A mosphe ic Bounda y Laye in he A lan ic: he dese dus impac
Ioanna Tsikoudi1,2, Eleni Ma inou1, Ma ia Tomb ou2, Eleni Giannakaki2, Emmanouil P oes akis1,
Kons an inos Rizos1, Ville Vakka i3,4, and Vassilis Ami idis1
1Na ional Obse a o y o A hens, IAASARS, G eece
2Depa men o Physics, Na ional and Kapodis ian Uni e si y o A hens, G eece
3Finnish Me eo ological Ins i u e, Finland
4A mosphe ic Chemis y Resea ch G oup, Chemical Resou ce Bene icia ion, No h-Wes Uni e si y, Po che s oom, Sou h
A ica
Co espondence: Ioanna Tsikoudi ([email p o ec ed])
Abs ac . We in es iga e he dynamics o he a mosphe ic Bounda y Laye (BL) o e he A lan ic Ocean, wi h a ocus on he
egion su ounding Cabo Ve de du ing he Join Aeolus T opical A lan ic Campaign (JATAC) and he ASKOS expe imen ,
using a combina ion o g ound-based PollyXT and Dopple lida s, sa elli e lida da a om Cloud-Ae osol Lida and In a ed
Pa h inde Sa elli e Obse a ions (CALIPSO), adiosondes, and he model ou pu s o he In eg a ed Fo ecas ing Sys em (IFS)
o he Eu opean Cen e o Medium-Range Wea he Fo ecas s (ECMWF). The compa ison o CALIPSO lida esul s wi h5
ECMWF/IFS eanalysis o 2012-2022, e ealed s ong co ela ions o BL op o e open ocean egions bu weake ela ion
o e dus -a ec ed a eas close o he A ican con inen . In hese egions, space lida indica ed lowe BL ops du ing day ime
han hose es ima ed by ECMWF/IFS. Obse a ions in Cabo Ve de highligh dis inc i e Ma ine A mosphe ic Bounda y Laye
(MABL) cha ac e is ics, such as limi ed diu nal e olu ion, bu also show he po en ial o BL heigh s o each up o 1 km, d i en
by ac o s like s ong winds ha inc ease mechanical u bulence. Addi ionally, he challenges in es ima ing he BL heigh using10
lida -de i ed ae osol mixing heigh e sus p o iling o me eo ological pa ame e s acqui ed om adiosondes a e illus a ed,
examining cases wi h s ong and weake in e sions ha a ec he e ical mixing and he pene a ion o dus pa icles wi hin
he BL. The indings unde line he need o u he imp o emen s in he ECMWF/IFS eanalysis model owa ds cap u ing
he complex in e ac ions be ween ma ine and dus -laden ai masses o e he A lan ic, which a e essen ial o cons aining he
dynamic p ocesses in BL and ae osol-cloud in e ac ions.15
1 In oduc ion
The a mosphe ic Bounda y Laye (BL) is cha ac e ized by complex in e ac ions be ween su ace-d i en o ces and me eo o-
logical condi ions, which de e mine i s heigh , s uc u e, and he deg ee o u bulen mixing wi hin (S ull, 1988). BL dynamics
a y conside ably ac oss di e en en i onmen s, p esen ing challenges o wea he modeling and p edic ion, especially in
ansi ional zones like hose be ween dese s and oceans (Seibe e al., 2000; Li e al., 2017).20
Moni o ing he BL op eliably is a challenge, pa icula ly in he e ogeneous en i onmen s whe e adi ional obse a ion
me hods may all sho . Lida sys ems ha e p o en aluable o con inuous p o iling o ae osol and a mosphe ic s uc u es, as
hei high e ical esolu ion enables de ailed moni o ing o BL heigh (Wiegne e al., 2006; Baa s e al., 2008). Ye , au oma ic
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iden i ica ion o he BL op om lida da a is challenging in complex a eas, because BL s uc u es can be in luenced by
su ace ype, ime o day, and a mosphe ic s abili y (Tsikoudi e al., 2022). Up o now, lida -based BL e ie als showed e y25
good pe o mance on ela i ely p edic able a eas wi h known BL pa e ns, such as open land su aces o s able a mosphe ic
condi ions (Tsaknakis e al., 2011; Seidel e al., 2012). Expanding lida BL e ie als o mo e complex en i onmen s, is an
ongoing challenge especially when i comes o oceanic and coas al BLs whe e g ound-based obse a ion si es a e limi ed.
O e he open A lan ic, he Ma ine A mosphe ic Bounda y Laye (MABL) is ypically shallow and in luenced by he el-
a i ely cons an sea su ace empe a u e, while bounda y laye s in coas al and island egions expe ience e es ial-ma ine30
in e ac ions ha inc ease hei a iabili y (Ga a , 1994; Wood, 2012). Few s udies o e yea s ha e add essed he de ec ion
and analysis o MABL using lida da a, la gely due o p ac ical and obse a ional challenges o e he ocean (e.g. A las e al.
