occlusions : Cold type occlusions and Warm Type occlusion(very rare).

How do we tell what kind of front it is ? If warm air replaces colder air, the front is a warm front. If cold air replaces warmer air, the front is a cold front. If the front does not move, it is a stationary front. Occluded fronts do not intersect the ground. Polar Front Theory

A polar front is a semi-continuous boundary separating cold, polar air from more moderate mid

latitude air. Where do mid-latitude cyclones form? Mid-latitude cyclones form on lee-side lows, Hatteras low, Alberta Clipper and in Explosive Cyclogenesis.

Formation of Mid-Latitude Cyclones Cyclogenesis – Development of a region of low pressure—a cyclone. Conditions:

• Cyclonic flow must be established at the surface. • Cyclonic flow is CCW in the Northern Hemisphere • Convergence at the surface must be supported by divergence aloft.

Formation of Cyclones

In order for cyclones to form we need a cold core. A cold will intensify with height. We also need an upper- level divergence stronger than convergence. More air is taken out of the top and the bottom. Surface pressure drops and low forms.

Convergence and Divergence Question: What initiates “cyclogenensis”? Answer: When upper-level divergence is stronger than lower-level convergence, more air is taken out at the top than is brought in at the bottom. Surface pressure drops, and the low intensifies, or “deepens”. Upper level divergence initiates and maintains a surface low.

Wave Cyclone Development

Cyclones mature when the shape of the cyclone looks like a comma.

Cyclone occludes when the cold front over takes the warm front. The cold air surrounds the cyclone. Gradients weaken and the low slowly dies.

El Niño Myths: El Niño Coming to California, All El Niños are the same , and El Niño means lots of rain for California.

Normal El Niño

La Niña

Normal Worldwide El Niño Impacts

El NiñoStorm Track Pattern

La Niña

Worldwide La Niña Impacts

Southern Oscillation Index (SOI) Pressure Difference between Darwin and Tahiti. Negative SOI closely related to El Niño.

La Niña / El Niño

El Niño: ONI ≥ +0.5°CLa Niña: ONI ≤ - 0.5°C

Oceanic Niño Index (ONI)• SST departures from average Niño 3.4 SST

• 3-month running mean values of SST departures

ENSO CONDITIONS

Strong La Niña1973-1974 1988-1989

Very Strong El Niño1997-1998 2015-2016

All El Niños (ONI > +0.5)

El Niño

ENSO Conditions

ENSO Forecast

ENSO vs. California Flooding

Summary :Top Ten Most Costly• 4 -- El Niño• 4 -- Neutral• 2 -- La Niña

Scripps ENSO Forecast