LaFísicadelaConvecciónAtmosférica

enlosTrópicosBenjaminR.Lintner(Rutgers)

DavidK.Adams(UNAM)KathleenA.Schiro(UCLA)

UniversidadNacionalAutónomadeMéxico

01/25/16-01/29/16

Coursesyllabus*01/25:Introduc8ontoandOverviewofTropical

Convec8on01/26:PartI:TheoriesofTropicalConvec8on

PartII:Convec8veCri8cality(KathleenSchiro)01/27:TropicalConvec8onanditsCouplingtotheLarge-

Scale01/28:PartI:Representa8onofTropicalConvec8onin

Models PartII:TheShallow-to-DeepConvec8ve Transi8on(DaveAdams)

01/29:TropicalConvec8onandtheLandSurface

*Thechoiceoftopicshereisbasedonourpar8cularsetofresearchinterests,sosomeobviouslyimportantaspectsoftropicalconvec8on,e.g.,tropicalcyclones,willnotbecovered.

Expecta8ons•  TolearnaliVleabouttropicalconvec8on(hopefully)

•  Tolinktoresearchareas/topicsofinterestamongthepar8cipants–  1)HighResolu8onNumericalModelling(WRF)[ArturoQuintanar/Carlos

Ochoa]–  2)WaterVaporTransportandMonsoonVariability[PaulinaOrdoñez]–  3)Land-AtmosphereInterac8on[FrisoHolwerda/LyseVeMuñoz]–  4)ColdPools/SquallLinesModeling[DiegoAlfaro]–  5)AtmosphericConvec8on/Thermodynamics[DavidAdams]

•  Tos8mulatediscussionandencouragecollabora8on

Lecture1Topics

•  Fundamentalconcepts,defini8ons,andequa8onsrelevanttounderstandingtropicalconvec8on

•  Overviewoftheprincipalobservedspa8alandtemporalfeaturesoftropicalconvec8onandtheirrela8onshiptoourunderstandingoftropicalweatherandclimate

Anvil

Cumulonimbus Hot Tower

Cumulus congestus

NASAImageoftheDayGallery

Cumulonimbus“HotTower”

NASAImageoftheDayGallery

Cloudsinthetropics

Cumulus convection In the troposphere, the vertical transport of relatively warm parcels from the surface to upper troposphere through the cores of large cumulonimbus clouds is partially responsible for determining the mean temperature profile.

Cumulus convection, from the Latin for “heap” and “to carry together,” represents the process by which an air parcel less dense than its surroundings rises and forms a convective cloud.

The physical basis of cumulus convection is Newton’s 2nd Law: a positively buoyant parcel, i.e., one that is less dense than its environment, experiences upward a c c e l e r a t i o n k n o w n a s a convective updraft.

Source:Myson(6yearsold)

Hydrosta8cequilibrium&buoyancy

€

F = m a

(ρδxδyδz)az =−(ρδxδyδz)g+ p(z)δxδy− p(z+δz)δxδy

Newton’s2ndLawofMo8oninthezdirec8on:

Considernow:

z

z

z+δz

ρ

x

yp(z)

p(z+δz)

gδy

δx

Inequilibrium(az=0),andapplyingthelimitδz→0,€

0 = −ρg− ∂ p∂z

ρ’

Considerfirstthecasewithequalenvironmentalandparceldensi8es,i.e., !ρ = ρ

!ρ ≠ ρ

(ρδxδyδz)az = −(ρδxδyδz)g+ p(z)δxδy− p(z+δz)δxδy

⇒ az = −g+1ρ∂ p∂z

= g ʹρ − ρρ

⎛

⎝⎜

⎞

⎠⎟ ≡ B Buoyancy

Hereprime(‘)referstothesurroundings(orenvironment)

1stLawofThermodynamics(alsoknownasenergyconserva8on)