1986; Flaman e al. 1997; Pena e al. 2015). Gi en hese cons ain s, sa elli e obse a ions, such as hose p o ided by he
Cloud-Ae osol Lida and In a ed Pa h inde Sa elli e Obse a ions (CALIPSO) mission, ha e become essen ial o s udying
lowe oposphe e cha ac e is ics o e emo e egions, o e ing a means o imp o e unde s anding o hese complex sys ems.35
The gene al ci cula ion o e he opical A lan ic is domina ed by he In e -T opical Con e gence Zone (ITCZ) and a ec ed
by he p esence o he Saha an Ai Laye (SAL). The SAL is a ypically wa m and d y ai laye ha equen ly occu s a
la ge scales in he opical No h A lan ic Ocean and can eside up o 5 km in al i ude, o en accompanied by dus ae osols
(Ca lson and P ospe o, 1972; Dunion and Velden, 2004; Wu, 2007). The ITCZ, mig a es seasonally be ween he no he n and
sou he n opics, in luencing ain all and con ec i e ac i i y, c ea ing condi ions conduci e o bo h he o ma ion o clouds40
and he ae osol con ec ion o e he A lan ic (Zhou e al., 2020). In andem, he SAL, comp ising o ho , d y ai laden wi h
dese dus om he Saha a, mo es wes wa d ac oss he A lan ic Ocean, especially in summe , d i en by he p e ailing ade
winds (P ospe o and Mayol-B ace o, 2013) and has consequences on he su ace adia ion budge (E an e al., 2009; Yu e al.,
2006). These ci cula ion pa e ns a e key in anspo ing dus om A ica o he A lan ic, a ec ing he adia i e balance and
po en ially impac ing cloud o ma ion, a mosphe ic s abili y, and he e o e BL beha io in he egion (Sun and Zhao, 2020).45
A ypical cha ac e is ic o he eas e n sides o he A lan ic, is ha he ai subsiding in o he sub opical no h-eas A lan ic
is wa me and d ie han he ai ha has been in con ac wi h he ela i ely cold ocean su ace in luenced by upwelling, and
a s ong in e sion o ms a he in e ace o he wo ai masses (Hanson, 1991). As such, anspo ed dese dus om A ica
in oduce ano he laye o complexi y in oposphe ic dynamics and clouds ac i i y by al e ing adia ion budge , a mosphe ic
s abili y, and mois u e dis ibu ion (e.g. Ma inou e al. 2021; Ansmann e al. 2017; Ma sham e al. 2008). This dual e ec 50
o dus —sca e ing and abso bing sola adia ion while in he same ime se ing as cloud condensa ion and ice nuclea ion
nuclei (CCN/IN)—leads o compe ing in luences on he BL (e.g adia i e cooling can supp ess u bulen mixing, ye CCN
ac i a ion can lead o inc eased cloud co e and associa ed eedback on su ace adia ion). These p ocesses ha e been obse ed
o in luence he e ical s uc u e and s abili y o he BL, bu hei o e all impac on BL dynamics is s ill no ully unde s ood.
Accu a ely ep esen ing BL-ae osol in e ac ions in clima e and chemical anspo models is c ucial because hese p o-55
cesses a ec su ace condi ions and la ge-scale a mosphe ic ci cula ion (Menu e al., 2009; Pé ez e al., 2006; Tomb ou e al.,
2015, 2007). Gaps in obse a ional da a o e complex en i onmen s, such as he dus -laden, dese -ocean ansi ion zone in
he A lan ic, limi he model’s abili y o accu a ely cap u e BL e olu ion and ae osol in luences (Rémy e al., 2019, 2021;
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Kallos e al., 2007). The need o obse a ional da a o alida e and e ine hese models is high, especially gi en he impac s
on cloud o ma ion, ene gy dis ibu ion, and su ace-ai in e ac ions. Add essing hese gaps h ough bo h g ound-based expe -60
imen al campaigns such as Join Aeolus T opical A lan ic Campaign (JATAC) and sa elli e senso s such as space Lida s can
signi ican ly enhance unde s anding and modeling o BL p ocesses in egions o c i ical clima ic impo ance. In addi ion o
in es iga ing BL-ae osol in e ac ions, his s udy aims o imp o e BL op de ec ion me hods in di e se and complex en i on-
men s. By add essing challenges inhe en o au oma ed BL de ec ion, pa icula ly in a eas a ec ed by ae osols and a iable
a mosphe ic condi ions, his wo k con ibu es o he de elopmen o mo e obus me hods o BL iden i ica ion.65
The s uc u e o his pape is as ollows: Sec ion 2 p o ides an o e iew o he da ase s and me hods used, including
g ound-based lida , space lida , adiosonde da a, and model ou pu s. Sec ion 3 examines he BL cha ac e is ics ac oss di e en
en i onmen s, beginning wi h he A lan ic Ocean (A ea 1) and he ocean-dese ansi ion zone (A ea 2), be o e ocusing on
Cabo Ve de, whe e dus in e ac ions wi h he BL a e in es iga ed. Finally, Sec ion 4 p esen s he main conclusions o his s udy.
2 F amewo k o Da a Sou ces and Analysis70
This s udy u ilizes da a om he ASKOS Campaign (Ma inou e al., 2023), which is he g ound-based componen o he JATAC
o ganised by he Eu opean Space Agency (ESA). ASKOS ook place a he Ocean Science Cen e Mindelo (OSCM), a he
island o São Vicen e, Cabo Ve de, du ing 2021-2022.
