Matching of the Fields-of-View (FOVs) of the sensors and the different capabilities of vertical resolution will have to be studied. The idea is to use active and the passive measurements to view the same atmospheric column, and together provide high vertical resolution and accurate properties of the region in common view (in the case of lidar this matched view will be along the satellite track). In the case of radar, this will be a 3D volume of atmosphere similar to the TRMM data set. The passive instruments also provide a wider cross-track scan and thus record a large region of the atmospheric and surface properties. In this way, the information retrieved across track can be continuously validated and enhanced using the high resolution profile observations obtained from active sensors. The vertical resolution of the active instruments will be vary according to the parameter derived. The nominal vertical resolution of the radar is 250 m. The nominal vertical and horizontal resolution for lidar varies according to the application. For clouds and aerosol, the vertical resolution is 50 m - 100 m; and 1 km for water vapor. The horizontal resolution will be 1-4 km for radar less than 1 km for lidar observations of clouds, 20 km for aerosol and 100 km for water vapor. The design goal is to insure easy matching in ground processing, including integer-multiple relationships of dimensions, similar centers of samples. Where technically feasible, some instruments (eg, vis-ir imager) will over-sample the FOV of other instruments to provide information about scene inhomogeneities at smaller scales.

An important issue for observing clouds, and to a lesser extent aerosol, is the diurnal or day/night variability. The preferred orbit would be a non-sun synchronous low altitude orbit to precess through the diurnal cycle. The altitude of the orbit should be maintained at 400-500 km to optimize sensitivity of the active sensors. Selecting the inclination of the orbit will involve some trade-off considerations. Lower inclination provides a more rapid sample through the diurnal cycle. However, the amount and distribution of polar clouds is also of critical importance to the topic of climate change. These clouds are difficult to detect and current retrieval methods are inaccurate. Active measurements are an important tool for studying these clouds and a polar or near polar orbit is also justified.

In view of the relatively short time-scales associated with atmospheric water processes and related heating processes, an adequate sample of different latitude and surface conditions, as well as seasonal and interannual variations, we can address the objectives of ARHM with a single research mission of a 5 year duration. Relevant to the objectives of the mission are the overlapping observations from weather satellites that will provide smaller-space-time scale context of ARHM measurements. Three years is a minimum (descope) duration but this is less likely to catch a typical ENSO climate anomaly. In 5 years ARHM will sample roughly 800,000 cloud systems which is a sufficiently large ensemble statistics of a wide range of dynamical cloud types from equatorial convective systems to polar cloud systems. The combination of sensors required to meet the goals of ARHM can be configured on a single platform or be accomplished through formation flying and/or coincident measurements with two (or more) satellite platforms.

For meaningful time-mean statistics it would be preferable to design the mission with two or more satellites to adequately sample the diurnal cycle. A two- satellite mission of comparable payload may be possible within the framework of international collaboration. Collaborations on missions similar to ARHM have been discussed with NASDA and ESA since the early 1990’s. For instance, ESA is currently studying an Earth Radiation Mission (ERM) which proposes a payload that has some similarity to the payload of this mission (backscattering lidar, cloud radar and radiative flux measurements but without precipitation radar). NASDA is also considering two missions, Atmos-A and Atmos-B, that have combined payloads similar to ARHM. The Atmos A is the TRMM follow-on mission and Atmos-B is a mission similar to ERM and the proposed CloudSat (ESSP).

The CloudSat mission proposed to ESSP is a pathfinding mission making first -of-a-kind observations of clouds from space with a 94 GHz nadir- viewing radar. The properties of clouds as a radar target at this frequency is well established theoretically. However, these properties are not as developed experimentally. The reflection properties of mid-latitude clouds is well established by surface and aircraft observations but much less data exist on the reflection properties of tropical and polar clouds. CloudSat will pathfind new knowledge about clouds providing the first global survey of their reflecting properties at 94 GHz. The next step is to produce a scanning version of this radar for ARHM. In addition to demonstrating the performance of a 94 GHz radar for cloud studies, CloudSat will demonstrate lifetime in space of 94 GHz components, and will demonstrate formation flying and coincident observations with other satellites thus opening options for implementing ARHM on multiple platforms. ICESat, flying in formation with CloudSat, will demonstrate the utility of combined lidar and cloud radar data from sensors carried on different satellites.

Although the instruments proposed have long heritage and are ready for space-borne application today, certain characteristics of the mission need to be addressed before finalizing requirements. At the mission level, the extent to which some of the measurements can be obtained from multiple satellites in orbit at the same time needs to be explored. For instance, the temperature and moisture sounding requirements might be satisfied by the operational systems. Similarly, TOA fluxes may be obtained from CERES or CERES-like sensors operating on other platforms. In addition to these considerations, issues such as FOV, pointing and overlap requirements require study.

