H31I-01 INVITED
Observation of the water cycle from space with the Atmospheric Infrared Sounder (AIRS)
AIRS is one of six instruments on board the Aqua satellite, part of NASA's Earth Observing System launched in a sun synchronous near polar orbit on May 4, 2002. AIRS and its partner microwave instrument, AMSU A, provide high quality data facilitating studies of the global water and energy cycles, climate variation and trends, and the response of the climate system to increased greenhouse gases. The exceptional stability of the AIRS instrument provides a climate record of thermal infrared radiance spectra spanning the 3.74 15.4 mm spectral band with 2378 channels at a nominal resolution of 1/1200. (Chahine et al, in BAMS, July 2006) Accurate knowledge of the vertical distribution of water vapor in the atmosphere is critically important to the determination of the warming the Earth will experience as a result of anthropogenic forcing. Comparison of the AIRS specific humidity product to state of the art climate models has shown most models exhibit a pattern of drier than observed (by 10 25%) in the tropics below 800 hPa and moister than observed (by 25 100%) between 300 and 600 hPa in the extra tropics (Pierce et al, GRL 2006). The AIRS water vapor measurements also reveal tropospheric moisture perturbations that are much larger than those depicted in previous NCAR/NCEP reanalysis and ECMWF analysis datasets, both of which have been widely used as observations to validate models. This suggests that the impact of convection induced downdrafts on the atmospheric boundary layer is significantly underestimated in both ECMWF and NCEP reanalysis (Fu et al., GRL 2006). AIRS data have led to the discovery of significant differences in the lower troposphere moisture and temperature fields during the spatial temporal evolution of the Madden Julian Oscillation (MJO). The anomalous lower troposphere temperature structure is observed in detail by AIRS for the Indian and western Pacific Oceans, while it remains much less well defined in the NCEP temperature fields (Tian et al,GRL 2007). Information about the AIRS mission, products and research may be found at the AIRS Project web site: http://airs.jpl.nasa.gov. AIRS data products are freely accessible world wide at the Goddard Earth Sciences Data and Information Services Center (GES DISC) web site for AIRS support: http://disc.gsfc.nasa.gov/AIRS/.
H31I-02
Annual variability of Tropospheric Water Vapour Isotopes as observed by TES
We present an overview of three years of tropical tropospheric measurements of HDO and H2O from the Tropospheric Emission Spectrometer aboard the Aura satellite. The TES water vapour isotope data set spans September 2004 through the present. Uncertainties in the tropical measurements are approximately 12 parts per thousand relative to SMOW for the free troposphere. This uncertainty allows for quantification of yearly and seasonal variations in the HDO/H2O ratio and how these variations depend on the balance between the evaporative supply of water, atmospheric transport patterns and condensation processes.
H31I-03
TES: An Unexpected Opportunity to Revolutionize Large Scale Hydrological Studies
Stable isotopes of hydrogen and oxygen have proven to be useful tracers in catchment scale hydrology. Water molecules involving heavier isotopes of hydrogen (deuterium) or oxygen (oxygen-18) tend to stay in lower phase (solid or liquid) during phase changes resulting in a unique labeling effect. The labeled water fluxes often go through further transformation via mixing in the various storage pools of the water cycle. The resulting isotope signals are useful indicators of the hydrological processes affecting the water circulation in the hydrological cycle. Isotopes often offer a different viewing angle to the hydrological processes providing much needed independent validation information. The application of stable isotopes is largely limited today to catchment scale hydrological studies due to the difficulties in carrying out the intensive "in-situ" isotope sampling campaigns in large watersheds. The adequate sampling of the precipitation is particularly critical for the successful applications. The International Atomic Energy Agency recognized the importance of the stable isotope applications in large scale hydrological studies and initiated two programs to gather isotope information about the precipitation (Global Network of Isotopes in Precipitation, GNIP) and river runoff (Global Network of Isotopes in Rivers). While these efforts opened the door to large scale stable isotopes studies, the operational use of stable isotopes is still limited by the availability of time varying isotope information. The Water Systems Analysis Group of University of New Hampshire pioneered application of stable isotopes in large scales by developing isotope enabled version of its Water Balance/Transport model suite (WBM/WTM) in collaboration with the Isotope Hydrology Section of the International Atomic Energy Agency. While this effort demonstrated the potential of isotope hydrology in large watersheds, the lack of isotope data beyond what GNIP and GNIR has to offer limits the extension of the stable isotope applications. Recent discovery of the Tropospheric Emission Spectrometer's ability to detect the deuterium composition of the atmospheric water vapor and the potential of estimating the deuterium distribution in the boundary layer (aka precipitation) is opening new opportunities for the wider application of isotope hydrology. The ability to estimate the stable isotope composition of the precipitation removes the biggest obstacle to wide adaptation of isotope hydrology. While the adequate "in-situ" monitoring of precipitation is a costly endeavor, the complementary sampling of integrated isotope signals in major rivers is much more feasible. Our presentation will demonstrate the significance of stable isotopes in large scale hydrological studies. We will discuss the key processes and the resulting hydrological signals at continental scale applications. The presentation will layout the expected opportunities in applying isotope hydrology to answer unresolved science questions like the determination of the accurate partitioning of the precipitation into evapotransporation and runoff, estimation of subsurface water storages, identifying source areas in large water catchments, etc.