2.1 Da ase s
Fo his analysis, we use he comp ehensi e ASKOS da ase ha , among o he s, includes ac i e emo e sensing obse a ions75
and adiosonde da ase s ha a e c ucial o unde s anding a mosphe ic dynamics in he egion. Mo e speci ically, adiosonde
p o iles, g ound-based PollyXT lida and Wind Dopple lida measu emen s as well he LIVAS (LIda clima ology o Ve ical
Ae osol S uc u e o space-based lida simula ion s udies) Clima e Da a Reco d (CDR) (Ami idis e al., 2015) using CALIPSO
a e examined. Addi ionally, he measu emen s-de i ed BL is compa ed o he ERA5 Re-Analysis da ase om In eg a ed
Fo ecas ing Sys em (IFS) o he Eu opean Cen e o Medium-Range Wea he Fo ecas s (ECMWF), a 0.25° × 0.25° esolu ion.80
2.1.1 G oundbased Lida
The g ound-based PollyXT Raman Lida (Engelmann e al., 2016), consis s o a compac , pulsed Nd:YAG lase , emi ing a
355, 532, and 1064 nm a a 20 Hz epe i ion a e, wi h he lase beam poin ed in o he a mosphe e a an o -zeni h angle o
5°. The backsca e ed signal is collec ed by a New onian elescope wi h a 0.9m ocal leng h, acqui ing p o iles wi h a e ical
esolu ion o 7.5 m, and a empo al esolu ion o 30 s. The sys em was ope a ed by TROPOS du ing he ASKOS Campaign,85
p o iding da a co e age o he en i e campaign. Figu e 1 p esen s PollyXT measu emen s, conduc ed du ing he ASKOS
Campaign. Speci ically, he a enua ed backsca e coe icien o he 1064 nm channel (A BSC, Fig. 1-le ) is examined o
de i e he BL op, and he olume linea depola iza ion a io (VLDR, Fig. 1- igh ) is in es iga ed o in e he ae osol shape.
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On ha day, he closes CALIPSO ajec o y poin was a 4:07 UTC, loca ed app oxima ely 240 km eas o he Mindelo si e.
The whi e poin s in he a enua ed backsca e indica e he p esence o clouds and we e no included in he BL analysis.90
Figu e 1. G ound-based PollyXT Lida a Mindelo (16.87°N, 24.99°W), Cabo Ve de, on he 10 h o Sep embe , 2021, depic ing he a enu-
a ed backsca e coe icien (A Bsc) a 1064 nm (le ), and olume linea depola iza ion a io (VLDR) a 532 nm ( igh ).
Addi ionally, complemen a y da a om a Halo Pho onics S eam Line scanning Dopple lida we e used o examine he
ho izon al wind speed and di ec ion, as well as he e ical wind componen . This lida is a 1.5 µm pulsed Dopple lida wi h
a he e odyne de ec o (Pea son e al., 2009). The Dopple lida has a ange esolu ion o 48 m and measu es he a enua ed
ae osol backsca e and Dopple eloci y along he beam di ec ion. Ho izon al wind p o iles we e e ie ed om a eloci y
azimu h display (VAD) scan wi h 12 azimu hal angles a 60° ele a ion angle e e y 15 minu es. O he wise, he Dopple lida 95
ope a ed in e ical s a e mode, e ie ing e ical wind p o ile ime se ies.
The Dopple lida da a was pos -p ocessed acco ding o Vakka i e al. (2019) and a signal- o-noise a io (SNR) h eshold
o 0.005 was applied o he e ically-poin ing measu emen s. Tu bulen kine ic ene gy (TKE) dissipa ion a e p o iles we e
calcula ed om he e ically-poin ing da a using he me hod by O’Conno e al. (2010). Ins umen al noise was calcula ed
om signal- o-noise a io acco ding o Pea son e al. (2009) and sub ac ed om he e ical wind a iance ime se ies be o e100
he TKE dissipa ion a e calcula ion. To es ima e mixed laye heigh (MLH) om he TKE dissipa ion a e p o iles a h eshold
o 10−4m2s−3was applied, simila o p e ious s udies (e.g. Vakka i e al., 2015).
