Observing the Global Water Cycle from Space
This paper presents an approach to measuring all major components of the water cycle from space. Key elements of the global water cycle are discussed in terms of the storage of water-in the ocean, air, cloud and precipitation, in soil, ground water, snow and ice, and in lakes and rivers-and in terms of the global fluxes of water between these reservoirs. Approaches to measuring or otherwise evaluating the global water cycle are presented, and the limitations on known accuracy for many components of the water cycle are discussed, as are the characteristic spatial and temporal scales of the different water cycle components. Using these observational requirements for a global water cycle observing system, an approach to measuring the global water cycle from space is developed. The capabilities of various active and passive microwave instruments are discussed, as is the potential of supporting measurements from other sources. Examples of space observational systems, including TRMM/GPM precipitation measurement, cloud radars, soil moisture, sea surface salinity, temperature and humidity profiling, other measurement approaches and assimilation of the microwave and other data into interpretative computer models are discussed to develop the observational possibilities. The selection of orbits is then addressed, for orbit selection and antenna size/beamwidth considerations determine the sampling characteristics for satellite measurement systems. These considerations dictate a particular set of measurement possibilities, which are then matched to the observational sampling requirements based on the science. The results define a network of satellite instrumentation systems, many in low Earth orbit, a few in geostationary orbit, and all tied together through a sampling network that feeds the observations into a data-assimilative computer model.
H21I-02 INVITED 08:15h
Temporal changes in continental water storage from GRACE observations
We present results on land hydrology based on the inversion of monthly GRACE geoids spanning over a 2-year period (mid-2002 to mid-2004).The method of separation we have developed is based on generalized least-squares inversion (Ramillien et al., 2004). It consists of solving for `water height equivalent' spherical harmonic coefficients of four separate fluid reservoirs (atmosphere, ocean, soil waters and snow) after inverting the observed geoid coefficients (expressed in terms of time-variable anomalies with respect to a mean reference geoid) and using global hydrological model outputs as constrainsts. A cutoff at degree 30 (half-wavelenth of 650 km) is considered. The solutions are compared to two global hydrological models : the Land Dynamics model (Milly and Shmakin, 2002) and the Watergap Global Hydrology model (Doll et al, 2003). The monthly GRACE solutions for total land waters (sum of soil water, including underground waters, plus snow) show patterns in rather good agreement with the models, especially at the seasonal frequency. The strongest signal is observed in large tropical river basins (Amazon basin in South America, Congo and Niger basins in Africa, Ganges and Bramhapoutra basin in North Western India, etc.), as well as in Northeast America, and Western Asia (Volga basin) and Central Asia (Ob and Lena basins). In these northern hemisphere regions, the observed signal results from the combined contributions of total soil water and snow. In terms of relative amplitude, the solution differs significantly from the models however, as do the models each other.
H21I-03 INVITED 08:30h
Means, Variability and Extremes of Precipitation in the Global Climate as Determined by the 25-year GEWEX/GPCP Data Set
The Global Precipitation Climatology Project (GPCP) 25-year precipitation data set is used as a basis to evaluate the mean state, variability and extremes of global and regional scales of precipitation. The uncertainties of these characteristics of the data set are evaluated by examination of other, parallel data sets and examination of shorter periods with higher quality data (e.g., TRMM). The global and regional means are assessed for uncertainty by comparing with other satellite and gauge data sets, both globally and regionally. The GPCP global mean of 2.6 mm/day is divided into values of ocean and land and major latitude bands (Tropics, mid-latitudes, etc.). Seasonal variations globally and by region are shown and uncertainties estimated. The variability of precipitation year-to-year is shown to be related to ENSO variations and volcanoes and is evaluated in relation to the overall lack of a significant global trend. The GPCP data set necessarily has a heterogeneous time series of input data sources, so part of the assessment described above is to test the initial results for potential influence by major data boundaries in the record. Regional trends are also analyzed to determine validity and correlation with other long-term data sets related to the hydrological cycle (e.g., clouds and ocean surface fluxes). Statistics of extremes (both wet and dry) are analyzed at the monthly time scale for the 25 years. The validity of a preliminary result of increasing frequency of extreme monthly values during the period will be a focus. Daily values for an eight-year period are also examined for variation in extremes and compared to the longer monthly-based study.
H21I-04 INVITED 08:45h
Observing the Ocean Water Cycle with GRACE
Monthly estimates of the Earth's gravitational field from the GRACE mission are used to construct a time-series of global mean ocean mass variations between August 2002 and August 2004. We discuss the methods used to convert gravity coefficients into water mass variations, pointing out several important corrections. The GRACE observations show a clear seasonal exchange of water mass with the continents ( amplitude ~8.5 mm, phase maximum in early- to mid-October). The largest continental contributors to the ocean mass variability are discussed. Using two years of observations, we begin to examine interannual variability, with comments on observing trends in eustatic sea level change.