Whenaparcelofairexpands,itdoesmechanicalpressure-volumeworkonitssurroundings.Thedifferen8alformofthis:Hereαisthespecificvolume[theinverseofdensity].Doingworkrequiresenergy.Whatisthesourceofthisenergy?From(1)theenvironment,intermsofheatflowintotheparcel;(2)aformofstoredenergy,or(3)acombina8onofthetwo.Wedenotethestoredformofenergy(2)asinternalenergyu.*

So,wehavethemathema8calstatementofenergyconserva8onorthe1stLawofThermodynamics:

δw = pdα

δq = du+ pdα

*Foranidealgas,uisafunc8onofTonly,withdu=cvdT,withcvthespecificheatatconstantvolume.

Adiaba8cprocessesAnadiabaUcprocessisoneinwhichheatexchangebetweenasystemanditsenvironmentisnegligible,i.e.,δq=0.Adiaba8cprocessesareofgreatimportanceinmeteorologicalapplica8onsbecausemanychangesaffec8ngmovingvolumesofairintheatmospherecanbeeffec8velyapproximatedasadiaba8c.DryadiabaUcreferstodryair.MoistadiabaUcreferstomoistair,whichisslightlymorecomplicatedbecauseitmayinvolveaphasechangeofwater(evapora8onorcondensa8on)withintheparcel.Notethatmoistadiaba8cprocessesarenotstrictlyadiaba8cinthesensethattheyinvolveaheatexchange(duetophasechange),butthisisopenapproximatedasremaininginternaltotheparcel.

Dryadiaba8clapserateConsiderthefollowingformtheofthe1stLawforaparcelunder(dry)adiaba8ccondi8ons:

Dividingthroughbydzandapplyinghydrosta8cbalancegives:

Undermostcondi8ons,theparcelandenvironmentaltemperaturesarecomparable;thus,thedryadiaba8clapseratecanbeapproximated:

δq = cpdT −αdp = 0

cpdTdz

−αdpdz

= 0⇒ cpdTdz

= −α #ρ g

ApplyingtheIGLforboththeparcel(toeliminateα)andenvironment(toeliminateρ’):

cpdTdz

= −RdTp

pRd "T

g⇒ dTdz

= −gcpT"T

Γd ≈gcp= 9.8Kkm-1

Poten8altemperatureStar8ngwithelimina8ngαusingtheIGL,andintegra8nggives:

T0T=

p0p

⎛

⎝⎜

⎞

⎠⎟

(Rd cp)

Thisrela8onshipisknownasPoisson’sequaUon,andfromit,wedefineanewthermodynamicvariable,thepotenUaltemperatureθ.Inpar8cular,seung(T0,p0)=(θ,1000mb)gives:

θ = T 1000 mbp

!

"#

$

%&

(Rd cp)

Thepoten8altemperatureisinterpretedasthetemperatureaparcelwouldhave,star8ngfromarbitrary(T,p),andthenundergoingadiaba8ccompressionorexpansiontoapressureof1000mb.Bydefini8on,foradryadiaba8cprocess,θisconserved.

δq = cpdT −αdp = 0

Foradryadiaba8cdisplacementConsideraparcelundergoingadryadiaba8cdisplacementfromanini8alposi8ontoanewposi8onseparatedbyadistanceδz.Atthenewposi8on

az =dwdt

= g ʹρ − ρρ

z

z0

z0+δz

ϑ(z0)

ϑ’(z0)=ϑ(z0)

ϑ’(z0+δz)ϑ(z0+δz)=ϑ(z0)

= g θ!θ−1

#

$%

&

'(= g

θθ +δz ∂θ∂z

−1#

$%

&

'(

≈ g 1− 1θ∂θ∂zδz

$

%&

'

()−1

*

+,

-

./= −

gθ∂θ∂zδz

⇒d 2δzdt2

+ N 2δz ≈ 0; N 2 =gθ∂θ∂z

NistheBrunt-Väisäila(orbuoyancy)frequency.