2.1.2 Space lida : CALIPSO–CALIOP
Towa ds in es iga ing he dynamics o he BL o e he A lan ic Ocean, obse a ions o he Cloud–Ae osol Lida wi h O -
hogonal Pola iza ion (CALIOP; Hun e al. 2009), he p ima y ins umen on boa d he join Na ional Ae onau ics and Space105
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Adminis a ion (NASA) and Cen e Na ional D’É udes Spa iales (CNES) Cloud–Ae osol Lida and In a ed Pa h inde Sa el-
li e Obse a ion (CALIPSO) mission (Winke e al., 2010), a e ex ensi ely used. Mo e speci ically, CALIOP p o ided as
in eg a ed componen o he A e noon-T ain cons ella ion o pola -o bi sun-synch onous sa elli es (S ephens e al., 2018),
p o iles o ae osols and clouds along he CALIPSO o bi -pa h be ween June 2006 and Augus 2023. In he amewo k o
he s udy, CALIOP Le el 2 (L2) Ve sion 4 (V4) ae osol p o iles (AP o) o backsca e coe icien a 532 nm and pa icula e110
depola iza ion a io a 532 nm a e used, p o ided a uni o m 5 km ho izon al esolu ion and 60 m e ical esolu ion o he
al i udinal ange be ween -0.5 and 20.2 km abo e mean sea le el (a.m.s.l.) a e used, o he domain encompassing he b oade
No h A lan ic Ocean - Wes e n Saha an Dese and o Sep embe 2021. P io implemen a ion o CALIOP op ical p oduc s,
igo ous quali y assu ance p ocedu es a e applied (Ma inou e al., 2017; P oes akis e al., 2024), ollowing also he quali y
con ols adop ed owa ds he gene aliza ion o he o icial CALIPSO Le el 3 (L3) p oduc s (Winke e al., 2013; Tacke e al.,115
2018). Towa ds his objec i e, he mos agg essi e quali y con ol p ocedu e applied in he amewo k o he s udy is he cloud-
ee condi ion, emo ing he en i e L2 p o iles when de ec ed a mosphe ic laye s (Vaughan e al., 2009) along he CALIPSO
o bi -pa h a e classi ied as clouds in he ea u e- ype classi ica ion algo i hm (Liu e al., 2009; Zeng e al., 2019). Figu e 2
p o ides an indica i e example o he conside ed CALIOP obse a ions and p oduc s, and mo e speci ically he Fea u e Type
(Fig.2 op le ) p oduc and he p o iles o pa icula e depola iza ion a io a 532 nm (Fig.2 op igh ), o al backsca e coe i-120
cien a 532 nm (Fig.2 bo om le ), and quali y-assu ed o al backsca e coe icien a 532 nm (Fig.2 bo om igh ), along he
CALIPSO o e pass on he 10 h o Sep embe 2021.
Figu e 2. CALIPSO nigh ime o e pass in he ESA-ASKOS campaign egion o in e es in he p oximi y o Cabo Ve de on he 10 h o
Sep embe , 2021, depic ing he Fea u e Type ( op le ), pa icula e depola iza ion a io a 532 nm ( op igh ), o al backsca e coe icien a
532 nm (bo om le ), and he quali y-assu ed o al backsca e coe icien a 532 nm (bo om igh ).
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2.1.3 Radiosondes and models
Radiosonde p o iles we e analyzed o examine he dynamic s uc u e o he lowe oposphe e and o e alua e he emo e
sensing measu emen s conduc ed du ing he ASKOS Campaign. The GRAW DFM-09 adiosondes we e launched o p o ide125
eal- ime, high- esolu ion measu emen s o empe a u e, humidi y, and wind, which a e essen ial o iden i ying he BL key
cha ac e is ics, such as heigh , s abili y, and he modynamic p ope ies. The senso s we e equipped wi h a GPS ecei e and
ansmi da a ia a adio equency link o he g ound s a ion.
The measu emen s-de i ed BL heigh was compa ed o alues ob ained om he ERA5 Reanalysis da ase , p oduced by he
ECMWF/IFS. The ERA5 da a, a ailable a a ho izon al esolu ion o 0.25° × 0.25° wi h 137 e ical le els (Vogelezang and130
Hol slag, 1996), o e s a consis en ep esen a ion o a mosphe ic condi ions. The BL heigh in ERA5 is de e mined acco ding
o ECMWF (2017), Chap e 3, inco po a ing he modynamic c i e ia.
Addi ionally, Hyb id Single-Pa icle Lag angian In eg a ed T ajec o y (HYSPLIT) is employed o analyse he backwa d
ajec o ies o ai masses a i ing a he si e o he ASKOS Campaign, Mindelo, Cabo Ve de. This model es ima es he acking
o ai pa cels o e ime, p o iding aluable in o ma ion abou he o igins o he ai pa cels and hei po en ial in e ac ions wi h135
dus and o he a mosphe ic cons i uen s (Rolph e al., 2017). By iden i ying hese pa hways, a clea e unde s anding o he
sou ces and anspo mechanisms o he he a mosphe ic condi ions a Cabo Ve de can be es ablished.
2.2 Bounda y Laye op e ie al Me hods
To e ie e he BL heigh s om he lida measu emen s, he Wa ele Co a iance T ans o m (WCT) and he G adien Me hod
a e applied (B ooks, 2003; Li e al., 2021) on he cloud- ee backsca e coe icien p o iles o he CALIOP (Fig. 2-bo om igh )140
and g ound-based PollyXT lida (Fig. 1, le ) o he channels o 532 and 1064 nm espec i ely. Fo he adiosonde da a, laye
de ec ion is achie ed wi h he g adien me hod. In some cases, de ec ing he BL op elies on isual inspec ion o accu a ely
loca e he in e sion cap, especially in cases whe e au oma ed me hods migh miss sub le ea u es. Figu e 3 shows p o iles
o he backsca e coe icien a 1064 nm om g ound-based PollyXT (le ), o he Rela i e Humidi y (RH) om adiosonde
(middle) and backsca e coe icien a 532 nm om CALIPSO sa elli e lida ( igh ) om he 23 d o Sep embe 2022, a ound145
19:30 UTC. The g ey lines ep esen he esul o he me hod applied o de ec ing BL op, namely WCT me hod o PollyXT
Lida and G adien me hod o he es wo. A local maximum o he wa ele p o ile o WCT me hod, and a local minimum
o he g adien o he g adien me hod, ep esen s eep educ ion in he in es iga ed signal ( ed dashed lines).
