H13H-01
Evaporation and Water Transport Dynamics over the Oceans Derived from Satellites
As part of the NASA Energy and Water Cycle Study (NEWS), we have derived a number of global products
related to the evaporation and water transport over the world's oceans. These products include monthly
average, 0.25-degree maps of the zonal and meridional components of water vapor transport, water vapor
transport divergence, evaporation, precipitation, and column-integrated water vapor. These datasets, which
span the last 20 years ( 1987-2007), were constructed by integrating microwave observations from a variety
of different satellite sensors, with due care put on precise intercalibration.
The development and evaluation of these hydrologic variables has generated many science questions. For
example, global precipitation seems to be increasing with global warming at a rate comparable to the
increase in water vapor. This is in contrast to climate models that generally predict a muted response of
precipitation to global warming. It appears that this increase in precipitation is mostly due to increases in
heavy rain. We will discuss the relative roles of wind changes and vapor changes to the global transport
changes. The implication of water cycle closure to mid- and high-latitude precipitation and ocean-land
exchanges will be also be addressed.
http://www.nasa-news.org
H13H-02 INVITED
Evaluating Land-Atmosphere Coupling Strength Over CONUS Using Satellite-based Remote Sensing
Understanding the coupling strength between land and its overlying boundary layer is important to establishing the role of the surface state in boundary layer development and related processes. Much of our current understanding has resulted from model diagnostics carried out by Alan K. Betts using the European Center's (ECMWF) forecast and reanalysis model outputs. Other model based analysis under the GEWEX Land Atmospheric Coupling Experiments (GLACE), lead by Randy Koster, has suggested that models with strong coupling have inferred "hot spots" that imply enhanced predictability of seasonal precipitation. Other analysis (Mitchell, personal communication) suggests that models with strong coupling fail to represent the observed diurnal cycle of precipitation across the central U.S. Dirmeyer et al. in 2006 compared the coupling strength (using Betts" measure that relates surface soil moisture to the lifting condensation level (LCL) pressure) for a number of models from the GLACE experiment, which showed a wide range of strength. This presentation utilizes space-based remote sensing (RS) observations to estimate the strength of warm season land-atmosphere coupling over the continental US. The remote sensing products are derived from the suite of sensors on-board NASA Aqua, including AMSR-E (soil moisture), AIRS (relative humidity, air temperature, skin temperature), MODIS (LAI, NDVI), and CERES (radiation). The relative strength of coupling is quantified in terms of observational diagnostics set forth by the work of Alan Betts, based on his work with the ERA40 model output data set, and Fendall and Eltahir, based on radiosonde data. While the analysis covers the continental US (CONUS), emphasis is placed on the southern Great Plains where dense in-situ measurements enable direct comparison between coupling strengths obtained from ground observations and those from remote sensing, and a region that previous studies by Koster et al. have inferred to be a coupling "hot spot".
H13H-03
A Modeling and Observational Framework for Diagnosing Land-Atmosphere Coupling and Impacts on Water and Energy Cycle Prediction
Land-atmosphere interactions play a critical role in determining the evolution of both planetary boundary layer (PBL) and land surface temperature and moisture states. However, the degree of coupling between the land surface and atmosphere in numerical weather prediction models remains largely unexplored and undiagnosed due to the complex interactions and feedbacks present across a range of scales. Further, uncoupled systems or experiments (e.g., the Project for Intercomparison of Land Parameterization Schemes, PILPS) may lead to inaccurate water and energy cycle process understanding by neglecting feedback processes such as those governing the relationship between soil moisture and precipitation. In this study, a framework for quantifying land-atmosphere interactions is presented using a coupled mesoscale model with a suite of PBL and land surface model (LSM) options along with observations during field experiments in the U. S. Southern Great Plains. Specifically, the Weather Research and Forecasting (WRF) model has been coupled to NASA-GSFC's Land Information System (LIS), which provides a flexible and high-resolution representation and initialization of land surface physics and states. Within this framework the land surface water and energy balance and mixed layer equilibrium established by each PBL-LSM pair are evaluated in terms of the diurnal temperature and humidity evolution. Results show how these variables are sensitive to and, in fact, integrative of the dominant processes involved in land-atmosphere interaction, and may be evaluated in terms of observable properties of the coupled system (e.g. soil moisture, PBL depth) measured by current and future remote sensing platforms. Overall, this work provides a pathway to improve water and energy cycle prediction using the LIS-WRF system, and will serve as the foundation for pilot experiments to evaluate coupled modeling efforts within the international community.