H31I-04 INVITED
Towards a Fully Integrated Rainfall Algorithm for GPM
The Global Precipitation Mission (GPM) will consist of a core spacecraft carrying a dual frequency (Ku/Ka) radar and a state-of-the-art passive microwave radiometer. In addition, the GPM concept includes a constellation of dedicated and existing spaceborne radiometers. Because existing radiometers are not under the configuration control of GPM, it is unwise to plan for algorithms that are optimized on planned or reported sensor characteristics. Instead, it is desirable for a single algorithm to accommodate any sensor in such a way that the information content of the sensor and not the specific sensor design (e.g., channel selection, view angles, spatial resolution) drive the final solution. A parametric algorithm for use by GPM is being constructed using current TRMM data. It begins by retrieving nonraining parameters when the radar detects no rainfall. An optimal estimation (OE) technique is used because it ensures that all sensor frequencies are matched between observations and retrieved geophysical parameters. This OE approach is quite robust over oceans but still requires refinement over land where theoretical models for the surface emissivity are still evolving. Synergy with other sensors and efforts is particularly critical over land. When the radar detects rainfall, cloud resolving models (CRMs) are matched to the radar reflectivity profile as a first guess. Radiative transfer computations are used to determine theoretical TMI brightness temperatures and these are in turn compared to the actual observations. Where there is no agreement, the DSD and ice density assumed by the radar are modified and the procedure is repeated until convergence between radar and radiometer is achieved. These products form an a-priori database, which radiometer only techniques can then exploit to create consistent rainfall products from diverse microwave sensors.
H31I-05
The Hydrological Cycle of the Madden-Julian Oscillation
The Madden-Julian Oscillation (MJO) is the dominant form of intra-seasonal variability in the Tropics and it impacts a wide range of phenomena, such as El Nino/La Nina, Asian-Australian monsoons, mid-latitude weather, and tropical cyclones. Despite the prominent impacts of the MJO and its potential predictability with lead times on the order of weeks, our weather and climate models have a relatively poor representation of the MJO and our environmental predictions suffer from this shortcoming. To date, the large-scale MJO convection and circulation characteristics have been relatively well documented and in some cases understood. For the most part, these studies have focused on quantities such as upper and lower level winds, outgoing longwave radiation and precipitation, and surface heat budget processes. In recent years, a number of studies have also documented aspects of the MJO's vertical structure impacts on biology and composition. In this study, we focus on the hydrological cycle of the MJO. With the addition of a number of new satellite products in recent years, it is possible to more completely describe most aspects of the hydrological cycle of the MJO. We build on recent work with AIRS water vapor and MLS cloud ice profiles to document and discuss the variations in rainfall (TRMM, CMAP), surface evaporation (derived via SSM/I etc), vertical profiles of moisture (AIRS), column moisture convergence (QuikScat, SSM/I), and cloud liquid (SSM/I) and ice water (MLS).