Estimating Water Vapor Transport Using a Multiple Satellite Network
The purpose of this study is to demonstrate the viability of estimating water vapor transport using polar orbiting satellite data. Columnar water vapor estimates are obtained from microwave radiometers with a high degree of accuracy using the Wentz retrieval algorithm. Feature tracking techniques are applied to the water vapor data to estimate the transport vector without resorting to the use of ancillary data (such as analysis wind fields). Successful use of feature tracking is made possible by the large number of satellites used in this study. We use data from 2003 when six radiometers were available: 3 SSMI on the DMSP satellites, TMI on the TRMM satellite, AMSR-E on Aqua, and AMSR on Midori-II. Feature tracking has been used with GOES water vapor imagery to estimate water vapor transport, however these estimates are more indicative of the upper-tropospheric water vapor. Our estimates are more representative of the vertically averaged water vapor transport. We will present 1-degree by 1-degree monthly maps of the zonal and meridional components of water vapor transport as well as water vapor divergence. We will include comparisons of our estimates of the zonal and meridional transport with estimates from existing model analysis, radiosondes, and GOES water vapor transport. We will also present comparisons between our estimates of water vapor divergence and satellite-derived evaporation minus precipitation estimates to assess how far we are from closing the atmospheric branch of the hydrological cycle over the oceans using satellite data alone.
Combining The Monthly-Mean MODIS And SSMI Data To Form A Global Water Vapor Data Set
Water vapor plays an important role in earth's hydrological processes. At present, the column amount of atmospheric water vapor over clear land surfaces and over oceanic areas with sun glint is operationally retrieved from radiances measured with near-IR channels of the Moderate Resolution Imaging SpectroRadiometer (MODIS) instruments on board the NASA Terra and Aqua Spacecraft platforms. TPW over oceans has also been derived operationally from radiances measured by the Special Sensor Microwave Imagers (SSMI) on board different spacecraft. By merging the monthly-mean MODIS near-IR TPW data over land and the SSMI TPW data over water surfaces, a global water vapor data product with improved quality is obtained. Major characteristics of the seasonal variations of TPW are described. Examples of water vapor anomalies associated with the 2002 El Nino event are presented. It is expected that the merged TPW data product will have important applications for the study of the global hydrological cycle.
H21I-07 INVITED 09:30h
A Space Lidar Monitoring the Global Water Cycle - NASA's ICESat Mission
Since it was launched and began operating in early 2003, NASA's Ice, Cloud, and land Elevation Satellite (ICESat) has produced valuable data on aspects of the global water cycle with its two-channel Geoscience Laser Altimeter System (GLAS) instrument. ICESat was specifically designed to provide precision elevation data through time across Greenland and Antarctica between ~86 deg. N-S latitude. This data, accumulated from repeat tracks and crossovers, will enable improved estimates of ice sheet mass balance and any ongoing changes. Because of its potential impact on the altimetry data, a second and complimentary mission goal was to collect data on clouds and aerosols and their distribution around the world. Other aspects of the global hydrologic cycle were also part of the initial mission plan including sea ice, vegetation cover, surface hydrology, and ocean applications. This presentation will highlight results from ICESat's operations in 2003 and 2004. Although technical issues with GLAS have more than halved the planned mission, the precision of the available altimetry data is excellent and glaciologically significant results are being derived. Cloud and aerosol layers are being resolved with unprecedented detail and indicate, for example, that deep convection reaches the tropopause far more frequently than had been previously thought. Sea ice cover and thickness maps are being derived for both Arctic and Antarctic waters. Characteristics of vegetation cover can also be retrieved for a limited range of surface slopes. Surface water results from ICESat include detection of river surface slopes along long sections of river channel or between multiple track crossings of a meandering channel with an accuracy sufficient for detection of stage changes over time, In summary, the precise laser altimetry data from ICESat and GLAS is establishing a valuable global framework. Due to its restricted operations plan, creative use of other earth-observing sensors as well as modeling will be needed to fully utilize the data being acquired by ICESat.
Enhanced snow cover products from MODIS for the hydrologic sciences
Near mountain ranges of the globe, over a billion people use melt of the seasonal snow cover as the dominant source of their water resources. These regions are increasingly experiencing the pressing, coupled implications of climate change, drought, and population/demand increase. Enhanced snow cover products have been developed using a multiple endmember spectral mixture analysis model that inverts MODIS surface reflectance products (MOD09) for fractional snow cover, plus the grain size and albedo of the fractional snow cover. Referred to as the MODIS Snow Covered Area and Grain Size/Albedo (MODSCAG) model, this tool is specifically aimed at providing an accurate estimate of snowcover for regional studies in mountainous areas across the globe. The model uses spectral libraries generated with a radiative transfer model for varying grain size snow, adapting the spectral library according to the specific scene solar geometry. Both the albedo and snow-covered area products stimulate advances in hydrologic forecasting by providing more accurate, spatially distributed data than are currently available for assimilation and model evaluation. Data are being developed and provided through an end-to-end NASA REASoN project that includes: (i) standard and custom product development, (ii) distribution through multiple user interfaces and (iii) user support for product evaluation and applications. Currently, MODSCAG is producing snow cover products for hydrologic research clients working in the Colorado River Basin, the Sierra Nevada of California, the Columbia River Basin, and central Tibet. Within the framework of the REASoN project, MODSCAG is designed to address `client' research needs in snow-covered basins around the globe. In this work, we present the introduction of the MODSCAG fractional snow cover products into a model of basin hydrology in the Sierra Nevada. We analyze the differences between the MODSCAG fractional product and the standard MODIS binary snowcover product for mountainous areas in the Southwestern U.S. relative to region, elevation, and time of year.