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Physics and Chemistry of the Mesopause Region Physics and Chemistry of the Mesopause Region

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Physics and Chemistry of the Mesopause Region Physics and Chemistry of the Mesopause Region 2001 CEDAR-SCOSTEP Longmont, Colorado June 17-22, 2001 Tutorial Lecture #6 by Anne Smith ACD/NCAR, USA Physics and Chemistry of the Mesopause Region Physics and Chemistry of the Mesopause Region A. K. Smith NCAR Processes involved in the coupling be tween composition, energy and dynamics Importance of external forcing Outline composition of the mesopause region: how it differs from other regions impact of dynamics and energy on com position impact of composition on energy and dy namics example: role of CO2 in mesopause struc ture example: photochemical destabilization of gravity waves Definitions • mesopause: altitude of the temperature mini mum, which varies on seasonal and shorter time scales; the talk will consider the range 80-100 km. • composition: mostly concerned with the variable part of the composition, making up <1% of the total • mixing ratio: density of species/total density • energetics: the thermal budget & the processes for converting between heat and other forms of energy • vertical scale: for the models, log(pressure) scaled to give approximate altitude Note: many of the illustrations are taken from numer ical models HRDI Mean Temperatures for July 160 140 -60 -40 -20 0 20 40 60 MSIS Mean Temperatures for July 100 90 80 160 70 140 -60 -40 -20 0 20 40 60 from Ortland et al., JGR, 1998 -80 -60 -40 -20 0 20 40 60 80 ^q I....I....I....I •!' inlunl'irliMilmiliiiiliiiiliiiil 110 CD c in r 100 =5 CD 100 C _JLS^S'ji ro YS o "O LZ YS *jr - r 90 o B I 90 •*-- LZ CD 2^ northern hemisphere so i- 80 P ro 70 ]iiii|i!iil\ii[|H winter |iiii|iiii|nn|nn|f summer i|iiii|nn|mi 70 CD 200 b- 200 LZ* - - ' - * YS =3 «C 4-» 180 6YS I 180 I• • — o \ \ 160 CD ~ 160 Q. 140 140 <3h 'r E 3-6h CD >6 h LZ* 11" " i' 120 4-> 120-3 -80 -60 -40 -20 20 40 60 80 geograph c latitude from von Zahn et al., GRL, 1996 Which atoms <& molecules are present? THERMOSPHERE • decrease in proportion of dominant molecules (O2, N2) • increase in several simple molecules (NO, CO) • increasing dominance of atoms • more excited states • reactions can be slow because of low density • ions and ion reactions are important STRATOSPHERE & LOWER MESOSPHERE • breakdown of larger molecules produced in the troposphere • larger role for atoms than in the troposphere • density is still high enough for 3-body reactions • sharp daytime maximum of some atoms & com pounds (O, CI, H, OH, NO, etc.) • only one excited species (O^D)) plays a signifi cant role • catalytic cycles of NOx, ClOx and HOx destroy ozone Which atoms <& molecules are present? UPPER MESOSPHERE • dominant molecules (O2, N2) still well mixed • concentrations of NO, CO can be large • excited states participate in chemistry (N(2D)) or energetics (e.g., 02(15I)) • a few tropospheric molecules remain: H2Of CO2 • increasing role for atoms, particularly O and H • decreasing density affects 3-body reactions • photochemically driven diurnal variations of most minor species • catalytic cycle of HOx dominates ozone destruc tion • ions (NO+) begin to have an effect on neutrals altitude (km) 05 CD CD o o o o O o o 1 i i t i 1 i i i i I i i i y I i i i i i i i i i i i i i i i i i |"-|~ 1 i i i i o w \ - 3 •-* : \ : o O — \ i C H- 1— - • ~ • - a - • w t—» • ^ • — o s z 1 o ^ o \ / o \ K ' ^"""^^ ^*^ \ ° s^*"^ - 1—»• \ x ' ^ P O z i—'• • ^>v^ 01 X \ ' j^ 0 // ~ \ «•» 1 ~~ ~~ "* s S \ \ / i—' Z \ -v /\\ / ^„• z z s. ** ^ z O /\\ / f^ Z. N ^* / \\ / **^^ i N "** «^ 03 V ** «» *• "' \\ / ^-'S^**- N \\/ -v*-**"^ x - \ viT -^^^ o - " \ ~ 1—' ->*c w — ~ ^ y^>^\ *"• z o =- V 1 C^"— \ \ ^^ \ ffi / / s \ X s •^ \ /> - jr _ •— ^. \ h-* N. \ J-^ — O z 1 S>v *- "*s^ O) VS.s*^^^^j. "" ^^"^ ,» "* ^s?— »* ^^^"^ ^~*~~ , l •*• "*— ^^^^ -' "' z O 1 ro ro X - X ro I—1 E o z O = i i i i i i i i i i i i i i i i i i i 1 i i i i 1 i i i i 1 i i i i 1 i t i i = What are the important species? 