Metr 356 Class Notes

11/12/2004

Anh Le

Announcements:

In next several weeks, we will be interspersing discussions of California weather patterns (i.e., Diablo Winds, Summer thunderstorms, Middle Latitude Storm type) with discussions of rainfall variability as an indicator both of climate and of climatic change.

Writing Assignment 2: Distributed Friday, 5 November, due Friday 19 November

Writing Assignment 1: Returned (Average 87/100)

Reading Assignments:

Gilliam, Completed

What You Have Completed So Far
Chapters 1-4 and part of 5); Chapter 9, 148-161
New Reading:
Chapters 5, 6, 7, 8

Reader

pp. 386-389, 538-543

Seasonal Climate Discussion:

• Look at the pattern of pressure in January, there are the Gulf of Alaska High, the Icelandic Low and the Great Basin High. During winter, the air in continent is colder than the air in the ocean.
• Moving northward along the coast, the air gets colder. In southern California is around mid 50°F, in the middle around 50°F and northern California is 45°F.
• Also, the air is colder when moving away from the coast. Central Valley is colder than along the coast due to elevation.
• Look at the Bay Area winter pattern, air is just moving in the exact opposite way of the summer pattern.

Great Basin High:

• While the Jet Stream does not present, the pressure around the area is high. In Medford, OR, heavy fog is observed, dew point temperature is 30°, and the temperature is 30°F, this is a typical model for the winter. In San Francisco, dew point temperature is around 36° while the temperature is 40°F, Tule fog flew outward from the Central Valley in the opposite direction of summer fog.
• The Polar Jet Stream in January is further south at this time and it affects the amount of rainfall across the State.

Rainfall

• The rainfall amount in winter is affected by:

_ The Latitude: closer to the Polar Jet Stream, get more rain.

_ Typographic:

ü In San Diego, the average rainfall is 10 inches/ year (peak in February)

ü The pocket behind Santa Barbara: very wet condition

ü In downtown San Francisco: 22 inches/year

à It is extremely wet in local pocket and dry in downhill mountain (rain shadow). The Great Basin is the rain shadow of the Sierra Nevada.

Slides (background of Diablo and Santa Ana winds)

• Diablo and Santa Ana winds are types of Foehn wind. Diablo wind is local Foehn wind occurred in the San Francisco Bay Area while Santa Ana wind occurred in Southern California.
• When there are:

ü Dynamic High

ü Offshore Flow

ü Fog along the coast

Foehn wind will form

Canyons:

• Major canyons in Southern California channel the winds.
• Direction of these canyons indicates direction of the winds: funnel from continent then spread out when moving towards the coast.
• Wind speed is roughly 1.5 times stronger through canyons.

Tule Fogs: (radiation fog)

1. Look on the Surface weather map: there’s a storm track going on now, isobars are farther from each other indicating an area of High pressure

à gets more of the night time, generally clear sky and enough cooling to form fogs.

• In smaller valleys, radiation fog is called valley fog
• Tule: is a local fog occurs in the Bay Area and the Central Valley.
• Locally, Tule is moving in San Francisco, the closer you get to the Delta, and the easier you see fog.

2. Look at the Satellite Image:

• Also see storm track
• Storm clouds cut right across the coast
• Fog fills up in California Valleys

National service publication of Diablo Winds:

• Major canyons are where winds are funneled (North, Northeast winds)
• Arrows indicate wind direction, biggest arrow is where the strongest wind is( in Sierra Nevada)

500 mb chart:

• There’s a trough, indicates a high pressure area at the surface, strong offshore flow, high risk for fire

Rainfall Variability:

• Way meteorologists conceptualize the rainfall amount by graphs.
• Be cautious to make any assumption based on graphs
• We look at San Francisco Rainfall from 1990-1991 to 1999-2000 and1980-1981 to 1989-1990: the 20^th century is dry compared to previous years.

Standard Deviation, Coefficient of Deviation:

(On class website http://virga.sfsu.edu/courses/courses.html)

è The smaller the Standard Deviation is, the more reliable we expect on the amount of rainfall in any given year.