Validation of two independent retrievals of SCIAMACHY water vapour columns using radiosonde data

摘要

Two independently derived SCIAMACHY total water vapour column (WVC) productsare compared with integrated water vapour data calculated from radiosondemeasurements, and with each other. The two SCIAMACHY WVC products areretrieved with two different retrieval algorithms applied in the visible andshort-wave infrared wavelength regions respectively. The first SCIAMACHY WVCproduct used in the comparison is ESA's level 2 version 5.01 WVC productderived with the Air Mass Corrected Differential Optical Absorption Spectroscopy(AMC-DOAS) retrieval algorithm applied in the visible wavelength range(SCIAMACHY-ESA). The second SCIAMACHY WVC product is derived using theiterative maximum likelihood method (IMLM) in the short-wave infraredwavelength range and developed by Netherlands Institute for Space Research(SCIAMACHY-IMLM). Both SCIAMACHY WVC products are compared with collocatedwater vapour amounts determined from daily relative humidity radiosondemeasurements obtained from the European Centre for Medium-Range WeatherForecasts (ECMWF) radiosonde network. The SCIAMACHY-ESA WVC product iscompared with radiosonde-derived WVC amounts for an 18-month period fromFebruary 2010 to mid-August 2011, and the SCIAMACHY-IMLM WVC amounts arecompared with radiosonde WVC amounts for the two individual years of 2004and 2009. In addition the WVC amounts from SCIAMACHY-ESA and SCIAMACHY-IMLMare also compared with each other for a 1-month period for June 2009.The AMC-DOAS method used to retrieve SCIAMACHY-ESA WVC is able to correct forwater vapour present below the clouds and can be used during cloudyconditions over both land and ocean surfaces. Results indicate a goodagreement between the WVC amounts of SCIAMACHY-ESA and that of radiosondes,with a mean difference of ?0.32 g cm?2 for all collocatedcases. Overall the SCIAMACHY-ESA WVC amounts aresmaller than the radiosonde WVC amounts, especially over oceans. For cloudyconditions the WVC bias has a clear dependence on the cloud top height andincreases with increasing cloud top heights larger than approximately 2 km.A likely cause for this could be the different vertical profile shapes ofwater vapour and O₂ leading to different relative changes in theiroptical thickness, which makes the air mass factor (AMF) correction method used in thealgorithm less suitable for high clouds. The SCIAMACHY-IMLM product's watervapour measurements are best used over land surfaces during cloud-freeconditions, and in these cases a good agreement is found when compared toradiosonde WVC amounts, with a mean difference of 0.08 g cm?2.It is shown that over ocean surfaces during cloudy conditions the partialSCIAMACHY-IMLM water vapour column above the cloud can be well estimated byusing the simultaneously retrieved methane column to calculate the cloud topheight. Comparing the two satellite WVC products with each other indicatesthat SCIAMACHY-ESA consistently measures higher WVC amounts than those ofSCIAMACHY-IMLM. Furthermore, the importance of the choice of cloud product ishighlighted, as intercomparisons between the two SCIAMACHY WVC productsindicate that using different cloud products to screen water vapour data forcloud-free conditions influences the data selection and may ultimately lead toa variation in results.In the last section of the paper, various options for filtering the twoSCIAMACHY WVC data sets are discussed and best selection criteria suggested.