Indian Journal of Radio & Space PhysicsVol. 16, February 1987,pp.11-24

Troposphere-Stratosphere CouplingB v KRISHNA MURTHY

Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695022

Received 30 October 1986

The troposphere-stratosphere coupling involves mass exchange, dynamical, radiative and electrical processes. Thesefour processes of coupling are reviewed.

1 IntroductionThe troposphere is the region of weather processes

where the main heat exchange is through convection.In the stratosphere, the radiative processes dominate.Interaction between the two regions plays an impor-tant role in their dynamical, chemical and radiativeproperties. The main processes of interaction aremass exchange processes, dynamical processesinvolving atmospheric wave motions, radiative pro-cesses, and electrical coupling.

2 Mass ExchangeStrong evidence for a continuous and effective air

mass exchange between troposphere and strato-sphere comes from the distribution of atmospherictrace constituents. Radioactive isotopes (generatedas products from nuclear tests) introduced into stra-tosphere are removed into the troposphere at ratesmuch greater than expected from small scale turbu-lent diffusion processes at tropopause level. Traceconstituents such as CH4 and SOz with troposphericsources are found in stratosphere, suggesting an up-ward flux of these through the tropopause. Thus, thetropopause acts as a permeable 'membrane' throughwhich continuous mixing between tropospheric andstratospheric air takes place.

For the study of troposphere-stratosphere mass ex-change processes, a proper identification of tropo-pause level and an understanding of its characteristicsare necessary.

Generally, a prominent change in the temperaturelapse rate demarcates troposphere and stratosphere.However, on occasions, especially in the vicinity ofsubtropical and polar jet streams, the tropopause po-sition becomes ambiguous and is obscured by frontalzones in the upper troposphere with prominentchanges in the lapse rate. Danielson Iproposed use ofpotential vorticity P,which is conserved in an adiaba-tic flow, to locate tropopause. P is given by

of)P=-Qz- ... (1)op

where Qz is the vertical component of absolute vorti-city, ()the potential temperature, and p the pressure.

Potential vorticity is generated in the stratosphereby diabatic radiative processes and destroyed in thetroposphere by diabatic mixing, overturning and fric-tional dissipation at the ground. Whereas strato-sphere is characterized by large values of P, tropos-phere is characterized by small values. Potential vor-ticity is a very useful index for separating the strato-spheric and tropospheric air masses especially in thejet stream regions. Notwithstanding the difficulty inthe identification of tropopause, the following can beconsidered to be the main processes responsible forair mass exchange between troposphere and strato-sphere:

(1) Seasonal adjustments in the altitude of themean tropopause level

(2) Organized large scale quasi-horizontal andvertical motions expressed by the mean meridionalcirculation

(3) Large-scale eddy transports, mainly in the jetstream region

(4) Meso-scale and small-scale eddy transport ac-ross the tropopause.

2.1 Seasonal Adjustments in the Altitude of theMean Tropopause Level

Tropopause altitude shows very prominent sea-sonal variation+'. The principal feature of the tropi-cal tropopause is a pronounced minimum in its alti-tude in the northern hemisphere summer and a broadrnaxirrrm in winter, as revealed by the stations inPacific zone (mainly inland stations)". KrishnaMurthy et at.' examined in detail the temporal var-iations (both seasonal and longer term) of tropicaltropopause in the Indian zone considering inlandstatioris also. Fig. 1 shows the temporal variations ofmonthly mean altitude (A), temperature (B) and pres-sure (C) of tropical tropopause at four stations, name-ly, Trivandrum (8.6°N), Madras (13.1 ON),

11

INDIAN J RADIO & SPACE PHYS. VOL 16, FEBRUARY 1987

Visakhapatnam (17.rN) and Delhi (28.6°N). Thelower latitude stations show the summer minimum inaltitude prominently while the higher latitude stationDelhi shows winter minimum with the transition oc-

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KRISHNA MURTHY: TROPOSPBERE - STRATOSPHERE COUPLING

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INDIAN J RADIO & SPACE PHYS, VOL 16, FEBRUARY 1987

spheric circulation. The three-cell pattern in the tro-posphere develops into a two-cell pattern in the lowerstratosphere with the Hadley cell extending furtherinto the middle latitudes because of the higher stabi-lity of the stratosphere and the Ferrel cell extendingupwards with its axis tilted towards the pole. In the up-per stratosphere the motions mostly respond to theradiative heating which heats the summer polar re-gions and cools the winter regions. This differentialheating produces a single-cell circulation in the stra-tosphere, with the air being lifted up in the summerhemisphere and being traps ported across the equatorinto the winter hemisphere where it is brought backdown into the troposphere.

