H23F-01 INVITED 13:45h
CO2-induced Changes in Extratropical Continental Hydrology
In this work, we revisit the question of continental summer drying in a doubled-CO2 environment using the latest version of the Geophysical Fluid Dynamics Labs (GFDL) model of the atmosphere and land surface coupled to a mixed layer (slab) ocean. The model differs substantially from previous GFDL models, notably in terms of the inclusion of enhanced vertical resolution, the inclusion of an explicit boundary layer, and a diurnal cycle of insolation. In the earlier models, the simulated response to a doubling of atmospheric CO2 included an increase in wintertime rainfall over most mid-latitude continental regions, an earlier snowmelt season and onset of springtime evaporation, and a higher ratio of evaporation to precipitation in summer. These factors led to large-scale increases in soil moisture in winter and decreases in summer in mid-latitudes in the doubled-CO2 experiment. The new model shows similar results, and the processes discussed above are important in this model as well. In addition, we find that changes in atmospheric circulation are playing an important role in driving regional hydrologic changes. We have conducted a set of additional experiments to isolate the relative roles of the land surface and the global ocean in generating the atmospheric circulation changes in the high CO2 simulation. We find that while tropical SST changes play the dominant role in driving the circulation changes, land surface feedbacks also play a crucial role. Such regional atmospheric circulation changes will play a critical role in projections of regional hydrologic changes in response to increasing greenhouse gases.
H23F-02 14:05h
An Off-Line Intercomparison of Soil Moisture Simulation by AOGCMs
Atmosphere-ocean general circulation models (AOGCMs) employ very different land surface schemes (LSSs) and as a result their predictions of land surface quantities such as soil moisture are often difficult to compare. Some of the disagreement in their results is likely due to differences in the atmospheric component; however, previous intercomparison studies have determined that different LSSs can produce very different results even when supplied with identical atmospheric forcing. A simple off-line LSS is presented that can reproduce the soil moisture simulations of various AOGCMs, based on their modeled temperature and precipitation. The scheme makes use of the well-established Thornthwaite method for estimating potential evapotranspiration combined with a variation of the Manabe `bucket' model. The model can be tuned to reproduce the control climate soil moisture of an AOGCM by adjusting the ease with which runoff and evapotranspiration continue as the moisture level in the bucket goes down. This produces a set of parameter values that can provide a reasonably good fit to each of several AOGCM control climates. As well, the parameter values can be set to imitate the LSS from one AOGCM while the model is forced with atmospheric data from another, providing an estimate of the magnitude of variation caused by the differences in land surface parameterization.
H23F-03 14:20h
Propagation of hydro-climatic fluctuations to soil moisture dynamics
Using field observation and analytical considerations, we investigate the propagation of temporal fluctuation in rainfall and potential evapotranspiration to the dynamics of soil moisture. In particular, we study a stochastic model of soil moisture with a marked Poisson process for rainfall, soil moisture-dependent infiltration, and Gaussian fluctuations in potential evapotraspiration. We derive and solve the equations for the probability distribution, mean, and variance of the fluctuations to elucidate the dynamic effect of external forcing on soil water balance. It is shown how hydroclimatic fluctuations are distorted and modified by the nonlinear soil moisture dynamics and how rainfall fluctuations are dominating soil moisture dynamics except when close to saturation, while evapo-transpiration fluctuations, being continuously modulated by soil moisture, tend to be relevant only for very shallow soils.
H23F-04 14:35h
How Do Precipitation, Evapotranspiration, and Runoff Contribute to Layered Soil Moisture Memory? - Results from A Climate Simulation
Previous studies have suggested that soil moisture memory is mainly controlled by four distinct factors: (i) nonstationarity (seasonality) in the atmospheric forcing, such as precipitation and net radiation; (ii) the effect of ET in removing soil moisture and hence its memory; (iii) the analogous dependence of runoff; and (iv) feedback of soil moisture on precipitation. The latter depends on how ET and runoff are partitioned between removal from the rooting zone soil column and from smaller stores located at or near the surface. This study quantifies the time scales over which these controls act globally using a 50-year climate simulation from CCM3-CLM. Our major findings are: a) soil moisture memory in warm climates can be at least several times longer for drier conditions than while it is sufficiently rainy; b) under wet conditions, the time scales of soil moisture appear to be controlled by temperature-dependent climatic demand but for drier conditions, appear to depend largely on increasing time scales for the coupling of soil moisture to ET and especially runoff; c) the longer soil moisture memory in deeper soil layers appears to respond and feedback to longer atmospheric variability.