1) from the point of view of controlling the composi tion: • O: produced locally from O2 and O3, transported downward from thermosphere; increases strongly with altitude • O3: produced from O+O2; rapidly destroyed dur ing the day • H, OH, HO2: produced during the day from H2O, or transported downward from thermosphere (H) 2) from the point of view of measurements • O3 • C02 • sodium and other metals • excited species that emit: 02(15I), OH, etc. Trace Gas Mixing Ratio 100 TTl MMI| TTTTTTIT] l\"TITmi| 1 I I illll| 1 I I lllil| l-ljrvmij TT r/lHij l"l ITIHI| 1 I I lllll| 1 I IIIIII \ 90 6 •8 80 CC 70 \ / \ 1 / • » ' / / \/ 60 I I I IMIll l/l I llllll 1 1 v I 1 1 1 mmiiI I 1 1 mml 1 1 "I"'! » ' 1mill 1 1 mint I I I Mill -12 10-n1 n-11 10-xu1 n-10 1Q-y1 n-9 10-a1 n-8 1Q-71 n-7 1Q-b1 n-6 10-d 10-* 1Q-j 1Q. 10 Which reactions occur? photolysis, e.g: O3+/11/ -» O+O2 • sun angle • solar flux • column of absorbing gases • weakly on temperature 2-body reactions, e.g: O+O3 -• O2+O2 • density of reactants • temperature 3-body reactions, e.g: O+O+M -• O2+M • density of reactants • temperature • density *M is any atom or molecule Day/Night Differences Daytime • Sunlight splits molecules: O2, O3, H2O • Absorption generates excited states of atoms and molecules that can react chemically or pass their energy along to another species Nighttime • at lower altitudes atoms & radicals recombine into more stable molecules (e.g., O -* O2, O3 and H, OH, H02 -• H20 • O and H persist at the mesopause & above Transport • dynamics is similar except for phase of the tides & processes affected by tides • vertical transport can be very different because of gradients Global log(mixing ratio) of 0 at four local times 0 LT 6 LT -nodel s;ep: 960 UT: 0.0 Doy: 196 nodel step: 960 UT: 0.0 Day. 196 12 LT 18 LT rro>: -1.07 mo: -M.0 »ai: -1.05 mn: -14.0 model step: 960 UT: 0.0 Doy: 196 model step: 960 UT: 0.0 Doy: 196 Impact of anthropogenic gases on composi tion • increased CO2 —>- cooling • increased hydrocarbons ->• ozone loss Impact of dynamics on composition • winds advect species • temperature affects reaction rates • winds and temperature control propagation and dissipation of waves Impact of energetics on composition • photolysis affected by incident radiative flux • atomic/molecular energy levels changed by ab sorption of radiation • energetic particles can generate ionized or excited states • temperature affects reaction rates temperature ABSOLUTE DIFFERENCE (K) -4 10 100- -2 _ --10 E ~3 a, CO bfl 10U £ o _-10' 90S 60S 30S Eq 30N 60N 90N latitudes (angular degrees) 03 relative difference (%) E M a 60- bfl G 90S 60S 30S Eq 30N 60N 90N latitudes (angular degrees) Diffusive transport molecular diffusion • diffusive component reduces gradients • "advective" component moves species differen tially • effects on mesopause H, H2, CO2, temperature • diffusion rate varies with temperature eddy diffusion • caused by turbulence or by any unresolved dy namical scales • important in the vertical because of strong mixing ratio gradients • limited measurements • mass mixing, i.e., all species (mixing ratio) and potential temperature have same diffusion coef ficient • extremely variable due to intermittent dynamics Fig. 5. Comparison of measured and calculated CO mixing ratios 120 110 - 100 - E Q> s 90 - --A--ISAMS •----EV18 80 •••••---- SL3-Mean TIME-GCM -GS 70 •' I I I I L L_j__i i i Ul i i—i—u 0 50 100 150 200 250 300 350 400 Volume mixing ratio (ppmv) UpeT- P*"-**-5 •*"*Xt Advective transport r variability gravity waves hours hours-seasonal tides hours days-seasonal planetary waves days weeks-seasonal mean circulation seasonal annual General rules: time scale of variability increases with spa tial scale propagation of low frequency waves (plan etary and gravity waves) strongly affected by zonal wind SUNRISE/SUNSET ANOMALY HALOE NO MARCH 199. 120p t F / i- 11oF- r u I- 1 ' / — o 10 10 ~ 10 10 VOLUME MIXING RATIO from Marsh & Russell, GRL, 2000 % Difference from Mean W 0600 HRS -40 -20 0 20 40 -40 20 0 20 LATITUDE (DEG) LATITUDE (DEG) % Difference from Mean W 1800 HRS Ml Hlkli iJiliU'i X LJ x -40 -20 0 20 40 LATITUDE (DEG) fx (m/s/day) ; January -0.001 .o 0.010 £ :-o.ioo 90S 60S 30S Eq 30N 60N 90N latitudes (angular degrees) (qui) amssaid o Z Q z o Z o en U So o ,8 ^ c o 3 Bd O d C3 00 O X co I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I [ I ! I o OS o o O O o o (ui>i) Qptuppg d-§0| log-p altitude (km) 3 C/3 B3 P H o Pressure (mb) log-p altitude (km) O m CD 3 CD >-. P 3 H! 0\ CD © P U3 o pa C D- o OQ W EL*0 p —.
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