ESE / GE 148a: Introduction to Climate

Transcription

1 ESE / GE 148a: Introduction to Climate Organizational Details - I TA: Xianglei Huang: <hxl@gps.caltech.edu> Xianglei (Luke) is a graduate student in Planetary Science and is working with Prof. Yuk Yung on a number of problems in atmospheric dynamics and chemistry. Xianglei will schedule office hour(s) after the next class

2 Organizational Details - II Course WebPage: Textbook: Global Physical Climatology, by Dennis L. Hartmann (Academic Press, 1994, ISBN ) P/F daily homework (aka Ticket) 25%; problem set (approximately every other week) 20%; an oral presentation 20%; final exam 35% Oral presentations: Each student gets a particular month, e.g., January 2003, from which to choose two published articles (journals such as Science and Nature, Climate Dyn., J. Climate, Geophys. Res. Lett., J. Atmos. Sci., J Phys. Oceanog., J. Geophys. Res., etc.). The student gives a 10-minute talk on each article. The goal is to instruct the class, and the class is responsible for the material presented. ESE 148b winter term w/ Andy Ingersoll details how energy, water, etc. are transported in the Earth Atmosphere/Oceans. ESE 148c spring w/ Jared Leadbetter and staff describes the cycling of elements by the plants, bugs, and animals. Daily P/F homework To help motivate the day s lecture, a short problem is to be worked and handed in at the beginning of class. These are listed on the class web page. The problems are not meant to be difficult, if the reading has been done. If you find yourself spending a lot of time on a problem, you are probably not thinking about it correctly! Here is the problem for Wed: Approximately what is the atmospheric pressure atop Mt. Everest (8848 m)? About what fraction of the mass of the atmosphere is below this altitude?

3 CLIMATE Climate is the synthesis of weather in a particular region. The climate variables of most interest: *Temperature - Figure 1 *Precipitation Wind Pressure Cloudiness Humidity => Climate is the expectation for a given month or season. Distinguish from synoptic scale weather. Climate is what we expect, weather is what we get Generally we are interested in surface climate, though recent observations provide measure of climate at other altitudes, e.g. stratospheric climate determined by ozone and temperature observations; recent mid-tropospheric satellite temperature measurements allow tests of warming theory. Figure 1. winter summer temperature difference. Ruddiman, 2001.

4 Temperature <T> surface = 288K (15 C). In lowest 10 km, the lapse rate, Γ, averages: T Γ z 6.5K / km The lapse rate varies with season, altitude, and latitude (See Figure 1.2, 1.3, 1.4). As we will discuss at length, the decrease of temperature with altitude in the troposphere is critical for the maintenance of the equitable climate. Atmospheric Composition The composition of Earth s atmosphere both controls the radiative environment and is significantly regulated by biological activity. On geological timescales, the recycling of carbon via plate tectonics is critical for Earth s climate (c.f. Mars & Venus). Table 1.1 lists, in order of abundance, the constituents of Earth s atmosphere.

5 The Hydrostatic Balance Jacob, "Atmospheric Chemistry" Atmospheric pressure is the weight exerted by the overhead atmosphere on a unit area of surface. Consider the mercury barometer: "vacuum" P atm H The weight of the mercury column, H, must equal the weight of the atmospheric column, or: P atm = ρ Hg g H = (at sea level) 13.6 g cm m s cm = kg m -1 s -2 in SI units. The SI unit is Pascal (Pa); 1 Pa = 1 kg m -1 s -2. Other units for atmospheric pressure in widespread use: the atmosphere, 1 atm = x 10 5 Pa, the bar (b) (1b = 1 x 10 5 Pa), the millibar (mb) (1mb = 100 Pa) and for Chemists, the torr = 1 mm Hg = 134 Pa. To be good internationalists, Pa are it! Often you will now see pressure in hectopascals, hpa, which are equivalent to mb. Consider the following forces acting on a slab of atmosphere: Z+dz Surface area, A Z At equilibrium (or hydrostatic balance!) the weight (acting downward) must be balanced by the pressure gradient force: ρ g A dz = A (P(z) - P(z+dz)) rearranging: dp/dz = -ρ g From the ideal gas law, ρ = P M a /RT Substituting yields: dp/p = - (M a g / RT) dz For an isothermal atmosphere (and this is true to ~20%), we can integrate: P z = P o exp(- [(M a g)/rt]z) = P o exp(- z / H) where H, the scale height, is about 7 1/2 km.

6 Let's calculate the mass of Earth s atmosphere: Mass = 4 π R 2 P(surface)/ g kg. (1 kg / cm 2 ). This is the same weight as a column of water 10 meters deep. Air is 78% N 2, 21 % O 2, and 1% Ar (+ some water and other things). The mean molecular weight, M a, is: ( ) + ( ) + ( ) = 29 g mole -1. For a column weight of 1 kg cm -2 we have ~35 moles cm -2 or molecules cm -2 in the column.