Contributors: Nabiz Rahpoe (DWD)
Issued by: DWD / Nabiz Rahpoe
Date: 22/10/2020
Ref: C3S_D312b_Lot1.1.3.3-v1.1_202102_ATBD_TCWV_SSMIS_TCDR+ICDR_v1.1.1
Official reference number service contract: 2018/C3S_312b_Lot1_DWD/SC1
History of modifications
List of datasets covered by this document
Related documents
Acronyms
Scope of the document
This document is the Algorithm Theoretical Basis Document (ATBD) for total column water vapour (TCWV) (product C3S_D312b_Lot1.3.3.14 v1.0 & v1.1) based on SSM/I & SSMIS measurement. It describes the algorithms used to generate the water vapour product, outlines the chosen approach, and provides a description of the assumptions and limitations of the algorithms.
The TCDR data products covered by this ATBD are provided as a brokered service from the EUMETSAT CMSAF. The original CM SAF ATBD, to which the present document refers, is the “CM SAF ATBD: HOAPS 4.0 1D-Var products [D1]. It can be found at the CM SAF web site http://www.cmsaf.eu.
The data products (ICDRs) covered by this ATBD are produced within the framework of C3S project and are the continuation of TCDRs brokered from the EUMETSAT CMSAF.
Executive summary
The project C3S_312b_Lot1 includes brokering of total column water vapour gridded monthly mean and 6-hourly data from the EUMETSAT CM SAF. This document refers to the CM SAF documentation [D1] describing the methods and algorithms that are used by the CM SAF to generate the total column water vapour data products.
The CM SAF documentation covers the TCDR with the time period January 1988 to December 2014, while the ICDR covers the time period from January 2015 – December 2020. The updated ICDR has the same characteristics as the CDR, which has been generated in a CM SAF reprocessing activity.
The ICDRs are based on the same retrieval scheme and algorithm as TCDR, and therefore no further changes has been implemented, except for the continuation and extension of the time series processing.
1. Instruments
1.1 SSM/I, SSMIS sensors
The sensors are described in Section 2.1-2.2 of the EUMETSAT CM SAF ATBD [D1]. The SSM/I and SSMIS sensors have been on different DMSP satellites (F8-F19) from 1987 and are still operational.
The ICDRs are part of this measurement system and extend the time series to 2015-2020. Hence a long-term & homogenous dataset based on same sensor configuration with a length of 38 years is available. Unfortunately, the DMSPs program was stopped, and F-20 will not be launched.
2. Input and auxiliary data
The input data to the CM SAF processing, as well as the auxiliary data (background for the construction of the 1D variational and statistical retrieval from NWP-SAF), are described in Section 2, 2.5, and 3.2 of the CM SAF ATBD [D1]. Essentially, 1D-Var requires a-priori information on the atmospheric state as input. The OISST (Optimal Interpolation of Sea Surface Temperature) is used in addition as input (Sec. 2.3 [D1]). These are in-situ observations of SST on buoys and ships, gridded and bias corrected.
The general scheme did not change for the production of ICDRs, and the information related to input and auxiliary data has the same characteristics as for TCDRs.
3. Algorithms
The methods and algorithms (1D-Var) that are used by the CM SAF to generate the total column water vapour data products, which are provided by the C3S_312b_Lot1 as a brokered service, are described in Section 3of the CM SAF ATBD [D1]. The HOAPS 1D-Var retrieval is an implementation of the NWP SAF MetOffice package with adaptions for the HOAPS processing chain. In a variational retrieval the a-priori or background information of the atmosphere and surface, and the measurements (observed brightness temperatures) are combined in a statistically optimal way to estimate the most probable atmospheric state. The retrieval requires background profiles as first guess, and the NWP SAF profiles are used here. The geophysical variables from the first guess are mapped into radiance space with the help of a radiative-transfer model (RTTOV). The model simulates the brightness temperatures for different wavelengths (sensor specific channels) based on atmospheric background fields.
The general scheme did not change for the production of ICDRs (2015-2020) and the algorithm and processor have the same characteristics as for TCDRs (1988-2014).
4. Output data
Post-processing and uncertainty estimation are described very briefly by the CM SAF ATBDs – in Section 3.4 of the CM SAF ATBD [D1]. The 1D-Var retrievals are characterized by a linear error propagation which takes into account model errors, instrument uncertainties and characteristics (i.e. signal to noise ratio) as well as uncertainties in auxiliary data (i.e. a priori knowledge) and atmospheric state via Look-Up-Tables of wind speed, near-surface humidity, sea surface temperature, and integrate water vapour (Section 3.4 [D1]). The systematic uncertainty component is selected from the look-up-tables and the random uncertainty component is the associated spread. Error propagation yields reliable error estimates for each retrieved value.
Data are provided in NetCDF format. More details of data format are found in the CM SAF PUM document Section 5 [D2]. Details on data format for TCDR+ICDR, can be found in the PUGS document [D3] which describes the format and related variables.