= gT − ʹTʹT

= gθ − ʹθʹθ

[Buoyancy]

[IGL+def.θ]

[Lapserate]

[Taylorexpansion]

*

*Forthemoregeneralcaseofmoistair,weshouldusevirtualtemperature: Tv = 1+ q ε−1 −1( )"

#$%T

Dryadiaba8cdisplacement,environmentallapserate,andstability

€

d2δzdt 2

+ N 2(z)δz = 0 N(z) = g Γd −Γ#T (z)

IfΓ<Γd,N2(z)>0,solu8onstotheaboveequa8onareoscillatory(sinusoids):thedisplacementoftheparcelawayfromitsequilibriumposi8onresultsintheparcelreturningtothatposi8on.Inthiscase,theequilibriumisstable.

IfΓ>Γd,N2(z)<0,solu8onstotheperturba8onequa8onareexponen8al(hyperbolicsine/cosine):thedisplacementoftheparcelfromitsequilbriumposi8onresultsintheparcelmovingawayfromthatposi8on.Inthiscase,theequilibriumisunstable.

Temperature

Height

StableSlope:-Γd

Slope:-ΓUnstable

≈ −g (Γd −Γ)$T

δz

T (z0 +δz) ≈ T (z0 )−Γdδz

!T (z0 +δz) ≈ T (z0 )−Γδz

dwdt

= g !ρ − ρρ

= gT − !T!T

Mixingra8oandspecifichumidityWewillopenusethera8oofwatervapordensitytodryairdensity,whichiscalledthemixingraUo,w:whereeisthevaporpressureandε=0.622isaconstant(thera8oofgasconstantsfordryairtowatervapor).Notethatmixingra8oisaconservedquan8tyaslongasnophasechangesoccur.Typically,wisexpressedinunitsof“gofvaporperkgofdryair.”Therangeofwvalueswithintheatmosphereis~afewgkg-1incool,dryairmassesto~20gkg-1inmoist,tropicalairmasses.Arelatedquan8tyisspecifichumidity,q:

w = ρvρd

≈εep

q = ρvρ=εep

[ApplyingtheIGLtoreplacedensi8esbypressures]

Satura8onIt is only possible for so much water vapor to occupy a givenvolumeofair,i.e.,theaddi8onofvaporbeyondacertainpointwillleadtotheforma8onofliquidwater(orice). Thepointatwhichthis phase change occurs is called saturaUon. (Note that thesatura8onpointsforliquidwaterandiceareingeneraldis8nct.)If the volume contains less water vapor than is required forsatura8on to occur, it is referred to as subsaturated orundersaturated. If it contains more vapor, it is referred to assupersaturated.In general, the atmosphere is subsaturated. For saturated orsupersaturated condi8ons, fog, clouds, and precipita8on mayoccur.

PeVy:Figure7.1

LatentheatWhat happens to the liquid as the energeMc molecules escape duringevaporaMon?Because these molecules “take” their kine8c energy with them, theaverage kine8c energy—and thus the temperature—of the remainingliquidisreduced.Thatis,evapora8onisacoolingprocess.Thetotalenergyrequiredtoconvertaunitmassofsubstancefromonephase to another, at constant temperature and pressure, is called thelatentheat.Notethatlatentheathasa(typicallyweak)dependenceontemperature.�Fortheliquidtovaporàlatentheatofvaporiza8onorcondensa8on,L[~2.5x106Jkg-1]� Forthesolidtoliquidà latentheatoffusionormel8ng,Lf[3.3x105kg-1]� Forthesolidtogasàlatentheatofsublima8on,Ls=L+Lf

Satura8onvaporpressureWedefinethesaturaUonvaporpressure,es,asthevaporpressureat which satura8on occurs. Thus, subsatura8on is the state forwhiche<es,whilesupersatura8onhase>es.Thesatura8onvaporpressurehasthefollowingproper8es:(1)Itisamonotonicfunc8onoftemperature,i.e.,esincreasesasTincreases;(2)Itdependsonlyontemperatureandisnotinfluencedbyothergases.The boiling point of water corresponds to a satura8on vaporpressure equaling the ambient atmospheric pressure. At meansea level, the boiling temperature is 100°C, since es(T=100°C) =1013 mb. With increasing al8tude, atmospheric pressuredecreasesandthustheboilingtemperatureisreduced.