Differential absorption/backscatter lidar: Backscatter lidar will be demonstrated by ICEsat and the MDS-2 mission of NASDA prior to ARHM. The technology for space-based water vapor DIAL is rapidly maturing with the development of high-efficiency, high energy, high spectral purity long-life lasers. The technological feasibility and capabilities of a space-borne water vapor DIAL has been addressed in a number of studies and LASE is a successfully demonstrated airborne prototype of a space-based instrument. The space-based DIAL system will operate in the 946 nm-band of water vapor, and it will have a simplified laser configuration based on a version of the Nd:YAG laser which can operate directly in this spectral region. This laser will be diode-pumped to meet efficiency and lifetime requirements for space operations. The receiver aperture will also be increased to at least 1-m diameter. A DIAL system based on this technology is proposed for demonstration under the Instrument Incubator Program (IIP), and it is expected that on an accelerated schedule this system could be ready for used in the ARHM mission as early as 2004. Power: 300 W, Weight 250 kg.

The 14/95 GHz radar: Measurements of precipitation using space-based 14 GHz radar have been advanced by TRMM. Cloud observations using 94 GHz radar from aircraft and the ground are also mature and space-borne measurements are expected prior to the launch of ARHM providing a level of maturity in cloud and precipitation observations at this frequency. Polarization capabilites of the radar and scanning requirements (as in TRMM) will have to be addressed. Frequency selection (e.g. 14 vs 35 GHz) needs consideration. Power 150 W. Weight 150 kg.

The Microwave Spectrometer: This instrument has a long heritage and represents a combination of the frequencies of the TMI, the frequencies used in humidity sounding and new higher sub-millimeter frequencies that are highly sensitive to ice content. An instrument with these higher frequencies will have flown in space before ARHM, although in limb- viewing mode. Aircraft measurements in support of CloudSat will clearly establish the technological feasibility and confirm the capabilities of these measurements. The requirements for this instrument are largely established. Some optimization of channels is required but these are likely to be those of the TMI, the MHS and new higher frequency channels. Selection of the relevant frequencies, while largely are established, will need to be addressed before setting final requirements. An airborne prototype of the sub-millimeter component of this instrument is being proposed under the recent the Instrument Incubator Program (IIP) AO and a smaller lightweight version of the TMI with reduced frequency capability is also being proposed under the IIP. Nominal power 100 W, weight 100 kg.

Visible/near infrared spectrometer: The approach of using spectrometer measurements to obtain cloud and aerosol will be firmly established prior to final ARHM design, partly through ongoing surface and aircaft measurement programs, partly through existing space-borne spectrometer measurements and partly through proposed ESSP and existing EOS instruments. For instance, OMI proposed for EOS-CHEM could serve as the basis for the proposed instrument. Spectrally- resolved measurements of oxygen absorption and water vapor/cloud particle absorption in the near-infrared are required. Additional polarization capabilities, such as demonstrated on POLDER and proposed by EOSP, as well as utility of UV measurements deserve careful consideration before finally establishing instrument specifications. Proposals to the current IIP are being developed for prototype aircraft instruments. Power 30W, weight 30 kg.

IR spectrometer: IR spectrometer measurements of clouds and water vapor are required to derive optical properties. The optimal spectral range and resolution for this purpose will be addressed but it is likely this spectral channels will be patterned after existing radiometer channels. Spectrometer measurements are also needed in conjunction with the microwave observations for sounding atmospheric temperature and water vapor. Sounding requirements need to be carefully identified within the context of existing operational IR sounding capabilities must be considered.

Because of the strong feedback effects of clouds on their corresponding atmospheric columns, it is necessary to retrieve the atmospheric temperature and water vapor profiles in the real environment of clouds, not just under cloudless conditions. Principally, the sounder will be the AIRS infrared spectrometer and a microwave radiometer. The two together will provide the necessary cloud clearing in order to allow for the retrieval of temperature and moisture profiles under a variety of cloud conditions in the fields of view. The combination of AIRS/AMSU will provide the necessary background information about the full atmospheric column below the clouds as well as above the clouds. AIRS has a spectral resolving power and wavelength coverage from 3.7 to 15.4 mm. The infrared measurements are complemented by two microwave instruments: AMSU-A & B. The infrared measurements provide the primary high accuracy sounding capability of 1 K/1 km for temperature and 15%/2 km for humidity in the presence of up to 80% clouds. The requirements on IR spectrometer needsto be carefully considered within the context of existing operational IR sounding capabilities and the extent to which these capabilities fulfill the needs of the mission. Power 150 W, Mass 120 kg.

Broadband fluxes: Utilization of CERES measurements from other platforms is preferred. Added spectral capability may be needed (e.g. visible and near-IR fluxes) is desirable). Power: 50W, weight 50 kg .