H13H-04
Use Of Mesoscale Model (MM5) Forecasts As Proxy Of Surface Meteorological Data To Estimate Reference Evapotranspiration
Integrated assessments and efficient operation of water resources at basin levels require detailed information about water availability and water demand. Temporal and spatial variability of water resources have to be characterized and included in any model whose aim is to provide with better insights about basin responses to human and climatic perturbations. While rainfall-runoff processes have been widely studied and incorporated, the dynamics of water withdrawals has received comparatively less attention. Water demand at a basin level is influenced by many factors such soil moisture, vegetation type and irrigation system. However, climate is by far the major driver because meteorological conditions determine energy balances and vapor pressure deficits that determine the magnitudes of vapor flow from surface to atmosphere. Monitoring evaporation is a great challenge since specific and costly equipments are required. As an alternative, we use of semi-empirical equations such as the Penman-Monteith formula to estimate potential evapotranspiration based on surface weather observations. Unfortunately weather stations are scarce and do not always record all relevant variables. In this work, we evaluate the use of MM5 forecasts as proxy for surface meteorological data with the specific objective of assessing its ability to estimate evapotranspiration (ETo) in the Maipo river basin. Four procedures to obtain ETo were compared: a) MM5 estimates of latent heat flux; b) ETo from Penman- Monteith equation using raw outputs of weather variables from MM5; c) ETo from Penman-Monteith using MOS-corrected MM5 weather data; and d) MOS-corrected MM5 latent heat flux. We used class A Pan evaporation observations and ETo from daily surface data to compare all methods. We found that the estimation of ETo based on MOS-corrected weather variables is usually the most effective method to estimate reference evapotranspiration. Since MM5 outputs in this region are available at 25 km grids, the number of monitoring points increases substantially and improves the ability to capture spatial variability of water demands in the basin.
H13H-05 INVITED
Robust Responses of the Hydrological Cycle to Global Warming
Using the climate change experiments generated for the Fourth Assessment of the Intergovernmental Panel on Climate Change, this talk examines some aspects of the changes in the hydrological cycle that are robust across the models. These responses include the decrease in convective mass fluxes, the increase in horizontal moisture transport, the associated enhancement of the pattern of evaporation minus precipitation and its temporal variance, and the decrease in the horizontal sensible heat transport in the extratropics. All of these robust responses are consequences of the increase in lower-tropospheric water vapor. Given our confidence that lower-tropospheric water vapor will increase as the climate warms, we can predict with nearly as much confidence that other changes which are coupled to the increase in water vapor will also occur.
H13H-06 INVITED
Remote sensing, field measurements and large eddy simulation: Tools for investigating evaporation and land-atmosphere dynamics
Understanding and predicting the impact of land surface heterogeneity on landscape and regional scale evapotranspiration estimates is one of the major unresolved research problems being addressed by hydrologists and atmospheric scientists. While land cover characteristics and surface states used as input to land surface models are available at patch and landscape scale (~100-1000 m), atmospheric properties are often assumed uniform at regional scales (10-100 km). This scale mismatch between land surface and atmospheric states has been noted as a significant source of error in regional flux estimation. To address this issue, a research approach was developed which combined a Large Eddy Simulation (LES) with a land surface scheme that used remotely sensed land surface boundary conditions to simulate the three dimensional turbulence in the Atmospheric Boundary Layer (ABL). This approach provided the opportunity to explore the impact of variation in both the size and magnitude of contrasts in key land surface states/conditions (i.e., moisture, vegetation density, surface temperature) on atmospheric surface layer properties (i.e., wind speed, air temperature, humidity) using observations from large scale remote sensing field experiments. Research results will be presented on evaluating the roles of air temperature and wind speed variability in heat flux estimation, and on techniques for estimating spatial variability in atmospheric properties, which directly address the issue of scale mismatch. In addition, combining these tools has allowed us to investigate the impact of ABL dynamics on water vapor and sensible heat flux transport that is seen to cause anomalies in source-area footprint predictions when comparing land surface model output with aircraft-flux observations in the atmospheric surface layer. The later findings, point to the development of footprint models that consider different source area/footprint contributions for so-called active scalars (temperature) versus passive scalars (water vapor) by using remote sensing information on the spatial variation of important land surface states and atmospheric conditions.
H13H-07
The impact of Humidity Fluctuations on Sensible Heat Fluxes Measured with Scintillometers
In scintillometery the temperature structure function is estimated with the structure function of the index of refraction. Atmospheric scintillations depend on both temperature and humidity fluctuations. When the sensible heat flux is estimated from the measured fluctuations in the index of refraction, the effect of the humidity is modeled, however, there is no verification of the humidity model from field data. In the summer of 2008 a field campaign was conducted to measure the index of refraction structure function, temperature structure function, and humidity structure function independently, with scintillometers and a temperature/humidity LIDAR. These data are used to ascertain the validity of the assumptions used to determine sensible heat fluxes with scintillometers, and the impact of water vapor on the flux estimates.
