Contributors: Nabiz Rahpoe (DWD), Rene Preusker (FUB), Jan R. El Kassar (FUB)
Issued by: Nabiz Rahpoe (DWD), Rene Preusker (FUB), Jan R. El Kassar (FUB)
Issued Date: 30/09/2020
Ref: C3S_312b_Lot1.3.6.2-v1.1_202008 _PUGS_v1.1.1
Official refence number service contract: 2018/C3S_312a_Lot1_FUB/SC1
History of modifications
List of datasets covered by this document
Related documents
Acronyms
General definitions
In the scope of the Copernicus Climate Change Service (C3S), a Thematic Climate Data Record (TCDR) has a fixed end point, whereas an Interim Climate Data Record (ICDR) is extended continuously.
The TCWV_MERIS_SSMI_TCDR product is a TCDR which consists of two satellite-based estimates of the Total Columnar Water Vapor (TCWV) of the atmosphere.
The two estimates are derived by two different instruments over two types of surface. Over land, data from the Medium Resolution Imaging Spectrometer (MERIS) are used. Over water, the Special Sensor Microwave/Imager (SSM/I) is used. All documents related to the TCWV_MERIS_SSMI_TCDR product are mostly limited to the processing of MERIS data by the Freie Universität Berlin (FUB). SSM/I data have been provided by the German Weather Service (DWD) and were not part of the processing at FUB and are thus not discussed in this or any of the other supplementary documents.
Scope of the document
This document provides a short overview of the Total Column Water Vapour (TCWV) product and gives a short introduction into the use of this product. Moreover some specifications of the TCWV product are provided as well.
Executive summary
The TCDR of Total Column Water Vapour (TCWV) from the Freie Universität Berlin (FUB) is an in-house product delivered to the Climate Data Store (CDS) of the Copernicus Climate Change Service (C3S).
The MERIS TCWV retrieval is based on the Cloud Aerosol and Water Vapor algorithm (CAWA) [Preusker et al., 2015] which was developed at the FUB within the Advanced Clouds, Aerosols and Water Vapour products for Sentinel 3/OLCI (CAWA) ESA Scientific Exploitation of Operational Missions Element Program (SEOM). The retrieval is applied to the MERIS instrument onboard the polar orbiting ENVISAT.
The SSMI TCWV is provided by DWD and is retrieved by SSM/I and SSMIS instruments onboard several polar orbiting DMSP satellites.
The MERIS TCWV and SSMI TCWV datasets are combined by the FUB team and comprises of 10 years (2002-2012) L3 monthly mean time series. The CDR provided here includes monthly means of TCWV on a regular global latitude-longitude grid, merged from MERIS (over land) and SSM/I (over ocean).
1. Total Column Water Vapour
1.1 Product description
Total Column Water Vapour is a measure of the integrated water vapour amount in a certain area of the atmosphere. It is a crucial variable for analyzing, understanding and predicting many processes in the atmosphere. These processes range from estimating the radiation and heat budget of the atmosphere to modelling cloud formation and cloud physics.
Instruments used for the creation of the TCWV product are the Special Sensor Microwave Imager (SSM/I) and Special Sensor Microwave Imager Sounder (SSMIS) and Medium Resolution Imaging Spectrometer (MERIS). MERIS TCWV was produced with a 1D-Var algorithm which utilizes the differential absorption technique at the 940 nm water vapor absorption band. Auxiliary data used were ECMWF ERA Interim TCWV, surface temperature and surface pressure, as well as surface winds. The available Processing Levels are the Level 3 monthly means at high resolution (= 0.05°) and low resolution (=0.5°) from May 2002 to March 2012.
1.2 Benchmark Requirements
Benchmark requirements were taken from the WMO OSCAR for the ‘threshold category’ with the precision (cRMSD) of 5 kg/m2 and accuracy (bias) of 3 kg/m2 (See D3-Table 1). The validation suggest that the current L2 MERIS TCWV meets these requirements at the second processing level (Level2, satellite projection at 1km resolution) for the ‘threshold category’ (See D3-Table 2). For SSMI/S the stricter benchmark, i.e. the ‘target category’ is met. The precision (cRMSD) lies between 1.54 and 2.56 kg/m2 with the accuracy (bias) of 0.13 to 2.24 kg/m2 for MERIS, and precision (cRMSD) between 1.0 and 1.1 kg/m2 with the accuracy (bias) of 0.30 to 0.39 kg/m2 for SSMI/S, respectively.
