Contributors: Rory Gray (Met Office), Christoforos Tsamalis (Met Office)
Issued by: Met Office / Rory Gray
Date: 18/10/2019
Ref: C3S_D1.3.4_201910_ATBD_v1.1.docx
Official reference number service contract: 2019/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), version 1.1, for the microwave upper tropospheric humidity (UTH) ICDR product, produced by the Met Office from the microwave humidity sounding instrument Microwave Humidity Sounder (MHS) on-board the series of NOAA satellites (18 and 19) (https://www.wmo-sat.info/oscar/satellites/view/340, https://www.wmosat.info/oscar/satellites/view/341) and the MetOp satellites (A and B) (https://www.wmosat.info/oscar/satellites/view/306, https://www.wmo-sat.info/oscar/satellites/view/307), from January 2016 to May 2019.For further details, refer to the EUMETSAT Climate Monitoring Satellite Application Facility (CM SAF) ATBD [D1].
Executive summary
This report describes the algorithms used to generate the Upper Tropospheric Humidity (UTH) product produced by the Met Office from the microwave humidity sounding instrument MHS, including the scientific justification for the algorithms selected and their assumptions and limitations. Measurements in a water vapour channel which probes the upper troposphere are approximately linearly dependent on the logarithm of the Jacobian weighted upper-tropospheric relative humidity (RH), hereafter referred to as UTH. At microwave frequencies, the upper tropospheric humidity channel is located at 183.31±1.00 GHz for the MHS instrument.
Threshold masks are applied in order to identify and remove from the ICDR data measurements contaminated by clouds and measurements contaminated by the surface. A correction is applied to account for limb darkening effects.
UTH is Jacobian weighted relative humidity (RH) in the upper troposphere. It is estimated by a retrieval scheme using relative humidity Jacobians. A daily UTH layer combining ascending and descending layers has been introduced to minimise diurnal cycle impacts due to satellite orbital drift. The coefficients used in the relationship between the natural logarithm of UTH and the brightness temperature of upper tropospheric water vapour emissions are determined by linear regression, using a training data set of atmospheric temperature and humidity profiles. Further details are provided in the CM SAF ATBD [D1].
1. Instruments
The instruments of concern here are cross-track scanning passive total power microwave radiometers intended for humidity sounding (surface to 200 hPa), with three channels in the 183 GHz water vapour line but at different distances from its centre. In addition, two channels in the atmospheric "window" at lower frequencies are used primarily for cloud clearing and quality control. Further details are provided in the CM SAF ATBD [D1].
2. Input and auxiliary data
The input data for this ICDR were from the NOAA CLASS dataset downloaded for each microwave humidity sounder (i.e. NOAA-18/19 and MetOp-A/B).MHS on NOAA-18 ceased in October 2018 and no data has been received since. Since July 2009, the relevant MHS channel of NOAA-19, channel 3, has exhibited erratic behavior and should not be used (Hans et al, 2017).
3. Algorithms
The natural logarithm of UTH, which is the Jacobian weighted relative humidity in the upper troposphere, can be expressed as a linear function of the radiance emanated from water vapour emissions in the upper troposphere, expressed in brightness temperature, Tb, (Buehler and John (2005)):
where the coefficients a and b are determined by linear regression.
The production of the UTH product includes some post-processing of the ICDR before applying the UTH retrieval.
Two criteria are applied to each measurement to identify cloud affected measurements. Firstly, negative brightness temperature differences between the 183.31±1.00 GHz and 183.31±3.00 GHz channels are assumed to be indicative of contamination by large ice particles or rain drops. Secondly, if the computed 183.31±1.00 GHz channel brightness temperature has a value lower than a minimum clear sky value for its particular viewing angle, then it is assumed to be affected by cloud. If both criteria are satisfied then the measurement is assumed to be cloud contaminated and is discarded from further processing.
Measurements contaminated by the surface are identified and discarded by assuming that in dry conditions of total column water vapour less than ~3mm, a positive brightness temperature difference between the 183.31±1.00 GHz and 183.31±3.00 GHz channel indicates that the former has a higher emission due to its proximity to the vapour line centre with both channels acting as surface channels.
Limb darkening effects are corrected for by applying a viewing angle dependent analytical equation to convert off-nadir brightness temperatures into nadir equivalents.
Further details are provided in the CM SAF ATBD [D1].
4. Output data
The UTH product is provided to users on a global, daily, 1.0° x 1.0° latitude-longitude grid.
UTH is retrieved for all cloud and surface cleared and limb-corrected brightness temperatures for each day. These are then separated for ascending and descending passes and binned into each 1.0° grid cell, and the UTH cell mean, median, and standard deviation is computed. Information such as number of measurements used and total number of measurements (before cloud or surface screening) for every grid box is also provided. Statistics for the 183.31±1.00 GHz brightness temperature in each grid cell are included as an intermediate product in the dataset.
In order to account for orbital drift, a daily mean UTH is included in the UTH data set, which is the weighted average of ascending and descending orbits for all grid cells with valid ascending and descending observations.
These daily gridded data, which includes both ascending and descending UTH fields and the daily mean UTH, are provided in NetCDF-4 (https://www.unidata.ucar.edu/software/netcdf/) files. Further details are provided in the CM SAF ATBD [D1].
Figures and Tables
For figures and tables refer to the CM SAF ATBD [D1].
References
Hans, I.; Burgdorf, M.; John, V.O.; Mittaz, J.; Buehler, S.A. Noise performance of microwave humidity sounders over their lifetime. Atmos. Meas. Tech. 2017, 10, 4927–4945.
For further references refer to the CM SAF ATBD [D1].
This document has been produced in the context of the Copernicus Climate Change Service (C3S).
The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation agreement signed on 11/11/2014). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.
The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view.
