MiRS - Algorithm Description

The Microwave Integrated Retrieval System (MiRS) is an iterative, physically-based retrieval algorithm (1DVAR), whose principle is to minimize a two-source penalty function. This cost function is composed of; (1) the departure of the simulated radiances from the actual measurements and (2) the departure of the retrieved parameters from their respective backgrounds. The retrieval process is performed in a reduced space and expects a number of degrees of freedom for each EDR in order to strike a balance between the stability of the matrix inversion and the optimal information content extraction. This mechanism allows MIRS to apply to both imaging and sounding sensors or any sensor that combines imaging and sounding capabilities. In the retrieval scheme used by MIRS, the departure from the measured radiances is normalized by the noise level (NEDT) impacting the measurements and the uncertainty in the forward modeling, making it possible to sometimes use the signal of a particular channel when the geophysical signature (through the derivative) is stronger than the noise (leading to a useful signal-to-noise level), and some other times dismiss the same channel when the signal in question is within the uncertainty/noise level.

The departure from the background is also scaled by the uncertainty placed on the background. This allows the retrieval to make it harder to depart from a background information if it is deemed accurate. The source of these backgrounds is currently a simple climatology (loose background errors).

The MIRS system was designed to be a flexible retrieval/assimilation tool and is therefore suited for applications in the microwave as well as other spectral regions, although it has been applied only to millimeter frequencies instruments so far. It allows the user to select which channels to use for a particular retrieval and which to-be-retrieved parameters to turn OFF or ON in a retrieval process.

These features (reduced space retrieval, integrated approach and flexibility), coupled with the advanced radiative transfer model (CRTM) used as the forward operator, allow MiRS to be a cutting edge algorithm, readily applicable to current and future sensors, both sounders and imagers.

The use of CRTM allows MIRS to perform retrievals in clear, cloudy and precipitating conditions, as this forward operator produces the radiances and the corresponding Jacobians in all these conditions. For more details, please see the MiRS development page: ( https://www.star.nesdis.noaa.gov/mirs/algorithm.php )