Phasediagramforwater

TheuniquenessofH2Oundercondi8onsofearth’satmosphere!

Rela8vehumidityThe relaUve humidity (or saturaUon raUo), RH, is the ra8o ofwatervaporpressuretosatura8onvaporpressure:RHisexpressedineitheradecimalorpercentagebasis.Supersatura8onoccursforRH>1(or100%).ThesupersaturaUon,s, isgivenbys=RH[in%]–100%. Supersaturatedcondi8onsinthe atmosphere generally occur when the rate of temperaturechange is so rapid that condensa8on cannot remove the vaporexcess.Inconvec8veupdraps,smayreachafewpercent.HumansarephysiologicallyresponsivetoRHbecauseit isrelatedto evapora8on. Thus, on a day with high RH, wemay becomeoverheatedsincethehumanbody’sprincipalcoolingmechanism,evapora8on from skin (perspira8on), is insufficient to dissipatebodyheat.

€

RH =e

es(T)

Satura8onmixingra8oRecallingthedefini8onofmixingra8o,,wecandefinethesaturaUonmixingraUoas:There is a unique value of ws for a given T and p. An importantconsidera8on for parcels undergoing ver8cal displacements is howwschangeswithheight.Consider: For adiaba8cascent, pressuredecreases, andby thePoissonequa8on, temperature also decreases. By the defini8on of ws, adecrease in temperatureshoulddecrease itsvalue,whileadecrease inpressureshould increase itsvalue. Whichchange“winsout”? Itturnsoutthatthetemperaturedecreaseprevails.Thus, the decrease of satura8on mixing ra8o with height means asubsaturated parcel will undergoing ascent will ul8mately reachsatura8on.

w = εep− e

≈εep

€

ws =εes(T)p − es(T)

≈εes(T)p

Ploungthermodynamicvariables:theskew-Tdiagram

skew-t.com

DewpointtemperatureSupposee<es(T).Thedewpointtemperature,Td,representsthetemperaturetowhichtheairwouldneedtobecooled(assumingfixedvaporpressure)forsatura8ontooccur,i.e.,Both dewpoint temperature and rela8ve humidity are related tothedegreetowhichediffersfromes.Meteorologistsopenpreferdewpoint temperature as ameans of characterizing atmospherichumiditybecauseitonlydependsontemperature.The dewpoint is related to the forma8on of condensa8on onsurfacesaswellastheforma8onofcloudsandfog.

€

es(Td ) = e

Graphicales8ma8onofLCLSincemixingra8o isconservedforadry adiaba8c process, it is alwayspossible to decreasep un8lws=w,i.e., satura8on is achieved. Thepressure at which satura8on isachieved is the li\ing condensaUonlevel (LCL). The LCL defines thecloud base under forced ascent orfreeconvec8on.On a skew-T, the LCL of a parcel attemperature T, dewpoint Td, andpressure p is determined by theintersec8on of the dry adiabatpassing through T and vapor linepassingthroughTd.

noaa.gov

QuesMon:HowwouldthesurfaceparcelenvisionedheregettotheLCL?

AbovetheLCLSupposewe have a parcel ascend toits LCL. What happens with furtherascent?Sincetheparcelissaturated,further ascent causes condensa8on.This condensa8on releases latentheat, which par8ally offsets thedecrease in internal energy withdecreasingtemperature.Theresultantlapseratefortheparcelwill thus be smaller than dryadiaba8c.

