Sentinel-5 precursor: TROPOMI

Cloud slicing retrieval: Program development and testing with

SCIAMACHY/GOME-2 WFDOAS data

Kai-Uwe Eichmann, Mark Weber, IUP Bremen

Background

TOZ: Total ozone / GVC: Ghost vertical column / CF: Cloud fraction

Input/output data• Level 2 data

– Total ozone TOZ [DU] (WFDOAS)– Ghost vertical column GVC [DU] (WFDOAS)– Cloud top height CTH [km] (SACURA or FRESCO)– Cloud fraction CF [-] (SACURA or FRESCO)

• Auxiliary data– Latitude/Longitude [deg] (Instrument)

• Derived data– Cloud top pressure CTP [hPa]

• Output data– Cloud slicing Ozone VMR (CSV) [ppmv] between CTH(min) and CTH(max)

CSL program development• IDL program (M. Weber) using an iterative approach to deal

with outliers in the data• Speed increase with packing all WFDOAS data into one file

per month, restricted to the tropics (25 deg): SCIAMACHY factor 12 (6 Min ->30 sec, GOME-2 factor 20: 20 to 1 min).

• FORTRAN 90 draft version is ready (linear least square only), unpacked data version takes about 3 Min.

• IDL version used for testing different parameters:– Cloud fraction = [0.8,0.9]– CTH(min) = [6, 7] km (5.5 km GOME-2) – Grid boxes: 5° Lat / [30°, 10°] Lon– Number of data per grid box: [15, 50]– Number of days: [6, 15, 30]

SCIAMACHY cloud top heights

Gridcell

Results from SCIAMACHY WFDOAS

CTH(min)=7km

2006/10: 30d of dataCF(min)=90%

Outliers

vmr=slope*1.27E3

• Two regimes of ozone• Negative VMR due to

change of ACCO during the month

• Thus monthly means are not the best choice

Problem: ACCO time-dependent

Dividing into 6 days periods

GOME-2 results• GOME-2 has about 4 times more measurements than

SCIAMACHY.• CTH is generally lower (FRESCO) than for SCIAMACHY

(SACURA). Differences are about 3 km on average. • This will have an effect on the calculated VMR (cloud slicing

volume mixing ratio).

Comparison SCIA / GOME-2

Results• Reducing the cloud fraction to 0.8:

– increases the number of cloudy pixels by up to 40% depending on altitude.

– The effect needs to be further analyzed.• The height to pressure conversion is error prone, as the

temperature profile is not taken into account. – The differences between pressures calculated with a scale

height of 8 km or using the international barometric height formula are, depending on the altitude, quite different (up to 40 hPa at 15km = 40%).

• Using data from a whole month for the CSL calculation is in a lot of cases not the best choice, as the above cloud ozone may change considerably during the month.

Outlook

• Extent the parameter study, – e.g. SACURA vs. FRESCO

• Use sonde data to decide which parameters are best for the retrieval (see E. Leventidou)

• Precalculated data input to FORTRAN program• Test the code with OMI data

– different input parameter, definition of ACCO