Validation and Update of SAO’s OMI Total Column Water Vapor (TCWV) product Huiqun Wang1 (hwang@cfa.harvard.edu) Gonzalo Gonzalez Abad1, Xiong Liu1, Kelly Chance1 1Smithsonian Astrophysical Observatory TEMPO Science Team Meeting, June 2016

Water Vapor Importance Participates in atmospheric chemistry Interfere with retrieval of several trace gases Active player in hydrological cycle Important factor for the weather Most abundant greenhouse gas Essential Climate Variable (ECV) Water vapor is highly variable in space and time. It is important to monitor its distribution.

Water Vapor Datasets Radiosondes – Reference Upper-Air Network (GRUAN) Sun-photometers - AERONET NCAR’s ground based GPS (SuomiNet, GEONET) GPS radio occultation - CHAMP, COSMIC Remote Sensing System (RSS) ’s Microwave data over the ocean – SSM/I, SSMI/S, TMI, AMSR-E, GMI, … Thermal IR – MODIS, TES, AIRS, IASI Near IR – MODIS, MERIS, SICAMACHY Visible – GOME, GOME-2, OMI Numerous datasets exist Challenge – long term, high quality, consistent

Water Vapor Observation Radiosondes, Microwave radiometer, Sun photometer, Aircrafts – higher temporal resolution, boundary layer sensitivity, sparse spatial coverage, calibration inconsistency GPS – accurate, available under all-sky condition, day&night non-uniform surface network coverage / extrapolation errors for GPS radio occultation. Microwave – reliable, routinely assimilated, all-sky, day&night non-precipitating ice-free ocean only Thermal IR – both land & ocean with profiles, day & night limited sensitivity to PBL, strongly affected by clouds Near-IR & Red – sensitive to PBL daytime only, strongly affected by clouds, low quality over ocean Blue – no saturation, land-ocean albedo uniformity affected by clouds, lower signal-to-noise, larger uncertainty

Water Vapor Spectrum Much stronger absorption at longer wavelengths. GOME, GOME-2 OMI SCIAMACHY Much stronger absorption at longer wavelengths. OMI visible spectrum covers weak water vapor features in blue range.

GOME annual mean Surface Albedo blue [Koelemeijer et al., 2003] red Large land-ocean contrast at longer wavelengths Ocean is brighter at shorter wavelengths

SAO water vapor SCD retrieval using Version 3 OMI data Algorithm 1.0.0 430-480nm HITRAN2008 @ 280K & 0.9atm Wavelength shift 3rd order polynomial Common mode Under-sampling Distinct spectral feature in the retrieval window

SAO OMI water vapor VCD retrieval VCD = SCD / AMF Scattering Weight Shape Factor VLIDORT GEOS-Chem

SAO’s Level 2 OMI TCWV retrieved with algorithm 1.0.0

SAO’s OMI H2O Validation Ground based network NCAR’s GPS data (SuomiNet) AERONET sun-photometer NearIR data 2. Satellite product Remote Sensing System (RSS)’s SSM/I microwave data over ocean long-term, global coverage and high quality

OMI versus GPS OMI tracks the seasonal and inter-annual variations of GPS for a wide range of climate regimes

Mean(OMI – GPS) (2005 – 2009) OMI agrees with GPS within 1.5 mm at 71% of the stations. OMI can be lower by up to 8 mm at a few ocean islands.

Mean(OMI – GPS) (2005 – 2009) OMI generally agrees with GPS over land mm OMI generally agrees with GPS over land OMI shows significant dry bias over ocean

Daily (OMI – GPS) (2005 – 2009) Mode = 0 mm Median = -0.4 mm

OMI versus SSM/I over the ocean for July 2005 OMI – SSM/I OMI cldfrac<25% SSM/I all-sky mean = -4.3mm OMI has a low bias over the ocean

OMI versus SSM/I for July 2005 OMI is lower than SSM/I over ocean

Common Mode SCD Fitting Ocean Land

Sensitivity study for SAO’s OMI water vapor algorithm 1.0.0 (430 – 480nm) It is essential to include liquid water in OMI water vapor retrieval

New OMI versus SSM/I for July 2005 OMI agrees with SSM/I over ocean much better than before

SAO OMH2O Retrieval Algorithm Update HITRAN2008 1.0 atm, 288K 428 – 465 nm [Thalman and Vokamer, 2013]

New OMI – Algorithm 1.0.0 OMI July 2005 Land mean = 0.4 mm Ocean mean = 2.9 mm New OMI retrievals increase slightly over land, but significantly over the ocean.

New OMI versus SSM/I for July 2005 All sky comparison mean = -4.3mm mean = -1.1 mm OMI agrees with SSM/I over ocean much better than before

New OMI versus SSM/I for July 2005 “Clear” sky comparison OMI agrees with SSM/I over ocean much better than before

Algorithm in Development Scattering Weight VLIDORT MERRA2 Shape Factor High-resolution (0.5°×0.5°) GEOS5 MERRA2 water vapor profile climatology New version OMI cloud information Better treatment of surface properties (albedo, pressure) AMF error analysis (albedo, cloud fraction, cloud top pressure, surface pressure, profile, aerosol…)

Summary Water vapor can be retrieved from the blue spectral range. Using the Version 1.0.0 OMH2O algorithm, OMI water vapor over land compares well with GPS & AERONET, OMI water vapor over the ocean has a significant low bias. The updated Version 2.0.0 algorithm leads to significant improvement over the ocean without affecting the land result much. New developments for AMF are being tested.