H23F-05 14:50h
Timescales of Land Surface Hydrology
The hydrological cycle between the atmosphere, vegetation, and soil is important but the temporal structure and persistence of water stored in the vegetation and soil are complicated. In our recent work (Wang et al. 2004), we have investigated the temporal variation and vertical structures of vegetation and soil in response to precipitation forcing. A simplified interactive model has been developed that includes the basic physical processes of land surface hydrology. A theoretical framework has also been developed to analyze the timescales of land surface hydrology in both the one-layer bucket model and the current three-layer model. It is found that different water reservoirs of the vegetation-soil system have different timescales. Precipitation forcing is mainly concentrated on short timescales, but it can cause long timescale disturbances in the soil moisture of root zone. Furthermore, this timescale increases with increasing soil depth and is also strongly influenced by soil properties. It is also found that the inherent timescale determined by the ratio of the field capacity to the potential evapotranspiration is only a first-order approximation for the response timescale of each land surface component. Furthermore, comparison of this three-layer model with the one-layer bucket model indicates that their timescales of evapotranspiration are quite different although those of soil moisture itself do not differ much. This suggests the need to consider the vertical structure in land surface hydrology reservoirs and in climate study. References: Wang, A., and coauthors, Timescales of land surface hydrology, J. Hydromet., Submitted, 2004.
H23F-06 15:05h
Investigating Land Memory Characteristics Using Observations and Modeling
It has been speculated that understanding land memory dynamics may provide a basis for hydrologic and climate prediction at seasonal to longer time-scales. In this study the characteristics of land memory are explored using observed soil-moisture data from the Illinois Climate Network stations and corresponding simulated soil temperature. The soil moisture data is observed for 11 layers from the surface to a depth of 2 meters and spans 23 years, 1981 to 2003. The soil temperature profile is obtained using the numerical solution of the heat transfer equation, constrained by observed soil-moisture, and driven by observed surface variables which include air temperature, relative humidity, solar radiation, wind speed, and snow depth. Spatio-temporal variability of soil moisture and soil temperature with depth is explored. Depth-wise variability of the magnitudes of amplitude, phase-lag, and temporal scales are quantified for both soil moisture and soil temperature. Dominant signals are identified using spectral analysis techniques and their variability with depth is investigated. Linkage between the inter-annual signals of soil moisture, soil temperature, and ENSO is explored. Further, the sensitivity of surface temperature and surface energy fluxes to the variability of soil moisture at different depths is demonstrated.
H23F-07 15:20h
Forty Five Years of Observed Soil Moisture in the Ukraine: No Summer Desiccation (Yet)
We present the longest data set of observed soil moisture available in the world, 45 yr of gravimetrically-observed plant available soil moisture for the top 1 m of soil, observed every 10 days for April-October for 141 stations from fields with either winter or spring cereals from the Ukraine for 1958-2002. We averaged the summer observations over the entire region to account for the observed scale of soil moisture variations, to enhance the portion of the variance that is related to meteorological forcing. The observations show a positive soil moisture trend for the entire period of observation, with the trend leveling off in the last decade. Although models of global warming predict summer desiccation in a greenhouse-warmed world, there is no evidence for this in the observations yet, even though the region has been warming for the entire period. While the interannual variations of soil moisture simulated by both the European Centre for Medium-range Weather Prediction and the National Centers for Environmental Prediction/ National Center for Atmospheric Research reanalyses are close to the observations, neither reanalysis simulates the observed upward trend. Climate model simulations for the period show the same general shape as the observations, but differ quite a bit from each other and from the observations. An observed downward trend in insolation may have produced a downward trend in evaporation and may have contributed to the upward soil moisture trend.
http://climate.envsci.rutgers.edu/soil_moisture/ruswet.agro.readme.html
H23F-08 INVITED 15:35h
Role of Soil Moisture in Shaping Regional Climates
Soil moisture conditions are known to reflect temporal and spatial variability in local and regional climatic conditions. However, during the last decade several studies suggest a new important role for soil moisture in shaping climate variability at the regional scale. This paper reviews recent results of research by the author and his group on how soil moisture conditions impact the surface radiation balance, turbulent heat fluxes, boundary layer moist static energy, and hence the energetics of convective storms. Field observations and numerical modeling results are presented to test a specific hypothesis on how soil moisture conditions may impact future rainfall. Examples will be drawn from studies on the hydroclimatology of North America and West Africa.