The Hadley cell penetrates well into the lower stra-tosphere, thus taking the tropospheric air into thestratosphere in circulation. In latitudes between 200Nand 30oS, the total annual mass input into the strato-sphere due to the tropospheric Hadley cell flux(essentially during the three seasons, namely autumn,winter and spring, the summer Hadley flux beingnegligible) amounts to 1633 X 1017g (Ref. 6) which is38% of the northern hemisphere stratospheric mass.Estimates of the Hadley cell circulation and mass fluxreported by Newell et al.1O and Manabe et al.llyielded similar values for these quantities. Thus itappears that the tropical branch of the Hadley cell isvery effective in introducing large amounts of tropo-spheric air into stratosphere. Same amounts of strato-spheric air in middle- and high-latitudes will returninto the troposphere because of continuity (exceptfor seasonal variations in tropopause altitude).

Rind and ROSSOWI2,through modelling, studied awide variety of processes that appear capable of influ-encing the mean Hadley circulation. The intensity ofcirculation is related to the coherence of thermal forc-ing and to the thermal capacity of the atmosphere.The latitudinal extent appears to be controlled pri-marily by eddy processes (Ferrel cell intensity).

The tropical convection transports lower tropos-pheric constituents to altitudes below the tropicaltropopause and from there the much weaker upwardmotion of the mean circulation performs the finalcross-tropopause transport into the stratosphere.Shenk':' presented an example of tropopause pene-trating cumulonimbus clouds in extra tropical lati-tudes. The relative importance of tropical and extra-tropical convective systems is not yet clearly esta-blished.

The upward branch of the tropical Hadley cell cir-culation may be viewed as a net effect of strong local-ized updrafts in large well-developed cumulousclouds, the so called hot towers!". However, laterevidence-'-" showed that less than 1% of the equato-

14

rial convective systems reach altitudes of tropicaltropopause.

Evidence for the existence of the hot towers wasprovided mainly in general terms by radar echo tops.The top of the ascent of the hot towers will vary withthe surface properties as the towers reach their ther-mal equilibrium with the surroundings 14. Further, un-der strong shear conditions, as discussed by Malkusand Riehl'? for the western Pacific and by Maddenand Zipser" for the Line Islands in the CentralPacific, the towers get eroded at altitudes lower thanthose of thermal equilibrium due to mixing with envi-ronmental air. Hitherto, most of the observations onthe tropical hot towers came from the Pacific and At-lantic zones (mainly from island stations). It is neces-sary to investigate the characteristics of these in theAsian zone in view of the large scale seasonal mon-soonal conditions present in this zone and also as alarge part of the zone is landlocked.

It may be added that the traditional concept of gen-eral meridional circulation (based upon point aver-ages of mean motions) comprising the three cells isfound to be not consistent with the chemical tracercirculation motions proposed by Brewer'? and Dob-son?", The Brewer-Dobson circulation (Fig. 8 fromWallace?' )contains a single direct Hadley cell extend-ing from equator to pole which defines the Lagran-gian (parcel trajectory) mean motions. The contribu-tion by the eddy motions (predominant at mid lati-tudes) is included in the Lagrangian mean motions.The effect of these eddy motions when added to theEulerian mean motions act to counter the effect of theFerrel cell. The resulting Lagrangian mean circula-tion (heavy arrows in Fig. 9 from Wallace21 ) is quite si-milar to the Brewer-Dobson circulation. The individ-ual contrbutions to the Lagrangian mean circulationsshown in Fig. 9 illustrate the complex processes whichforce the Lagrangian mean circulation. Studies byKida-? and Dunkerton+' suggest that it is useful toview the general circulation between the troposphereand mesosphere in terms of the mean Lagrangian mo-tions.

2.3 Large Scale Eddy TransportsThe mass transfer from stratosphere to tropos-

phere in the jet stream region (tropopause folding)has been studied quite extensively by manyscientists 1.24,25.