Se e al signi ican challenges a ise when s udying he BL wi h lida s (bo h g ound-based and sa elli e), pa icula ly in
complex en i onmen s. Fo a sa elli e-based lida like CALIOP, he signal can become highly a enua ed as i app oaches150
he Ea h’s su ace, due o he exis ence o clouds abo e he BL. This can comp omise he eliabili y o de ec ing lowe
oposphe ic ea u es and lead o inaccu a e iden i ica ion o he BL op. To mi iga e his, only cloud- ee p o iles we e selec ed
o ensu e da a quali y, hough his es ic ion educes he da ase and in oduces obse a ional limi a ions. Addi ionally, in
ma ine en i onmen s, cumulus clouds equen ly o m a he BL op, which can se e as a use ul, albei indi ec , ma ke o
BL heigh o g ound-based lida s ha can de ec he cloud base. Mo eo e , i a hin cumulus cloud is p esen abo e he BL155
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Figu e 3. 23 d Sep embe 2022, a ound 19:30 UTC: P o iles o a mosphe ic a iables and hei co esponding de ec ion me hods o de e -
mining he bounda y laye (BL) op. The blue lines ep esen he obse ed signals, while he g ey lines co espond o he applied me hods
o BL op de ec ion. Le : Backsca e coe icien a 1064 nm om he g ound-based PollyXT lida , Middle: Rela i e Humidi y (RH) om
adiosonde, and Righ : backsca e coe icien a 532 nm om he CALIPSO sa elli e lida . The selec ed BL op is highligh ed by he ed
dashed lines.
op and allows pa ial lase pene a ion, he WCT may inco ec ly iden i y he cloud’s uppe bounda y as he BL op ins ead o
he ac ual BL heigh . A simila issue occu s in he p esence o dus laye s, as he WCT de ec s educ ions in he lida signal
caused by hese laye s. This can lead o misclassi ica ion o he dus laye bounda ies as he BL op, complica ing he accu a e
iden i ica ion o he a mosphe ic s uc u e. These limi a ions unde sco e he need o isual inspec ion o ensu e accu acy in
iden i ying he BL op in such se ings, as au oma ed me hods may s uggle o loca e he co ec laye ing.160
3 Bounda y Laye Cha ac e is ics in di e se en i onmen s
The cha ac e is ics o he BL du ing JATAC Campaign a e examined ac oss he con as ing en i onmen s depic ed in Figu e
4: o e he A lan ic Ocean (blue ec angle - A ea 1), wi hin he ocean-dese ansi ion zone (o ange ec angle – A ea 2), and
a he a ea o São Vicen e in Cabo Ve de ( ed ci cle). The Saha a Dese and he A lan ic Ocean a e cha ac e ized by dis inc
condi ions in e ms o wea he , ae osol concen a ions, and he e o e a mosphe ic dynamics. These a ia ions a e an icipa ed165
o in luence espec i ely he s uc u e and e olu ion o BL in he A eas o Figu e 4.
The lowe oposphe e abo e he A lan ic Ocean is ich in ma ine ae osols, and p esen s ela i ely s able me eo ological
condi ions, ypical o open-ocean b oad-scale ci cula ions (C o e al., 2021). In con as , he lowe oposphe e o e he
dese is cha ac e ized by high dus ae osol concen a ions, in ense sola hea ing, and a iable a mosphe ic s abili y (Giménez
e al., 2010). The bo de egion be ween ocean and dese in oduces an in e ac ion zone whe e di e en ae osols co-exis in170
big concen a ions, p oducing unique BL cha ac e is ics due o he con e gence o hese di e ing ai masses. Mo eo e , he
exis ence o SAL has an impac on he on he su ace adia ion budge (E an e al., 2009) and hence on he sea su ace em-
pe a u e (SST). Fol z and McPhaden (2008) ound ha Saha an dus ou lows a he T opical No h A lan ic, we e consis en ly
associa ed wi h a educ ion in sola adia ion, wi h app oxima ely 35% o SST a iabili y a ibu ed o dus ou b eaks, while
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o he SST cooling anomalies we e linked o wind s ess. The dus ae osol e ec on SST depends on se e al ac o s, such as175
he empe a u e con as be ween he dus laye and SST, he cha ac e is ics o he dus laye , concen a ion and al i ude (Luo
e al., 2021).
Figu e 4. Map displaying he s udy a eas o BL analysis: The blue ec angle (A ea 1) ep esen s he open-ocean Ma ine A mosphe ic
Bounda y Laye (MABL) discussed in Sec ion 3.1. The o ange ec angle (A ea 2) ma ks a ansi ion zone a he ocean-dese in e ace,
analysed in Sec ion 3.2. The ed ci cle is he g ound-based measu emen s si e a he Ocean Science Cen e Mindelo (OSCM) in Cabo Ve de
(3.3).
