Lecture 2: Composition & Structure of the Atmosphere (Ch1)

• Composition (continued)

• Vertical structure of the atmosphere

Hurricane Florence, a problem for Bermuda on Monday, was expected to bring strong winds and heavy rain to eastern Newfoundland on Wednesday. (NOAA)… by that time, Florence will probably be gearing down from a hurricane to a "very intense post-tropical storm," Environment Canada's Canadian Hurricane Centre said in a statement issued early Monday.

OO33 (ozone) and the ozone hole (ozone) and the ozone hole

• radiatively/climatologically active (accounts forpeak T in mid-atmosphere)

• inactive relative to weather

• essential relative to life… absorbs ultraviolet radiation

• peak concentration of up to about 15 ppm in mid-stratosphere

• depletion in spring over poles (esp. Antarctica)• reactions on surface of polar stratospheric ice clouds• over long term, size of ozone hole governed by emissions of CFCs, lifetime

order 100 years• year-to-year variability determined mostly by temperature variations (NASA)

Cl + O3 O2 + ClO

ClO + O O2 + Cl

AerosolsAerosols

• size 0.1 m 100 m or larger (smallest formed from sulphate gases)• influence visibility (VV see Appendix C)• increase shortwave reflectivity • trap outgoing longwave radiation• form cloud condensation nuclei

UA farmEllerslie28 May 2001

*An uncertain feedback in climatic modelling: DMS (dimethyl sulphide) gas released by decay of ocean biota generates aerosol with radiative impact as well as acting as cloud condensation nucleii (CCN)

1oC reduction in N. hemisphere sfc temp a year after Pinatubo eruption 1991

Vertical StructureVertical Structure

Fig 1-8

Fig 4-3

Density () and pressure (p) both decrease with increasing elevation

Air column,base area Ap=0

Ground- or sea-level

p=p0

Top of atmosphere

p0 = weight of air divided by area A = M g / A [Pascals]

Relates to Fig. 4-2

Vertical StructureVertical Structure We shall neglect these layers

Fig. 1-9

Ideal Gas Law (equation of state)Ideal Gas Law (equation of state)

TRp (p96, 4(p96, 4thth edition) edition)

p , pressure [Pa]

, density [kg m-3]

R = 287 [J kg-1 K-1] , specific gas const. for dry air

T , temperature [K]

Question: these paragliders are Question: these paragliders are flying at a height of 1000 m above flying at a height of 1000 m above sea-level. A pilot’s instrument sea-level. A pilot’s instrument reportsreports

]C[25

],mb[900o

T

p

so the air density at flight level is?so the air density at flight level is?

The Hydrostatic LawThe Hydrostatic Law

Gives the change in pressure (p) associated with an increase (z) in height

1mPaor

m

Pag

z

p

= air density [ kg m-3 ] (approximately 1, near ground)g = grav. accel’n = 9.81 [ m s-2] (approximately 10)

Thus: Thus: near ground, pressure increases by an amount p = - 10 Pa for each 1 m increase in height

100 Pa (= 1 mb) per 10m100 mb per 1 km

(p104, 4(p104, 4thth edition) edition)

Question: if those paragliders Question: if those paragliders descend 100 m, estimate the descend 100 m, estimate the pressure at their new flight level:pressure at their new flight level:

]C[25

],mb[900o

1

1

T

p

p1, T1 (known) p2,T2

To find: p2

Question: if those paragliders Question: if those paragliders descend 100 m, estimate the descend 100 m, estimate the pressure at their new flight level:pressure at their new flight level:

]C[25

],mb[900o

1

1

T

p

p1, T1 (known) p2,T2

To find: p2

Need to use

with z = - 100 m. But we need the density …

gz

p

Approximate the density as

gz

p

25273287

10x900 2

1

11 RT

p

10100 11 gzp

(note the unit conversions; we work in MKS units)

(implies)

A negative step in height z gives us a positive step in pressure p