Fig. 10 (from Danielson-") shows the division be-tween stratospheric and tropospheric air massesschematically. The vertical component of vorticity(Qz) shows strong horizontal gradients in the core ofthe jet stream, and so the dividing surface between thetwo masses goes through the jet core. The mean circu-lation relative to the jet stream position causes exten-

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streams (J), planetary waves and small scale motions (thin lines)

sion of stratospheric air into the upper portion of thefrontal zone and return flow of tropospheric air intostratosphere above the jet core. This leads to the ap-pearance of folded tropopause (Fig. 10).

The tropopause folding, in itself, is not evidence forstratospheric-tropospheric exchange. This is becausein the absence of diabatic and turbulent mixing pro-cesses, potential vorticity is conserved. However,there is substantial evidence for such mixing in thetroposphere-t" where the potential vorticity is des-troyed.

Shapiro's" observations give strong evidence thattropopause folds are regions of mixing of ozone andcondensation nuclei. It is shown that the air within thefold contains both stratospheric levels of ozone andtropospheric concentrations of condensation nuclei.Cadle et al/" presented evidence from aircraft mea-surements, showing tropospheric concentrations ofcondensation nuclei within the stratosphere in the vi-cinity of upper-level jet-stream system. These evi-dences show that the small-scale turbulent mixingprocesses within tropopause folds act to entrain tro-pospheric chemical constituents into the stratos-

tz

NORTH

Fig. 10- Vertical cress-section showing tropopause with verticalslope through axis of the jet. Potential temperature isotherms arecontinuous lines. West wind speed (isotachs) are dashed lines.J de-notes the core of the westerly jet. Gray denotes stratospheric circu-

lation and white tropospheric circulation.

phere, while stratospheric ozone is simultaneouslyexpelled into the troposphere.

It is estimated" that approximately 20% of strato-spheric air mass is exchanged by the cyclonic eddies(large scale eddies) in the northern hemisphere. Thisvalue may be compared with 38% due to Hadley cellcirculation and 15% from summer to winterhemisphere.

2.4 Small Scale eddy Transport Across Tropopause

The vertical flux F, of an admixture to the atmos-phere of mixing ratio m is given by

omF, = w.m - Kzih, ... (2)where w is the mean vertical motion and K, the verti-cal component of eddy diffusivity.

The first term on the right hand side gives the fluxthrough mean motion (described earlier), and the sec-ond term the effects of small scale turbulent motionthrough tropopause. The horizontal fluxes Fx and Fycan be defined similarly. The local changes in the mix-ing ratio due to fl uxconvergence or divergence can berepresented by

e); of, of, om-+~+-=--ox oy OZ ot ... (3)

Except in the tropopause folding region, horizon-tally homogeneous conditions can be assumed.Then

om om-----=-=0iJx OV

andox, ox,-----= -~= 0ox oy

19

INDIAN J RADIO & SPACE PHYS, VOL 16, FEBRUARY 1987

Assuming at tropopause level

OU OV ow-+-+--=0ox oy iJz

(where u and v are the mean motions along the twohorizontal axes x and y)

Eqs (2) and (3) can be combined to obtain

oK, om iim omaz . a;- al =Tt ... (4)

Taking (oK/iJz) as negative and assuming (om; oz)as negative and ( (Jl ml iJz2)as zero around tropopauselevel, it is seen from Eq. (4) that mwill increase at tro-popause level. This would lead to a sharp ledge of pol-lutants (aerosols) at tropopause levels.

The effect of small-scale eddy transport on theoverall stratosphere-troposphere mass exchangewould be quite smail, in the range between 1 and 5%.

From the above discussion of mass exchange pro-cesses, it appears that the relative importance of thedifferent processes involved in the mass exchangethrough the tropopause is not well understood atpresent and considerable ambiguity exists in thisaspect. It is therefore essential to make better esti-mates of the mass exchange through the tropopausefolds. Co-ordinated research efforts on a global scaleinvolving observations of the meteorological andchemical properties in the vicinity of the tropopauseon different scales are required in order to arrive atquantitative evaluations of the stratosphere-troposphere mass exchange processes.

3 Dynamical CouplingDynamical coupling between troposphere and

stratosphere involves transport of momentum and/or energy. This is achieved by the propagation of at-mospheric waves of different spatial scales and timeperiods.