3.1 Analysis o A ea 1: The BL in he A lan ic Ocean
The A lan ic Ocean is cha ac e ized by dynamic wea he sys ems and cyclonic ac i i y, inco po a ing con inuous exchange o
hea and mois u e be ween he sea su ace and he adjacen ai pa cel (Schni ke , 1982). In open ocean a eas such as A ea 1,180
he e is no di ec in e ac ion o he lowe oposphe e and he land, allowing o he de elopmen o a MABL. The MABL
con ains highe humidi y le els and he ai low is smoo he due o educed ic ion om he wa e su ace, compa ing o land.
Wind and empe a u e p o iles in he MABL a e mainly in luenced by sea su ace empe a u e, oceanic cu en s and la ge-scale
a mosphe ic ci cula ion.
In his sec ion, we ocus on he MABL cha ac e is ics wi hin he blue ec angle o A ea 1 (Figu e 4). 10 yea s o CALIOP185
da a (2012–2022) a e examined, using only he p o iles eco ded in mon h Sep embe . By limi ing he da a o one mon h, we
aim o achie e mo e homogeneous condi ions o be e cap u e he p e ailing en i onmen al cha ac e is ics (e.g. ela i ely con-
sis en sea su ace empe a u es). Figu e 5-le illus a es he concep ual ajec o ies o he CALIPSO sa elli e ac oss he s udy
a ea. The analysis a ge s cloud- ee p o iles measu ed wi hin app oxima ely 40 km a ound la i ude 16.87° N, co esponding
o he la i ude o g ound-based measu ing si e in Cabo Ve de, as ep esen ed by he ed poin s in Figu e 5-le . A o al o 6392190
nigh ime and day ime p o iles (concep ually indica ed in g een and pu ple, espec i ely) a e analyzed ac oss longi udes om
60° W o 25° W. The spa ial ange o 40 km is sui able o cap u ing ep esen a i e MABL cha ac e is ics in he s udy a ea be-
cause he selec ed p o iles a e cloud- ee and measu ed o e he ocean su ace, main aining gene ally homogeneous condi ions
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o empe a u e, and humidi y. Fo each p o ile, he de i a i e o he backsca e -coe icien p o ile a 532 nm is calcula ed (as
in Fig. 3- igh ) and he minima a e cons ained a he lowe 3 km.195
Figu e 5. Le : Concep ual illus a ion o he ajec o ies o he CALIPSO sa elli e ac oss he s udy a ea. Righ : Compa ison o BL op
de i ed om CALIPSO (blue poin s) and ECMWF (magen a poin s) o 10 yea s (2012-2022) in A ea 1.
The esul s o he MABL analysis om he space lida da a a e compa ed wi h BL heigh s de i ed om he ECMWF/IFS
da ase . To accoun o longi udinal ime di e ences, each p o ile’s measu emen ime is con e ed o local ime based on i s
longi ude. Fo each lida p o ile, a empo ally and spa ially ma ched ECMWF poin a he same local ime is selec ed o di ec
compa ison. The indings a e p esen ed in Figu e 5- igh .
The blue ci cles display he MABL op heigh s de i ed om CALIPSO p o iles, a e aged hou ly in local ime. The magen a200
poin s ep esen he co esponding hou ly-a e aged BL op heigh s om ECMWF. The da a poin s a e clus e ed wi hin he
00:00–04:00 and 12:00–16:00 local ime windows, because hey co espond o CALIPSO’s nigh ime and day ime o e passes
in he A lan ic egion. The esul s show e y good ag eemen o e all, hough lida -de i ed BL heigh s ca y g ea e unce ain y
and sensi i i y. This is expec ed, as unce ain ies in he lida p o iles, occu no only om ime a e aging bu also om he
g adien me hod used o de i e heigh s om CALIOP p o iles, which can be challenging o au oma e due o low signal o noise205
a ios especially du ing day ime. In con as , he model is less sensi i e o small-scale a ia ions, as i p o ides an a e aged
ep esen a ion o e a ela i ely la ge g id (0.25° o 27.83 km a ound 16°N). The BL op in A ea 1 unde cloud- ee condi ions,
is ound o consis en ly ange be ween 600 and 800 me e s abo e sea le el. Al hough unce ain ies in de i ing he BL and
ime a e aging b oaden his es ima e, hese indings align well wi h expec ed MABL beha io ha ypically do no show a
signi ican diu nal e olu ion.210
3.2 Analysis o A ea 2: The BL in he Ocean-Dese T ansi ion Zone
A ea 2, highligh ed by he o ange ec angle in Figu e 4, spans wi hin longi udes o 35°W-0°: om he eas e n A lan ic Ocean
o he Wes e n A ica, including he egion a ound Cabo Ve de. This a ea lies a he in e ace o wo signi ican ly di e en
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he ma ine and coas al ai masses, impac ing ae osol concen a ions and BL dynamics. In Figu e 11a, he alues o VLDR
inside he BL a e close o 20%, indica ing he exis ence o dus pa icles in he MABL, mixed wi h ma ine pa icles. The315
adiosonde p o iles o i ual po en ial empe a u e and ela i e humidi y e eal weake in e sions han hose obse ed in
Sec ion 3.3.1, wi h a no able in e sion a ound 500 m, which may indica e he app oxima e BL op in his case. This weakened
in e sion also sugges s ha he BL may be mo e suscep ible o e ical mixing, acili a ing dus in usion om highe al i udes
in o he BL. On his pa icula day, he wind speed p o ile (magen a line) shows milde condi ions, eaching speeds up o 10 m/s
(~5 on he Beau o scale). The di ec ion o he wind is no he n (black s a s) ela i ely o he p e ious case. A he no he n320
side o he measu emen s’ si e, he e is he neighbou ing San o An ão island ha could ac as an obs acle o he wind’s low,
shaping local dynamics ha a ec he e ical mixing.