LCL

Subsaturated

Saturated

Equivalentpoten8altemperature

Δq = −Ldqs

Theamountoflatentheatreleaseduetocondensa8onisgivenby:

cpTd lnθ = cpdT −αdp

FromthePoissonequa8onandthe1stLawandequa8ngwiththelatentheatrelease,whichisassumedtowarmtheparcel:

AssumingthechangeinqsislargecomparedtoeitherchangesinTorLcandintegra8ngbetweentwostates(θ,qs)and(θe,0)gives:

d lnθ ≈ −d LcqscpT

⎛

⎝⎜⎜

⎞

⎠⎟⎟⇒ ln θ

θe

⎛

⎝⎜

⎞

⎠⎟= −

LcqscpTLCL

⇒θe ≈θ expLcqscpTLCL

⎛

⎝⎜⎜

⎞

⎠⎟⎟

ConsiderasaturatedparcelofairatatitsLCL(q=qs)undergoingpseudoadiabaUcascent,i.e.,ascentinwhichanycondensa8onproductsareassumedtofallout(carryingonlysmallamountsofheatwiththem).

= −Ldqs

θe canberegardedasthemaximumθthatcouldbeachievedifallofthevaporintheparcelcondensedout.

Determiningϑeonaskew-TConsideraparcelat temperatureT, dewpoint Td, and pressure p.To find ϑe, one first locates theLCL as before. Above, one thenfollowsthemoistadiabatpassingthroughtheLCLuptosufficientlyh i g h a l 8 t ude s o t h a t w sapproaches0. Thisresults inthemoist adiabat asympto8ng to aunique dry adiabat. One thenfollows this dry adiabat down to1000mb.

noaa.gov

Pseudoadiaba8clapserateStar8ngwiththe1stLaw,Poissonequa8on,andcondensa8onalhea8ngandconsideringthechangeinthever8cal:

cpd lnθdz

= cpd lnTdz

− Rdd ln pdz

= −LTdqsdz

Sinceqsisafunc8onofTandp,wecanexpandinitsfirstpar8alderiva8vesandapplythehydrosta8crela8onshiptoget:

No8ngthatand[i.e.,Clausius-Clapeyron]gives:

€

qs ≅0.622es(T)

pdesdT

=0.622LesRdT

€

∂qs∂p T

≅ −0.622esp2

= −qsp

€

∂qs∂T p

≅0.622p

desdT

=0.622LcqsRdT

2

dTdz

+Γd = −Lcp

∂qs∂T p

dTdz

⎛

⎝⎜⎜

⎞

⎠⎟⎟+Γd

LpRdT

∂qs∂ p T

Thus,thepseudoadiaba8clapserateisgivenby:

Γs ≡ −dTdz

= Γd1+ (Lqs ) / (RdT )[ ]

[1+ 0.622L2qs / (cpRdT2 )]

MoistStability

Temperature H

eigh

t

Absolutely Stable Γ=Γd

Absolutely Unstable

Forthestabilityofmoistair,wehave3possibili8es:

AbsolutelyStable

AbsolutelyUnstable

Forunsaturatedair,thepurpleregionisstable;forsaturatedair,itisunstable.

Γ=Γs

Conditionally Unstable Γ<Γs

Γ>Γd

Γs<Γ<Γd CondiUonallyUnstable

CAPEandCIN

CAPE = BdzzLFC

zEL

∫

= g "ρ − ρρ

dz =zLFC

zEL

∫ gθ − "θ"θdz

zLFC

zEL

∫

Convective available potential energy (CAPE) represents the energy reservoir to “drive” deep convection.

CAPE is defined in terms of the vertically-integrated buoyancy between the LFC and EL and is, thus, by definition positive.

CAPE can be estimated from a skew-T diagram as the positive area between the environmental T profile and the moist adiabat passing through the LCL.

The convective inhibition (CIN) represents the energetic barrier to deep convection; is is defined as the integral over the negative buoyancy region between the surface and LFC.