Atmospheric waves originating in the tropical tro-posphere are responsible for the quasi biennial oscil-lation (QBO) in the lower tropical stratosphere andplay an important role in forcing the semi-annual os-cillation (SAO) in the tropical upper stratosphere. Ex-cellent reviews exist on this subject in literature31,32,dealing with the explanation of QBO and SAO in thetropics. The QBO in the tropical stratosphere is ex-plained theoretically+-" involving wave-mean flowinteraction. The theory is based on the vertical mo-mentum.fluxes carried by the eastward propagatingKelvin waves carrying westerly momentum and thewestward propagating mixed Rossby gravity (MRG)waves carrying easterly momentum to explain thedownward propagation of alternating westerly and

20

esterly wind regions of the QBO. The evidence for theexistence of these waves in the lower stratospherecame initially from the works of Yanai andMaruyama" and Wallace and Kousky ".

It is generally believed that the upward flux of waveenergy from the troposphere is capable of providing acontinuous source of excitation for the equatorialwaves. The upward flux of wave energy through thetropopause per unit area is estimated to be - 0.04W/m2 and - 0.005 W/m2 for the Kelvin and MRGwaves respectively (Ref. 31 ).These fluxes are small incomparison with the typical ranges of kinetic energygeneration in the tropical troposphere, which rangefrom - 0.02 W/m2 in synoptic scale disturbances '?to - 1 W/m2 in the monsoons. Observationalstudies ofYanai and Hayashi", Nitta'", and Yanai andMurakamr" provided further evidence for the tro-pospheric energy source for these waves. From theobservational evidence", it appears that the MRGwave in the lower tropical stratosphere may be a re-sponse to tropospheric forcing with the same periodbut with a different horizontal structure.

The tropospheric soruce for Kelvin wave genera-tion is less certain. Modelling studies'? suggest thatthe stratospheric Kelvin wave activity does not resultfrom a similar spectral distribution of disturbances inthe troposphere. It appears that the natural band passselectivity of the atmosphere combined with a forcingred noise spectrum in time and/or space may accountfor the stratospheric Kelvin waves. The model alsoshows that long period oscillations in the upper tro-posphere have no vertical propagation.

Results of the recent equatorial wave campaign ofIMAP (Ref. 43) show some interesting characteristicsof the equatorial disturbances. In the region 16-21 km, an upward phase propagation of disturbancesin the period range 12-20 days has been reportedfrom a study of rocket chaff data and balloon mea-surements at Trivandrum (8°, 6°N), SHAR (13.7°N)and Balasore (21SN) [Fig. 11]. Krishna Murthy eta/,43 have also reported strong disturbances (in thewind field) in the altitude region 23-30km with nophase propagation. They suggested that this regioncould be a source region for the disturbances prob-ably due to corresponding fluctuations in ozone. Theeffect of this source combined with the effect of strongeasterly jet present around tropopause level (Fig. 10)could cause changes in the phase propagation charac-teristics of wave disturbances from the troposphere.This may result in the reported upward phase propa-gation. Theoretical and modelling studies are neededto understand these aspects in a quantitative way. It islikely that different longitude zones behave different-ly in respect of tropospheric disturbances because ofthe different weather circulation systems (e.g., mon-

KRISHNA MURTHY: TROPOSPHERE - STRATOSPHERE COUPliNG

50 '..~..'(....

"

t-,-,'.j) I••• I.'J'I~.\

•••••• BALASQRE

-- - SH ••••- THUNU40

I t I • I ! I , I . I

-60 -50 -40 -30 -20 -10ZONAL WIND (m'S-')

10 20

Fig. l l=-Mean profile (over 20 days) of zonal wind at Balasore,SHAR and Trivandrum

soon in the south-east Asia). These aspects need fur-ther observational and theoretical studies. Thoughthe presence of various wave modes (Kelvin andMRG) has been experimentally observed, there is stillconsiderable amount of uncertainty in the observedcharacteristics in the sense that these are not consist-ent among all the experimental studies. Some of thedifferences are probably due to zonal differences re-lated to changes in forcing or changes in mean wind aspointed out earlier. There is a great need for coordi-nated observations in this regard. Also wave-meanflow interaction needs to be further studied both the-oretically and observationally.