Figu e 11. Same as Figu e 9 o 23 Sep embe 2022. a) The adiosonde launch ime a 19:38 UTC on 23 Sep embe 2022. b) ECMWF BL
heigh (blue dashed line) a 720 m, WCT maximum ( ed dashed line) a 500 m. c) Halo Wind Dopple Lida TKE dissipa ion a e o he
same ime pe iod as PollyXT, wi h he black hexagons ep esen ing he MLH.
The WCT me hod (g ey line) applied o he a e aged ¯
β1064 p o ile (Fig. 11b) iden i ies mul iple maxima, none o which a e
pa icula ly dominan . The mos p onounced ea u e below 1.5 km appea s a 500 m ( ed dashed line). This me hod p esen s
limi a ions when he lida signal is in luenced by o e lying ea u es, such as ele a ed ae osol laye s o hin ci us clouds325
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(B ooks, 2003). In such cases i is essen ial o c oss-check he esul s wi h independen measu emen s. Acco ding o Wind
lida TKE dissipa ion a e, u bulen mo ions ex end up o 600 m (Fig. 11c), while he ECMWF BL op o he same ime
is loca ed a 720 m (blue dashed line). The wo g ound-based lida s show be e ag eemen , bo h indica ing a BL op a ound
500–600 m. The weake in e sion obse ed in he adiosonde p o ile suppo s his lowe BL heigh , howe e , he ECMWF
BL op is no ably highe (a 720 m), highligh ing a la ge disc epancy be ween he model and obse a ions compa ed o he330
p e ious case.
4 Conclusions
This s udy highligh s he c i ical impo ance o unde s anding he BL in he A lan ic, o be e cha ac e ize he complex in-
e ac ions be ween he ocean and he a mosphe e, pa icula ly in he p esence o anspo ed Saha an dus . These in e ac ions
go e n undamen al p ocesses such as e apo a ion, sea su ace empe a u e a iabili y, and cloud o ma ion, all o which ha e335
signi ican implica ions o clima e modelling and ma ine ecosys em p oduc i i y due o dus nu ien deposi ion.
Ou indings demons a e ha , based on 10 yea s (2012-2022) o CALIPSO measu emen s o e he open A lan ic (A ea 1),
he BL heigh anges om 600 m o 800 m o bo h day ime and nigh ime ajec o ies, wi h cloud- ee p o iles conside ed
du ing Sep embe . Fu he mo e, a s ong co ela ion is obse ed be ween he CALIPSO measu emen s and ECMWF/IFS model
ou pu s o his A ea du ing he decade. Howe e , he a ia ions associa ed wi h CALIPSO BL a e signi ican ly la ge han340
hose o he ECMWF/IFS model ou pu s. CALIPSO’s sensi i i y o small-scale ea u es such as ae osols and clouds leads o
mo e a iabili y in he da a. In con as , ECMWF/IFS model ou pu s a e based on global a mosphe ic simula ions ha p o ide
a mo e gene al, smoo hed es ima e wi h less sensi i i y o local dis u bances.
The analysis o A ea 2 e eals dis inc BL cha ac e is ics o e he ocean and land, shaped by di e ences in su ace p ope ies,
me eo ological condi ions, and ae osol composi ion. O e he ocean (35°W–17°W), CALIPSO and ECMWF show s ong345
ag eemen in BL heigh s, ypically anging om 500 o 800 me e s, consis en wi h indings om A ea 1. Howe e , o e
he examined a ea o land (17°W–0°), disc epancies eme ge, pa icula ly du ing he day ime, when ECMWF es ima es a
signi ican ly highe BL op han CALIPSO, likely due o di e ences in how he model and lida cap u e he mixing laye ,
esidual laye , and en ainmen zone. The unde - ep esen a ion o ae osols in ECMWF/IFS model (Mo c e e e al., 2008;
Bozzo e al., 2020; Rémy e al., 2024) may also con ibu e o hese di e ences. A nigh o e land, CALIPSO gene ally epo s350
highe BL han ECMWF, which is mos p obably due o he p esence o he esidual laye ha p o ides an ae osol laye heigh
signi ican ly highe han he he modynamically de ined BL op.
In Cabo Ve de, colloca ed da a om CALIPSO, PollyXT, Halo Lida and adiosondes we e analyzed o Sep embe 2021–2022.
