Information content on hydrometeors from millimeter and sub-millimeter wavelengths

Camille Birman,Jean-François Mahfouf,Mathias Milz,Jana Mendrok,Stefan A Buehler,Manfred Brath

doi:10.1080/16000870.2016.1271562

Abstract

ABSTRACTThis study examines the information content on hydrometeors that could be provided by a future HYperspectral Microwave Sensor (HYMS) with frequencies ranging from 6.9 to 874 GHz (millimeter and sub-millimeter regions). Through optimal estimation theory the information content is expressed quantitatively in terms of degrees of freedom for signal (DFS). For that purpose the Atmospheric Radiative Transfer Simulator (ARTS) and its Jacobians are used with a set of 25 cloudy and precipitating profiles and their associated errors from the European Centre for Medium-range Weather Forecasting (ECMWF) global numerical weather prediction model.In agreement with previous studies it is shown that frequencies between 10 and 40 GHz are the most informative ones for liquid and rain water contents. Similarly, the absorption band at 118 GHz contains significant information on liquid precipitation. A set of new window channels (15.37-, 40.25-, 101-GHz) could provide additional information on the liquid phase. The most informative channels on cloud ice water are the window channels at 664 and 874 GHz and the water vapour absorption bands at 325 and 448 GHz. Regarding snow water contents, the channels having the largest DFS values are located in window regions (150-, 251-, 157-, 101-GHz). However it is necessary to consider 90 channels in order to represent 90% of the DFS. The added value of HYMS has been assessed against current Special Sensor Microwave Imager/Sounder (SSMI/S) onboard the Defense Meteorological Satellite Program (DMSP) and future (Microwave Imager/Ice Cloud Imager (MWI/ICI) onboard European Polar orbiting Satellite – Second Generation (EPS-SG)) microwave sensors. It appears that with a set of 276 channels the information content on hydrometeors would be significantly enhanced: the DFS increases by 1.7 against MWI/ICI and by 3 against SSMI/S. A number of tests have been performed to examine the robustness of the above results. The most informative channels on solid hydrometeors remain the same over land and over ocean surfaces. On the other hand, the database is not large enough to produce robust results over land surfaces for liquid hydrometeors. The sensitivity of the results to the microphysical properties of frozen hydrometeors has been investigated. It appears that a change in size distribution and scattering properties can move the large information content of the channels at 664 and 874 GHz from cloud ice to solid precipitation.

Highlights

Satellite observations have in the last decades become the major source of information for numerical weather prediction (NWP) models
This study has examined the information content on temperature, water vapour and hydrometeors provided by a theoretical hyper-spectral microwave sensor HYperspectral Microwave Sensor (HYMS) over the frequency range between 6.9 and 874 GHz, that has been compared to microwave frequencies of Sensor Microwave Imager/Sounder (SSMI/S) and future similar instruments onboard EPS-SG
Since Atmospheric Radiative Transfer Simulator (ARTS) is a line-by-line model where scattering is solved with a rather computationally expensive algorithm (DOIT) and for which the linearised versions are not available, the computations of the Jacobians by the brute force method have been cumbersome, preventing from examining more than 25 profiles, that have been chosen in order to have a maximum of variability of cloud properties

Summary

Introduction

Satellite observations have in the last decades become the major source of information for numerical weather prediction (NWP) models. The assimilation of cloudy radiances has progressed more slowly but there is an operational use of several microwave instruments in the European Centre for Medium-range Weather Forecasting (ECMWF) four-dimensional variational (4D-Var) system since 2006 (Bauer et al, 2006b, 2010; Geer et al, 2010). Such operational usage has required the development of a fast and accurate radiative transfer model describing absorption and scattering by atmospheric hydrometeors: the Rapid Radiative Transfer for TOVs including SCATtering effects (RTTOV-SCATT) model (Bauer et al, 2006a) together with its tangent-linear and adjoint versions. Recent improvements in RTTOV-SCATT have concerned a more realistic description of cloud fraction (Geer et al, 2009), a new representation of observation errors accounting for displacement errors in the background field (Geer and Bauer, 2010) and revised radiative properties of solid precipitating hydrometeors allowing the assimilation of high frequency channels (Geer and Baordo, 2014; Geer et al, 2014)

Methods

Results

Discussion

Conclusion