It is now generally recognized that the lower strato-spheric equatorial disturbances are excited by theconvective latent heat release associated with the in-ter tropical convergence zone (ITCZ). Satellite imag-ery shows that the tropical convection is not regularand steady. The heating variance is mostly concen-trated spatially within three localized centres, name-ly, Indonesia, the Amazon and the Congo, and theconvective activity within these regions undergoesirregular development over a span of couple of days.Salby and Garcia'" studied the stratospheric re-sponse to such convective activity in the troposphereby modelling and obtained the characteristics of thewave components to be in agreement with the theo-

retically expected ones for the Kelvin and MRGwaves. Such modelling studies incorporating thesource characteristics are needed to understand thetroposphere-stratosphere dynamical coupling in thelow latitudes. Another area of dynamical couplingwhich has been studied quite extensively is suddenstratospheric warming phenomenon. This appears asa strong increase of temperature in middle and highlatitudes of the winter-spring hemisphere, associatedwith planetary zonal wave-I and/or 2 disturbances inthe stratospheric and tropospheric circulation. Ex-tensive descriptions of the phenomenon are given byQuiroz et al.4s, Labitzke'" and Shoeberl" amongothers, while model simulations have been made byMatsuno?", Holtorr" and Shoeberland Strobel'". Themodels essentially start with the establishment of ablocking type of circulation pattern in the tropo-sphere. This pattern causes planetary zonal wavenumber 1 and/or 2 to grow to large amplitudes. Thelarge amplitude waves propagate into the stratos-phere and cause deceleration of the mean zonalwinds. The polar night jet (which is a characteristic ofthe winter stratospheric circulation] weakens andgets distorted due to this interaction. When the plane-tary waves are sufficiently strong, the mean zonal flowdecelerates sufficiently to form critical level (the levelwhere the wave phase speed is equal to the mean windspeed). Then, further upward transfer of energy getscompletely blocked and a very rapid easterly acceler-ation and polar warming occurs at the critical levelwhich must then move downward until eventually thewarming and zonal wind reversal affect the entire po-lar stratosphere. High latitude warming is accompan-ied by cooling at low latitude (summer hemisphere)consistent with the concept of poleward heat flux in-corporated in the model simulations. Observationalevidence for low latitude cooling/warming has re-cently come from the sudies of Mukherjee et al.50and Appu 51.

The models of stratospheric warming reproducemany of the observed features. However, there arestill some important questions to be understood, re-lated to (1) relationship between stratospheric warm-ing and large planetary wave amplitudes in the tro-posphere (since strong tropospheric wave amplitudeis not a sufficient condition for strong stratosphericwarming), and (2) relevant heat transport patterns inthe troposphere and stratosphere involving knowl-edge of the vertical structure of the responsible eddiesand origin of amplifying waves in the troposphere.

In order to explain the observed features of timevariation of stratospheric wave activity, Shiotani andHirota-? presented two hypotheses involvingtroposphere-stratosphere coupling, namely, (1) a di-rect excitment of wave activity by the enhanced wes-

21

INDIAN J RADIO & SPACE PHYS, VOL 16, FEBRUARY 1987

terlies in the high latitude troposphere, and (2) a 'shut-ter' mechanism at around.tropopause level to affectthe stratospheric activity.

The abov~ discussion mainly pertains to tropo-spheric disturbances affecting the stratospheric con-ditions. That changes in stratosphere can significantlyaffect the troposphere (dynamically) has been sug-gested by Hines", The tropospheric ally generatedplanetary waves propagate their energy upwardwhere the stratospheric winds play an important rolein determining the 'refractive index' (Ref. 54), thus af-fecting the transmission and reflection of these waves.Many modelling studies have been carried out on thisaspect with apparently different results": Jung andLindzen " indicated that changes must take place inthe middle stratosphere, where dissipative processesare small, to affect tropospheric disturbances.Krishna Murthy et a1.43, from stratospheric andtropospheric wind observations, suggested that stra-tospheric disturbances may cause the observed phasechanges in the upper tropospheric waves.Ramanathan " suggested that a combined dynamical-radiative coupling process may be important. He sug-gested that during a stratospheric warming, the highlatitude stratosphere emits enhanced infrared radia-tion to significantly decrease the pole to equator tem-perature gradient in the troposphere.