Co ela ions be ween all measu emen s and ECMWF wi h CALIPSO da a we e assessed. 66% o he PollyXT poin s, 50%
o he ECMWF poin s and 30% o he Halo poin s indica e ag eemen wi hin 20% e o when compa ing wi h CALIPSO BL355
heigh . The weakes co ela ion is obse ed be ween CALIPSO and Halo Lida , due o me hodological di e ences —CALIPSO
p ima ily de ec s laye ing ha may include he esidual laye , while Halo Lida es ima es he mixing laye heigh based on TKE
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dissipa ion a e. Mo eo e , sys ema ic di e ences could a ise om a ia ions in measu emen echniques, e ie al algo i hms,
o e en inhe en model biases in ep esen ing BL p ocesses.
To u he in es iga e he si ua ion in Cabo Ve de, wo cases wi h dis inc he modynamic condi ions we e examined. The360
i s case (12 Sep embe 2022) is cha ac e ized by s onge in e sions and dus ae osols p ima ily abo e he capping laye .
Tempe a u e and humidi y in e sions a e obse ed a app oxima ely 1 km; howe e , PollyXT and Halo Lida de ec he BL
op a 650 m and 520 m, espec i ely, aligning mo e closely wi h he ma ine BL. While he wo g ound-based lida s show
good ag eemen , he adiosonde p o ile indica es a highe BL op nea 1 km. This disc epancy likely a ises because he lida s
p ima ily cap u e he well-mixed laye , whe eas he adiosonde in e sion ma ks a mo e s able uppe bounda y, po en ially365
co esponding o he en ainmen zone o a decoupled esidual laye . Such di e ences a e an icipa ed in complex en i onmen s
like Cabo Ve de, whe e he in e play o dus , ma ine ae osols, and a iable me eo ological condi ions in oduce challenges o
unde s and he BL dynamics. In he second case (23 Sep embe 2022), cha ac e ized by a weake in e sion and dus ae osols
wi hin he BL, he BL op was ound lowe , wi h bo h g ound-based lida s de ec ing i a ound 500 m. The key di e ences in
his case a e he smoo he wind speed and a mo e no he ly wind di ec ion. The no he n wind low may ha e been in luenced370
by he p esence o San o An ão island o he no h, impac ing local dynamics. Addi ionally, he slope o he i ual po en ial
empe a u e p o ile sugges s less uns able condi ions, p omo ing be e e ical mixing and enabling dus o en e he BL.
The obse ed di e ences in BL heigh can be mainly a ibu ed o he mechanical u bulence d i en by s ong winds in
he i s case, highligh ing he a iabili y o he a mosphe ic condi ions in his egion, ha is in luenced by a combina ion o
ma ine and dus ae osols, as well as he complex sea-land in e ac ions in be ween, ha con ibu e o he di e se a mosphe ic375
condi ions. SST eme ges as a key ac o , d i ing BL e olu ion, os e ing an uns able lowe oposphe e. This s udy sugges s
ha when hese complex condi ions a o less ins abili y, dese dus om he SAL is mo e e icien ly pene a ing o he BL.
This mechanism should be u he examined on i s impo ance as a acili a o o dus deposi ion o he ocean. Expe imen s
such as JATAC b ing he obse a ional syne gies needed o s udy complex BL dynamics go e ning dus anspo .
Da a a ailabili y. Visualized da ase s o he ASKOS Campaign and addi ional in o ma ion a e a ailable a h ps://askos.space.noa.g /da a;380
he ERA5 ECMWF Re-analysis Da ase is a ailable a h ps://www.ecmw .in /en/ o ecas s/da ase /ecmw - eanalysis- 5; he LIVAS CALIPSO
da a a e a ailable upon eques .
Au ho con ibu ions. IT and EM conduc ed he analysis and d a ed he manusc ip ; MT, EG and VA p o ided me hodological guidance
and con ibu ed o he in e p e a ion o he da a; EM, MT and VA designed he s udy amewo k and de ined he esea ch objec i es; EP, KR
and VV con ibu ed o he da a cu a ion, p ocessing, isualiza ion and e isions o he esul s; EM and VA p o ided unding acquisi ion and385
p ojec adminis a ion; All au ho s edi ed and e iewed he o iginal d a , p o ided c i ical eedback and helped shape he esea ch, analysis
and manusc ip .
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Compe ing in e es s. The au ho s decla e ha hey ha e no compe ing in e es s.
Acknowledgemen s. This esea ch has been suppo ed by he Hellenic Founda ion o Resea ch and Inno a ion (H.F.R.I.) unde he “3 d
Call o H.F.R.I. Resea ch P ojec s o suppo Pos -Doc o al Resea che s” (P ojec Ac onym: REVEAL, P ojec Numbe : 07222). We also390
acknowledge he suppo by he PANGEA4CalVal p ojec (G an Ag eemen 101079201) unded by he Eu opean Union . Emmanouil
P oes akis acknowledges suppo by he AXA Resea ch Fund o pos doc o al esea che s unde he p ojec en i led “Ea h Obse a ion o
Ai -Quali y – Dus Fine-Mode (EO4AQ-Dus FM)”. Funding was also ecei ed om Ho izon Eu ope p og amme unde G an Ag eemen
No 101137680 ia p ojec CERTAINTY (Cloud-aERosol inTe ac ions & hei impAc s IN The ea h sYs em). Finally, he au ho s would
like o acknowledge he p ojec o ASKOS (G an ag eemen 4000131861/20/NL/IA) om he Eu opean Space Agency.395
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