CAPEandconvec8veupdrapsSince CAPE is effec8vely poten8al energy per unit mass, by energyconserva8on,itcanrelatedtothemaximumpossiblekine8cenergyperunitmass aparcelmayachieve. Thus,we canuse it to es8mate themaximumconvec8veupdrapvelocityby: Underrealis8ccondi8ons,actualupdrapveloci8eswillgenerallybelessthanwmax, by up to a factor of 2, since factors such as entrainment ofenvironmental air in ascending air masses can reduce buoyancy and thusCAPE. CAPEinthunderstormsistypically1000-2000Jkg-1[orm2s-2]butcanexceed4000Jkg-1.NotethatforCAPE=1500Jkg-1,wmax=55ms-1! Ontheotherhand,fortropicaloceanicconvec8on,whichtendstohavesmalldifferences between parcels and their environmental, CAPE and convec8veupdrapveloci8estendtobelowerthaninmidla8tudesoroverland.

wmax = 2CAPE

Dryandmoiststa8cenergiesTwo other quantities to define for later reference are the dry and moist static energy, s and h, respectively, which are given by:

s and h are related to θ and θe, respectively. For example, taking the natural logarithm of θ gives:

The differential of this expression is:

lnθ = lnT − Rdcp(ln p− ln p0 )

s = cpT + gdzh = s+ Lcq

d lnθ = dTT−Rdcp

dpp

s is conserved (and h approximately conserved) for a dry adiabatic (moist adibatic) process.

=dTT+Rdcp

gdzpα

=dscpT

Fieldcampaignsforsamplingthetropics

Comet

LecturebyProfessorRobertHouze,2011SummerSchoolonSevereandConvec8veWeather

Meanrhprofiles

Comet

Meanϑandϑeprofiles

Comet

MostpronouncedΔθbetweenconvec8veandfairweathercondi8onsseeinthemid-troposphere(~500mb),withΔwmostlyresponsibleforthedifference.

Comet:Lucas&Zipser[2007]

Thermodynamicdifferencesbetweenconvec8veandnonconvec8vecondi8ons

Asseverityofconvec8onandprecipita8onamountincrease,theminimainlowerfreetomid-tropospherearereduced.

Comet:Aspliden[1976]

Barbadosθeprofilesclassifiedbyrainamount

Pressure(m

b)

EquivalentPotenUalTemperature(K)

sandhthroughaconvec8vesystem

Bothdryandmoiststa8cenergiesdropsharplyattheleadingedgeandslowlyrecoverinthewakeregion. Comet:Barnes&Sieckman[1984]

h(kJk

g-1 )s(kJk

g-1 )

Distancefromleadingedge(km)

Height(k

m) PropagaUon

Tropicalconvec8onandmoisture

Asynop8c-scaleincreaseintroposphericmoistureisevidentroughly24-36hoursbeforetheonsetofstrongprecipita8oneventsatNauruinthetropicalwesternPacific.Amesoscaleincreaseoccurs~6hoursbefore.

Holloway&Neelin[2010]

High-frequencyprecipita8onsta8s8cs

Usingsatelliteobserva8ons,Peters&Neelin[2006]firstiden8fiedapower-lawrela8onshipbetween“instantaneous”precipita8onratesandcolumnwatervapor.Foroceanicregions,the“cri8calvalue”ofcolumnwatervaporscaleswithtropospherictemperature.

Thebehaviorhasbeendescribedintermsoftheconceptof“self-organizedcri8cality.”MuchmoretofollowwithKathleen’spartoftomorrow’slecture! Sahanyetal.[2012]followingPeters&Neelin[2006]

Anatomyofacumulonimbus

Important features here include: The lifting condensation level (LCL, not shown), which is the level at which an initially unsaturated parcel reaches saturation, i.e., it’s relative humidity reaches 100%. The level of free convection (LFC) is the level at which the parcel first becomes warmer, less dense, and positively buoyant, with respect to its environment. The equilibrium level (EL) is the level at which the parcel becomes as cool (dense) as the environment. Parcels reaching the EL do not stop abruptly but will oscillate [N2 changes sign at this level]. Lateral motions near the cloud top—associated with outflow of the updraft air mass—may form an anvil cloud.