H13H-08
A new two-Dimensional Physical Basis for the Complementary Relation Between Terrestrial and pan Evaporation
Archived global measurements of water loss from evaporation pans constitute an important indirect measure of evaporative flux. Historical data from evaporation pans shows a decreasing trend over the last half century, but the relationship between pan evaporation and moisture-limited terrestrial evaporation is complex, leading to ambiguities in the interpretation of this data. Under energy-limited conditions, pan evaporation (Epan) and moisture-limited terrestrial evaporation (E) increase or decrease together, while in moisture- limited conditions these fluxes form a complementary relation in which increases in one rate accompany decreases in the other. This has lead to debate about the meaning of the observed trends in the context of changing climate. Here a two-dimensional numerical model of a wet pan in a drying landscape is used to demonstrate that, over a wide range of realistic atmospheric and surface conditions, the influence that changes in E have on Epan (1) are complementary and linear, (2) do not depend upon surface wind speed, and (3) are strikingly asymmetrical, in that a unit decrease in E causes approximately a five-fold increase in Epan, as found in a recent analysis of daily evaporation from US grasslands (Kahler and Brutsaert, 2006). Previous attempts to explain the CR have been based on one dimensional diffusion and energy balance arguments, leading to analytic solutions based on Penman-type bulk difference equations. But without acknowledging the spatially complex multidimensional humidity and temperature field around the pan, and specifically how these fields change as the contrast between the wet pan and the drying land surface increases, such integrated bulk difference equations are a priori incomplete (they ignore important divergence terms), and thus these explanations must be considered physically incomplete. Results of the present study improve the theoretical foundation of the CR, thus increasing the reliability with which it can be applied to estimate water balance and to understand the pan evaporation record of climate change.
H13H-09
Long-term Observations of Ecohydrology, Climate, Energy Fluxes, and Eddy Covariance Error in a Large, Semiarid Floodplain
Some of the highest rates of water and energy fluxes between terrestrial ecosystems and the atmosphere
occur over large floodplains in arid and semiarid areas. Often located in high-pressure zones near 35
degrees latitude, abundant radiation and easily accessible groundwater contribute few limitations on growth
and production in desert phreatophytes. Desert regions typically undergo cycles of drought and floods, and
phreatophytic communities wax or wane in cover, density, and structure with cumulative species responses to
timing and severity in these regional weather cycles. The Rio-ET Laboratory at the University of New Mexico
has been collecting long-term data from a flux network of riparian monitoring stations, mounted on towers
along the Middle Rio Grande. Ongoing measurements of energy, water and carbon dioxide fluxes,
groundwater dynamics, meteorology, leaf area index, and community dynamics began at some locations in
1999. Recent reanalysis of the flux dataset was performed in which error correction procedures were
compared to each and other and in relation to an irrigated crop under advection. Most riparian sites
exhibited stable atmospheric stratification and an energy balance consistent with evaporative cooling.
Evaporative cooling was more prominent in the late afternoon and evening, during wet conditions. Reduced
latent heat fluxes were observed in a cottonwood forest following restoration and fire, but only in years when
the forest floor was not re-vegetated by opportunistic annuals or target removal species. Water use by
riparian phreatophytes was 1) non-responsive to drought during the monsoon season (non-native Russian
olive and monospecific saltcedar communities), 2) responded negatively to monsoon-season drought
(xeroriparian saltcedar and saltgrass mosaic community), or 3) responded positively to monsoon-season
drought (cottonwood forests). Water salvage related to ecological restoration is dependent upon restoration
strategy, emphasizing the importance of due diligent followup to prevent unintentional re-vegetation of the
site. Restoration of monospecific saltcedar provides the greatest opportunity for water salvage although
restoration of cottonwood forests through removal of densely-packed non-native understory results in
marginal water salvage. Benefits of ecosystem restoration increase with drought and during the period of
explosive growth following a period of prolonged drought.
http://bosque.unm.edu/~cleverly
H13H-10
Soil moisture feedbacks on convection triggers: the role of soil-plant hydrodynamics
The linkages between soil moisture dynamics and convection triggers, defined here as the first crossing between the boundary layer height and lifting condensation level, are complicated by a large number of interacting processes occurring over a wide range of space and time scales. To progress on this problem, a soil-plant hydrodynamics model was coupled to a simplified ABL budget to explore the feedback of soil moisture on convection triggers. Using a simplified homogenization technique, the soil-plant hydraulics formulation solves the intrinsically 3-D soil water movement equations by two 1-D coupled Richards' equations. The model is able to account mechanistically for features such as root water uptake, root water redistribution, and mid-day stomatal closure, all known to affect diurnal cycles of surface fluxes and consequently ABL growth. The ABL model considered the convective boundary layer as a slab with a discontinuity at the inversion layer. The coupled model was parameterized using the wealth of data already collected for a maturing Loblolly pine plantation situated in the Southeastern United States. Previous studies, which made use of surface flux measurements to drive an ABL model, have postulate that a negative feedback was possible, which could award the ecosystem with some degree of self-regulation of its water status. According to model simulations, this negative feedback is unlikely. However, drastic changes in external water sources to the ABL are needed for triggering convection when soil moisture is depleted. The apparent negative feedback originated from a decoupling between the water vapor sources needed to produce convection triggers and surface water vapor fluxes.