ACE is a proposed Tier 2 NASA Decadal Survey mission that will focus on clouds, aerosols, and precipitation as well as
ocean ecosystems. The primary objective of the clouds component of this mission is to advance our ability to predict
changes to the Earth’s hydrological cycle and energy balance in response to climate forcings by generating observational
constraints on future science questions, especially those associated with the effects of aerosol on clouds and
precipitation. ACE will continue and extend the measurement heritage that began with the A-Train and that will continue
through Earthcare. ACE planning efforts have identified several data streams that can contribute significantly to
characterizing the properties of clouds and precipitation and the physical processes that force these properties. These
include dual frequency Doppler radar, high spectral resolution lidar, polarimetric visible imagers, passive microwave and
submillimeter wave radiometry. While all these data streams are technologically feasible, their total cost is substantial
and likely prohibitive. It is, therefore, necessary to critically evaluate their contributions to the ACE science goals. We
have begun developing algorithms to explore this trade space. Specifically, we will describe our early exploratory
algorithms that take as input the set of potential ACE-like data streams and evaluate critically to what extent each data
stream influences the error in a specific cloud quantity retrieval.
The new NASA Enhanced MODIS Airborne Simulator (eMAS) is based on the legacy MAS system,
which has been used extensively in support of the NASA Earth Observing System program since
1995. eMAS consists of two separate instruments designed to fly together on the NASA ER-2 and
Global Hawk high altitude aircraft.
The eMAS-IR instrument is an upgraded version of the legacy MAS line-scanning spectrometer,
with 38 spectral bands in the wavelength range from 0.47 to 14.1 μm. The original LN2-cooled
MAS MWIR and LWIR spectrometers are replaced with a single vacuum-sealed, Stirling-cooled
assembly, having a single MWIR and twelve LWIR bands. This spectrometer module contains a
cold optical bench where both dispersive optics and detector arrays are maintained at cryogenic
temperatures to reduce infrared background noise, and ensure spectral stability during high altitude
The EMAS-HS instrument is a stand-alone push-broom imaging spectrometer, with 202 contiguous
spectral bands in the wavelength range from 0.38 to 2.40 μm. It consists of two Offner
spectrometers, mated to a 4-mirror anastigmatic telescope. The system has a single slit, and uses a
dichroic beam-splitter to divide the incoming energy between VNIR and SWIR focal plane arrays.
It will be synchronized and bore-sighted with the IR line-scanner, and includes an active source for
monitoring calibration stability.
eMAS is intended to support future satellite missions including the Hyperspectral Infrared Imager (
HyspIRI,) the National Polar-orbiting Operational Environmental Satellite System (NPOESS)
Preparatory Project (NPP,) and the follow-on Joint Polar Satellite System (JPSS.)
With a global frequency of occurrence near 30%, cirrus clouds wield a strong influence over the radiation budget of the Earth’s climate system due to their location in the upper troposphere. Currently, global climate models (GCMs) are unable to accurately represent cirrus cloud feedbacks on the radiation and hydrological cycles due to a lack of understanding of how to parameterize the effects of cirrus. This inability to parameterize the microphysical properties of cirrus clouds can be attributed to a general lack of observations of these clouds and their dynamical environment in the upper troposphere. While aircraft provide direct measurements in this region, their use is limited due to expense, and ground-based remote sensors such as radars and lidars, while also quite useful, are limited to just a few locales. Satellite measurements, on the other hand, are global in nature but limited in the sense that the cloud properties must be derived through the use of complicated inversion algorithms. One of the newer satellite instruments currently on board the NASA Earth Observing System Terra and Aqua platforms, is the moderate resolution Imaging Spectroradiometer. MODIS observes upwelling reflectance and radiance from the Earth's atmosphere and surface in 36 narrow spectral intervals ranging from .62 μm to 14.385 μm. By combining measurement channels that are non absorbing and thus sensitive to total cross sectional area with other channels that are absorbing and include sensitivities to particle size, the observed radiances can provide estimates of optical depth (τ) and effective radius (r<sub>e</sub>). Ice water path is calculated directly from these values. Validation of the retrievals is essential for eventual development of parameterizations that can be assimilated into GCMs.
Operational Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals of cloud optical thickness and effective particle radius employ well-known solar reflectance techniques using pre-calculated reflectance look-up tables. We develop a methodology for evaluating the quantitative uncertainty in simultaneous retrievals of cloud optical thickness and particle size for this type of algorithm and present example results. The technique uses retrieval sensitivity calculations derived from the reflectance look-up tables, coupled with estimates for the effect of various error terms on the uncertainty in inferring the reflectance at cloud-top. The error terms include the effects of the measurements, surface spectral albedos, and atmospheric corrections on both water and ice cloud retrievals. Results will deal exclusively with pixel-level uncertainties associated with plane-parallel clouds; real-world radiative departures from a plane-parallel model are an additional consideration. While we demonstrate the uncertainty technique with operational 1 km MODIS retrievals from the NASA Earth Observing System (EOS) Terra and Aqua satellite platforms, the technique is generally applicable to any reflectance-based satellite- or air-borne sensor retrieval using similar spectral channels.
Multiple photon scatterings inside an integrating sphere can result in significant path lengths compared with line-of- sight sources. In strong water vapor absorption channels, such as those on MODIS and the MODIS airborne simulator, these internal path lengths can result in a significant reduction in sphere output radiance. Path length probability distributions for photons exiting a sphere are determined using Monte Carlo calculations. Approximate analytic expressions are also derived. Results are used to determine the effect of water vapor absorption on integrating sphere sensor calibrations in several pertinent channels.
Over the past few years, the MODIS airborne simulator (MAS) has been providing imagery for EOS scientific algorithm development. Primarily flown aboard NASA's ER-2 aircraft, the MAS provides high spatial resolution (50 m at nadir) in 50 spectral channels from 0.55 to 14.2 micrometer, overlapping many MODIS and ASTER channels. This paper focuses on calibration of the short-wave (0.55 - 2.38 micrometer) channels, both radiometric and spectral, and calibration of the integrating sources. Also discussed is the dependence of the short-wave calibration on instrument temperature, showing significant reduction in the thermal sensitivity after recent instrument enhancements and upgrades. The procedures for intercomparison of MAS and AVIRIS (airborne visible/infrared imaging spectrometer) data are also discussed. Some limited comparisons for flights over Alaska (June 1995) are presented, although this analysis is in its initial stages and quantitative results are preliminary.
The MODIS airborne simulator (MAS), a scanning spectrometer built by Daedalus Enterprises for NASA Goddard Space Flight Center and Ames Research Center, is used for measuring reflected solar and emitted thermal radiation in 50 narrowband channels between 0.55 and 14.3 micrometers . The instrument provides multispectral images of outgoing radiation for purposes of developing and validating algorithms for the remote sensing of cloud, aerosol, water vapor, and surface properties from space. Nineteen of the channels on MAS have corresponding spectral channels on the moderate resolution imaging spectroradiometer (MODIS), an instrument being developed for the Earth Observing system (EOS) to be launched in the late 1990s. Flown aboard NASA's ER-2 aircraft, the MAS has a 2.5 mrad instantaneous field of view and scans perpendicular to the aircraft flight track with an angle of +/- 43 degree(s) about nadir. From a nominal ER-2 altitude of 20 km, images have a spatial resolution of 50 m at nadir and a 37 km swath width. We report on the status of the instrument, discuss recent design changes, and provide comparisons with MODIS. We also summarize MAS calibration work, especially efforts to calibrate those channels with strong water vapor absorption.