H31I-06
The NASA Earth Observing System (EOS) Evapotranspiration Product: the New MOD16
A multi-sensor Earth Observing System evapotranspiration (ET) algorithm and product, referred to as MOD16, will be presented here. The original MOD16 algorithms were based on remote sensing data solely from the Moderate Resolution Imaging Spectroradiometer (MODIS) and evolved first from an empirical relationship between MODIS vegetation surface temperature and ET to a Penman-Monteith type approach driven by the NASA GMAO global model output and MODIS vegetation parameters. An alternative MOD16 algorithm is based on the Surface Energy Budget System (SEBS) scheme of Su (2002) followed. This latter algorithm depends on a surface temperature – air temperature gradient as a core parameterization of the surface heat flux. The low temporal resolution of the MODIS land surface temperature (MOD11) product, coupled with the lack of a consistent MODIS-based surface radiation product made the use of advanced algorithms using only MODIS data to generate a MOD16 product quite challenging. In this study, we use a blended approach that utilizes both SEBS and the Penman-Monteith (PM) ET algorithms (depending on available information) to generate the "NASA Earth Observing System (EOS) Evapotranspiration Product". The algorithms use multi-sensor datasets from AQUA and TERRA that include CERES coarse resolution (20 km) surface radiation, AIRS surface meteorology and surface temperature, and MODIS vegetation and land surface temperature, when available. To evaluate the impact of using the CERES coarse resolution surface radiation, a high spatial resolution MODIS-based radiation product has been developed by the University of Maryland. The extensively validated SEBS algorithm serves as the primary ET estimator. When SEBS required inputs (primarily surface temperature) are not available, the secondary PM approach is implemented. To assure consistency and accuracy of the mixed-model ET output, the PM approach is calibrated to best-fit the climatology of the SEBS retrievals. This combined approach is used to generate daily ET estimates over North America for 2003. Regional and site-scale comparisons with observations, including the point-scale FLUXNET sites, demonstrate the potential of this approach to monitor land surface ET on a global basis at daily time scale.
H31I-07
Estimating Hourly Land Surface Temperatures at 1-km Spatial Scales over the Southwestern U.S.
Observations of land surface temperatures (LST) can be important for water cycle modeling because of its relationship to evapotranspiration and other surface energy fluxes. LST data have the potential to improve accuracies of such models by constraining soil moisture estimates and detecting water stress in vegetation. Considering studies at landscape to global scales, LST observations would ideally be provided at frequent time intervals (hourly) and at fine spatial resolution (100 m). Unfortunately, however, such observations are not currently possible. LST data are either provided hourly at coarse (4 km) spatial scales from geostationary satellites such as GOES, or episodically at moderate (1 km) to fine (60-120 m) spatial scales from polar-orbiting satellites such as MODIS, Landsat and ASTER. To improve LST sampling, a method has been developed that combines remote sensing observations into an LST data set at hourly time steps with 1-km spatial resolution. The approach relies upon accurate LST estimates from MODIS observations, a few of which are verified against ground observations in Oklahoma, Nevada, and New Mexico. The approach also relies upon the ability to screen cloud cover and to model the diurnal LST cycle with GOES observations. Results from synthesis of LST data collected in 2002-3 over the U.S. Southwest will be discussed, showing that estimation accuracies are often better than 2°C.
H31I-08
GRACE-Based Estimates of Terrestrial Freshwater Discharge from Basin to Continental Scales
Consequences of a rapidly changing climate have become a major concern for scientists and policymakers alike. It has become increasingly clear that pragmatic, near real-time information on freshwater water flux, at varied spatial scales, over the globe is of paramount importance in assessing changes in the global water cycle. Current capabilities for making such assessments are stalled by various issues. Large scale flow diversification in the deltaic regions, floodplain inundation, direct groundwater flows, drainage into wetlands are some of the pathways of water outflow that are not registered by the conventional streamgauges. Thus, in-channel streamflow, often used as a surrogate of net basin outflow, may in reality represent only a part of the net freshwater discharge and are therefore incomplete for proper budget analysis. Here we present large-scale estimates of freshwater discharge using Gravity Recovery and Climate Experiment (GRACE)-derived monthly terrestrial water storage changes in a combined land-atmosphere water mass balance. The computed estimates of freshwater discharge are subsequently analyzed in the context of global climate and compared with previously published estimates. This method has been previously tested on the Amazon, Mississippi and several large Arctic river basins. Results and comparisons to observations indicate that the method has important potential for global-scale discharge monitoring of combined surface water and submarine groundwater discharge at near- real time.