class #32: friday, november 13

Class #32: Friday, November 13 1

Class #32: Friday, November 13

Weather Forecasting

(conclusion)

Climate types of the present


The Comeback of Analog Forecasts

• ENSO has characteristic weather patterns that are statistically more common in El Niño, La Niño, and neutral conditions.

• Some of the uncertainty of numerical weather forecasts (the 6th type of forecasts) can be assessed using a type of analog forecast.


A New Method of Forecasting

• Spectral models view the atmosphere as having a number of waves of different wavelengths and amplitudes (wave heights).

• Spectral models are faster than grid point models.

• Spectral models are used for mid and long range forecasts.


Another New Forecasting Method: Ensemble Forecasting

• Involves running the same forecasting model many times, using very slightly different conditions.

• Is a research tool, because it is too slow to show in real time

• Slight changes in initial conditions can produce very different forecasts

• 50 in test ensemble that follows• Resembles the old analog method


Climate

• Is to weather what a friend’s personality is to his/her mood

• Sums up the weather’s long-term behavior

• Is the collective state of the atmosphere for a given place over a specified interval of time


Climate is defined by

• Location– Globe, continent, region, city– Chapter 14: regional and global scale

• Time: a specified interval– 30 year average is normal– 100 years or longer for history of climate

• Averages and extremes of variables– Chapter 14: temperature and precipitation


Controls on climate

• Similar to controls on temperature in Chapter 3

• Latitude: solar energy input

• Elevation: air temperature, snow vs. rain

• Topography: moist vs. dry, temperature, distribution of cloud patterns, solar energy reaching the surface


Controls on Climate (continued)

• Proximity to large bodies of water– Thermal properties of water (absorption, heat

capacity, transparency, mixing) moderate temperature downwind

• Prevailing atmospheric circulation– Intertropical Convergence Zone (ITCZ)– Subtropical Highs


Classifying Climate

• Hard to do– No clear boundaries– Complex natural systems

• How it is done– Important to life: hot or cold; moist or dry– Most common system: based on vegetation

• Köppen climate classification scheme based on vegetation and temperature


Köppen (KEPP-in) Scheme

• Modified by Trewartha and Horn

• Related to geography and global circulation

• Uses letters (1, 2 or 3) in Chapter 14

• Has many other subdivisions within the categories shown in our book

• Has 6 major groups: A, B, C, D, E, H


The 6 major climate groups

• A: Tropical moist• B: Dry (can be subtropical or mid latitude)• C: Moist with mild winters (mid latitude)• D: Moist with severe winters (mid latitude)• E: Polar (high latitude)• H: Highland (rapid climate change with

elevation)• 2nd letter: usually latitude (except B)• 3rd letter: differences in temperature


Tropical Humid Climates: Af, Aw, Am

• All tropical (A) climates are humid

• Letter “f” means no dry season, rain year round, usually closest to the equator

• Letter “m” means “monsoonal”, with a short dry season and a very rainy season

• Letter “w” means “winter dry season” except no real winter in tropics, just cool


Tropical humid climates (continued)

• Af – Closest to the equator– Smallest annual range of temperature– 6.9-10 inches of rain per month– Most thunderstorms in afternoon– Linked to ITCZ– Tropical rain forests

• Am– Seasonal onshore winds during summer monsoon– Climates with most yearly precipitation– Jungle vegetation


Tropical moist climates (continued)

• Aw– Farthest A climate from the equator– Often border Af– Tropical wet and dry– Wet summers, dry, cooler winters– Linked to the seasonal migration of the ITCZ– Vegetation is savannah or tropical grasslands with

scattered deciduous trees, as in the grasslands of Africa.


Dry (B) climates

• Potential evaporation minus precipitation is greater than 0.

• More land of this climate type than any other

• Lubbock has a B climate

• Descending branch of the Hadley circulation near the subtropical highs or

• Rain shadow of a mountain range


Dry climate subtypes

• 2nd letter– “S” for steppe or semi-arid (like Lubbock)– “W” for true desert (extremely dry)

• 3rd letter– “h” for low-latitude, hot (yearly average

temperature >= 64ºF)– “k” for mid latitude, cool (yearly average

temperature <64ºF)


Dry climate subtypes (continued)

• BWh Extremely dry and hot; can have large sand dunes; Sahara, Arabian peninsula, central Australia, most extreme B climate

• BSk Least extreme B climate; midlatitude steppe, often high plateau, Lubbock, Denver, San Diego; often rain shadow

• BSh Much of Mexico, lower latitude, subtropical steppe

• BWk Central Asia, very dry, midlatitude rain shadow, continental interior

class #32: friday, november 13

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