4 Radiative CouplingStratosphere and troposphere are radiatively cou-

pled through transfer of radiant energy through thetropopause. The stratospheric effects due to radia-tion from the troposphere are not yet well under-stood, whereas the tropospheric effects of strato-spheric changes (radiative) have received consider-able attention in recent years. The stratosphere re-ceives short wave radiation reflected from the sur-face, clouds and tropospheric aerosols (includingscattered part) and long wave radiation emitted by allthese and by tropospheric CO2 and H20. Absorptionby ozone in the long wave region (9.6 .urn) is of specialinterest since it allows effective radiative coupling be-tween surface/cloud tops to the lower stratosphere.The stratosphere affects the radiation reaching thetroposphere, thus affecting its radiative properties.

4.1 Radiative Effects oro)Reduction in stratospheric ozone allows more so-

lar radiation to reach the troposphere but because ofcooling in the stratosphere, less infrared radiation isemitted downwards. Studies by Ramanathan andDickenson" showed that these two balance eachother. However, they find that infrared effect is ap-proximately uniform with latitude, while the solar ef-fect is stronger at the equator than at the poles on an

22

annual average basis, so that there is likely to be a netcooling in high latitudes.

4.2 Radiative Effects of AerosolsAerosols play an important role in the radiative

coupling between the stratosphere and troposphere.Aerosols scatter, absorb and emit radiation dependingupon their refractive index (complex) and size distri-bution. While the characteristics of stratospheric aer-osolare reasonably known (though still gaps exists inour knowledge in respect of their size distribution, re-fractive index and variability on short time scales), thecharacteristics of the tropospheric aerosols are notwell understood mainly owing to their highly variablenature. Extensive modelling studies have been carri-ed out on the radiative effects of aerosolsv'?", How-ever, in order to obtain realistic estimates of their ra-diative effects, their physical and chemical propertiesneed to be studied on a global basis,

The effects of the recent eruption of El ChichonVolcano (in Mexico) on the stratospheric aerosolhave been well documented through SAGE II Satel-\.itestudies and Lidar and other ground-based observ-ations?'. The impact of the EI Chichon cloud onearth's radiation has become evident through satelliteobservations. The NOAA's Advanced Very High Res-olution Radiometer (AVHRR) indicated changes insea surface temperature (SST) due to infrared ab-sorption by the volcanic cloud. The radiative effectsof the volcanic cloud on the atmosphere were also im-mediately observed with significant changes in thestratospheric temperatures'", The magnitude of er-uption is so high (the total global stratospheric in-crease due to the cloud is estimated to be about 12magatonnes) that it provides a real test for the variousmodels on aerosol radiative effects.

Coordinated observations on tropospheric andstratospheric aerosol properties using different tech-niques (ground-based and space-borne) at differentlocations are needed to characterize the aerosols andto develop realistic models useful in estimating theirradiative effects.

5 Electrical CouplingElectrical coupling between troposphere and stra-

tosphere involves perturbations in the global electri-cal circuit'". Fig. 12 (from Markson=') shows the glo-bal electrical circuit. In Markson's model, the loadportion of the global circuit in the fair weather regionhas a global resistance of - 102 ohms. Assumingthere are about 1500 thunderstorms (with typical cur-rent output ofa thunderstorm as 1Alovertheearth'ssurface at any given time, the total global currentworks out to be 1500 A. The tops of the thunderstormclouds are at potentials of 1OH-109V,giving a charging

KRISHNA MURTHY: TROPOSPHERE - STRATOSPHERE COUPLING

Fig. 12- The atmospheric electrical global circuit. Large arrowsindicate flow of positive charge.

resistance of 105-106 ohms over the thunderstormgenerator (considering all of them to be acting inparallel). So, the thunderstorm generator acts as a cur-rent generator. The resistance between the thunder-storm and the earth is estimated to be of the order of104-105 ohms, the reduction being the result of coro-nal discharges under the thunderstorm clouds due tovery high electric fields. The controlling resistance inthe global electrical circuit is the resistance betweenthe top of the thunder clouds and the ionosphere.Thus changes in the global electrical circuitry can bebrought about by changes in the stratospheric con-ductivity. Stratospheric conductivity changes can becaused by events such as solar proton events on ashort time scale and by cosmic ray modulation onlonger scales. Herman and Goldberg'" suggested thatif appropriate meteorological conditions exist or de-velop during a solar proton event, the atmosphericelectric field enhancement may be sufficient to triggerthunderstorm development.

To understand the global electrical circuit in detail,and the electrical coupling between the troposphereand stratosphere and its effect on troposphericweather, coordinated experimental efforts involvingsatellites and ground-based experiments at differentlocations covering different regions are required.

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