Further assessment of the Level 3 monthly product is performed due to comparison with GUAN stations and the ‘target category’ are met [D3 – Section 2.2] with the accuracy (bias) of 0.06 kg/m2 and precision (cRMSD) of 3.14 kg/m2, respectively.
The stability (decadal bias trend) for L3 TCWV, by using the GUAN TCWV as the reference, is around 0.18 kg/m2 /dec, where the ‘target category’ for stability is met (<0.2 kg/m2 /dec) as it is stated in Table 1 of PQAR document.
The details of the methods and results can be taken from the PQAR document [D3].
1.3 Data usage information
The data format is CF-1.7-conform netCDF-files that can be read with applications such as Panoply. The filenames have a straight-forward structure containing all necessary information such as the date and the resolution and version.
Included in the files is the main product, monthly mean of TCWV. Also provided are the associated average error estimate and standard deviation, as well as the background TCWV, used as a base of the retrieval for both, the SSM/I and MERIS. The TCWV from the ERA Interim reanalysis is also given.
Further more we include additional variables such as the number of valid observations per month for SSM/I and MERIS. Above that, for MERIS we offer a valid fraction and a cloud fraction in order to assess the influence of cloud-free bias on the water vapor content in a given grid box.
Finally, the product contains a Level 3 Flag field that includes bit-wise flags for the instrument used, failed retrievals and the surface type. The flags can also be used to separate MERIS (flag=16) and SSM/I (flag=128) TCWV respectively (See Table 1).
In the high resolution (0.05°) one file has the size of approx. 140 MB, in the low resolution (0.5°) the size is approx. 4 MB.
Table 1: Variables found in the TCWV netCDF-Files.
Variable | Long name | Unit | Valid range | Comment |
tcwv | atmospheric water vapour content | kg/m2 | 0-75 | Total Column Water Vapor (also called integrated Water Vapor (IWV) or Precipitable Water Vapor (PWV)) is the integrated amount of gaseous water in the total column of the |
tcwv_err | atmospheric water vapour content | kg/m2 | 0-60 | Error of total column water vapor from 1D-Var retrieval |
tcwv_stdev | atmospheric water vapour content | kg/m2 | 0-50 | Standard deviation of total column water vapor |
tcwv_back | atmospheric water vapour content | kg/m2 | 0-75 | Background of total column water vapor |
n_obs | number of observations | 1 | 0-580 | sum of observations over a whole month |
valid_fraction | valid pixels/all pixels | 1 | 0-1 | only for MERIS grid boxes |
cloud_fraction | cloud pixels/ all pixels | 1 | 0-1 | only for MERIS grid boxes |
l3_flags | surface & product flags | 1 | 0-265 | Level 3 Flags are bits that contain masks for surface type, quality level and instrument in bits. Flag Codings are:
|
Figure 1: Example for TCWV file opened in Panoply.
Figure 2: TCWV field plotted with Panoply.
2. Data access information
The dataset is available in the C3S Climate Data Store (CDS, see Figure 3) (https://cds.climate.copernicus.eu). The product is provided by C3S.
Figure 3: Copernicus CDS Search (https://cds.climate.copernicus.eu/cdsapp#!/search).
2.1 Product ordering process
You need to be registered and logged in to order products in the CDS. A login is provided upon registration, all products in the CDS are delivered free of charge.
2.2 Visualization tool
The data will be available in the CDS Toolbox (https://cds.climate.copernicus.eu/user/login?destination=/toolbox-user) as soon as the data sets are online.
To prepare, analyze, and visualize the datasets, different tools can be used, as the data are stored in user friendly NetCDF format. The CM SAF R Toolbox can be used to work with the data sets (https://www.cmsaf.eu/EN/Products/Tools/R/R_node.html).
References
Preusker R., H. Diedrich and J. Fischer, 2015: Retrieval of Total Column Water Vapor from MERIS/OLCI and MODIS above Land- and Ocean Surfaces ATBD
