## 4 Community Radiative Transfer Model (CRTM)  
CRTM is a sensor-based radiative transfer model. It supports more than 100 sensors including sensors on most meteorological satellites and some from other remote sensing satellites. The CRTM is composed of four important modules for gaseous transmittance, surface emission and reflection, cloud and aerosol absorption and scattering, and a solver for radiative transfer. The CRTM is designed to meet users' needs. Many options are available for users to choose from. The CRTM provides surface emissivity to the users; allows users to select a subset of channels for a given sensor; turns off scattering calculations; computes radiance at aircraft altitudes; computes aerosol optical depth only; and completes parallel calculations of the CRTM. The public interface contains the initialization modules for choosing sensor/sensors and full channels or a subset of channels. The CRTM forward model module is used to simulate satellite measured radiances, which can be used to verify measurement accuracy, uncertainty, and the long-term monitoring of radiances. The K-matrix module can be used to compute Jacobian values (i.e., radiance derivative with respect to geophysical parameters), which are used for the inversion processing in retrieval and radiance assimilations. Using the tangent-linear and adjoint modules are equivalent to using the K-matrix module, which is also applied to radiance assimilation at some numerical prediction centers. The CRTM is a FORTRAN library for users to link to their own code instead of a graphic user interface. At the CRTM initialization step, the user selects the sensor/sensors and surface emissivity/reflectance look-up tables. Developers may incorporate their own expertise into the CRTM for any desired application. The gaseous transmittance describes atmospheric gaseous absorption so that one can utilize remote sensing information in data assimilation/retrieval systems for atmospheric temperature, moisture, and trace gases such as CO\({}_{2}\), O\({}_{3}\), N\({}_{2}\)O, CO, and CH\({}_{4}\)[18]. The aerosol module is fundamental to acquiring aerosol type and concentration for the study of air quality. The cloud module contains optical properties of six cloud types, providing radiative forcing information for weather forecasting and climate studies [19]. The CRTM  
Figure 3: Optical depths of absorbing gases: O\({}_{3}\), SO\({}_{2}\), NO\({}_{2}\), O\({}_{4}\), Bro, and CIO for U.S. standard atmosphere. The column ozone amount is 300 Dobson Units. surface model includes surface static and atlas-based emissivity/reflectivity for various surface types. There are two radiative transfer solvers that have been implemented in the CRTM. The advanced double-adding (ADA) solver [20] was chosen as a baseline. The advanced matrix operator method (AMOM) is a current baseline, and ADA is optional. The successive order of interaction (SOI) for radiative transfer [21] developed at the University of Wisconsin has also been implemented in the CRTM. The latest released CRTM version is 2.4. The CRTM version 3.0 beta version has also been released for testing. The CRTM 3.0 beta inherits the new features from the version 2.4. Users can now use cloud and aerosol optical tables in either binary or netCDF format. In previous CRTM releases, an on-line aerosol modeling in Goddard Chemistry Aerosol Radiation and Transport (GOCART) was developed and implemented within GMAO's GEOS-5 earth system model [22] and was later coupled with NCEP's NEMS version of GFS to establish the first interactive atmospheric aerosol forecasting system at NCEP, NGAC [23]. The recently added aerosol model is the Community Multi-scale Air Quality (CMAQ) model, which is more focused on regional areas. In previous releases, the CRTM was developed for un-polarized scattering atmosphere. The polarization calculation in the CRTM only comes from polarized surface emissivity and reflectivity.