Cloud Screening and Characterization of UV/VIS Spectral Radiances for MAX-DOAS Observations

Abstract

Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) retrievals can be strongly influenced by the presence of clouds. Thus, the identification and characterization of clouds and their optical properties are essential in order to obtain MAX-DOAS products of higher quality (Gielen et al., 2014; Wagner et al., 2014).
The methodology presented here, is based on calculations of a color index, defined as the ratio of the spectral radiances at two wavelengths (347 and 420 nm), which is used to detect and define different cloud conditions, i.e., clear-sky under low Aerosol Optical Depth (AOD), clear-sky under high AOD and overcast conditions. The effect of aerosols as a function of Solar Zenith Angle (SZA) is investigated and parameterized.
This cloud-screening method is applied to zenith-sky observations for the period March-June 2022 performed by a MAX-DOAS system at the rooftop of the Faculty of Sciences building of the Aristotle University of Thessaloniki (40.634o N, 22.956o E), about 60 m above sea level.

Full text

Cloud Screening and Characterizaon of UV/VIS Spectral Radiances
for MAX-DOAS Observaons
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Dimitrios Nikolis*, Dimitris Karagkiozidis, Natsis Athanasios and Alkiviadis F. Bais
The Phaethon system
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Data and Methodology
Zenith Radiance Observaons Ancillary AOD Data Model Simulaons
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8>#!98###8
"   "    ( )     *;
  " "! J "     " FJ  
08'"FJ")K3!3B!
 F J'  /   %      " "  8
8#  # %       ! * "# 
% #""# 8L#8" 
8"FJ1"%+!2345-!
""*;#4CG&ABG!E8"*;MCBG
8'"#*;&
+!2345-!FJ'+FJ-"%95:
52308'""%!'"
FJ8""#88)9C3N3!3B!>#!48
0FJ"%*;81""%"#
"!
F""%!J
#%8O0&"8P
4B"!Q94!B""&&
% "% "    "   "   8
"&%!>#!28'
8F)#)9C3
!"0%8FJ"#)"!
J""0&%"##%+HR;FE4-"""#!>
"#8"""""##"!J/#"8'%#("S
%!
References
Conclusions
•Color Index "#88&/"%"3!B'#""3!?!
".#$,&)*"%"!
•cloudy condions *;'##!
• clear skies    # *;  8  "  " #  %!  "
%""'8#)!
•>low aerosol ''08#*;!
•F!$!QI0>!E!!Q'Q!TE!TF!>F!E!!7#!+2345-U%"&#
$,&)*V$"1"!
•#"!T !>! ;"J!H"%@! ""!@! F!H""$!!T! $! F#"& @)2
$,&)* "   % 8      1"       ! F! ! 4: BC2ADBC5A
SLL!#L43!B4A5L&4:&BC2A&234:234:!
•H#0!>$!$!T*!T!E>!Q""H!!>""J!H!0"!QI0$!T!7#!I
@)2EFE)%/$,&)*%%8#!
$!!4B42?AD4934SLL!#L43!B4A5L&4B&42?A&23222322!
•H"%@!!T*!H00$!T"I!*SF#&"#")%5:+43-
4BA5233C!!
•7# ! ! " *! T *! W ! > <! I  I! *#!+2345- UF" %  /%   $,&)* )%V 
$"1"!
Results and Discussion
Fig 3! F J'   0& %  F )   9C3   
"%  *;! *"    "%    '  /' )
"8#!
Fig 2! F J'  0& %  8 F )   9C3   
"%*;!
Fig 1!FJ'0&%"%*;1"
"%+=-!
Clear sky – Low AOD condions
J&+24L3BL2322-"8)%+"3!4499C3-!J
'##"8'""3!A:""3!?4!/"8
""&#8"/4253F!
Fig 4!FJ'0&%88)%"%*;81""%+=-"
"/+"&#-#+S&#-/%
FJ*;!
Clear sky – Higher AOD condions
J+4?L3BL2322-8#)+3!92A9C3-!J'#8
"8)%8'""3!C5""3!BB!
#+4253F-/"+8&'%-!
Fig 5!*>#58#)"!
Fig 6! Same with Fig 4 but for an overcast day.
!
Overcast condions
J+4:L35L2322-8!'#8"8'""3!B:
""3!5B!/8""#8&#
/%FJ*;424BF!