The schematic below illustrates a mature cumulonimbus hot tower.

Comet

Precipita8onandconvec8vedowndrapsWith condensation, water or ice droplets form and after these have acquired sufficient mass, they begin to fall. The relatively cold—and negatively buoyant—air “dragged” with falling precipitation comprises a convective downdraft. Downdrafts are faster under cooler and/or drier conditions, e.g., the inflow of relatively dry environmental air and evaporation of falling rain both increase downdraft velocities. A cold pool forms beneath the precipitating cloud, spreading out laterally in the planetary boundary layer.

Comet

Mesoscaleconvec8vesystems

Mosttropicalprecipita8onoccursinorganizedconvec8vesystemscalledmesoscaleconvecUvesystems(MCSs).MCSshavecharacteris8cspa8alscalesororder102-103kmandlife8mesofseveralhourstoaday.

NASAEarthObservatoryImageoftheDay

COMET

Distribu8onofMCSsizesandlife8mes

Chen & Houze [1997]

MCSstructureWe have largely developed the thermodynamic framework for convection in terms of individual elements (buoyant parcels) ascending from the ABL. However, organization of convection into an MCS involves another process, called layer lifting, in which a conditionally unstable low-level air layer (albeit typically deeper than the ABL) enters and rises through the MCS. Precipitation in an MCS is both convective and stratiform. The former is generally more intense. As the MCS ages, stratiform fraction tends to increase.

Comet

MoredetailedanatomyofanMCS

Zipser [1977]

Tropicalsqualllinelifecycle

MCSHovmollerplot

Propagating w/ wave velocity

“Bifurcation” into 2 MCSs?

Chen et al. [1996]

Convective vs. stratiform precipitation

Convec8veprecipita8onratesaretypically3-6xlargerthanstra8formrates(andhigheroverland),whilestra8formprecipita8onoccursoveralargerareafrac8on.Withinthetropics,stra8formrainfrac8onasapercentageoftotalaccumulatedprecipita8onis~40%;thisfrac8onincreasesbeyond~20°,reaching~50%beyond30°.

Schumacher & Houze [2003]

Shallowconvec8veprecipita8on

Anothercategoryoftropicalrainfallwhichoccursinisolatedcumulusclouds[i.e.,notassociatedwithMCSs]Shallowconvec8veprecipita8onisgenerallyweakandislargelyconfinedtotheoceanbasinsadjacenttostrongerconvec8on.

LandversusoceanMCSsOnecategoryofMCS,mesoscaleconvec8vecomplexes(MCCs)⎯whichareintense,long-las8ng,quasi-circularMCSswithextremelyhigh,coldcloudtops⎯occursmorecommonlyoverland.

Liang & Fritsch [1997]

From S. Nesbitt (U. Illinois)

Lightningstrikesaremorecommonoverlandthanocean,reflec8ngdifferencesinicepar8clecharacteris8cs.

Diurnalcycles:landversusocean

Tropicaloceanicregionsexhibitweakdiurnalcycleswithmorning-8memaximaaroundlocalsunrise.Tropicallandregionsexhibitstrongdiurnalcycleswithlateapernoonmaxima.MoretofollowwithDave’spartofThursday’slecture! Comet

Regionaldiurnalcycles:Mari8mecon8nent

1 2

3 4

1.  Late evening: maximum precipitation near high topography

2.  Midnight

3.  Morning: max rainfall over ocean/min over land

4.  Early afternoon

Comet

Annualmeanprecipita8on

NCARClimateDataGuide

Precipita8onseasonality

Comet