Search Results

You are looking at 1 - 7 of 7 items for :

  • Author or Editor: Siegfried D. Schubert x
  • Journal of Hydrometeorology x
  • All content x
Clear All Modify Search
Michael G. Bosilovich and Siegfried D. Schubert

Abstract

Precipitation recycling has been computed for 15 yr of reanalysis data from the National Aeronautics and Space Administration Goddard Earth Observing System (GEOS-1) Data Assimilation System using monthly mean hydrological data and a bulk diagnostic recycling model. This study focuses on the central United States and the extreme summers of 1988 (drought) and 1993 (flood). It is found that the 1988 summer recycling ratio is larger than that of 1993, and that the 1988 recycling ratio is much larger than average. The 1993 recycling ratio was less than average during the summer, but it was larger than average during the springtime, when the soil was being primed for flooding. In addition, the magnitude of summertime recycled precipitation was smaller than average in both 1988 and 1993. During the summer of 1993, the extremely large moisture transport dominates evaporation as the source of water for the extreme summer precipitation. The diagnosed recycling data show that the recycled precipitation is large when moisture transport is weak and convergence and evaporation are large. The analysis identifies the summer of 1989 as having the largest magnitude of recycled precipitation, resulting from a combination of low moisture transport and high moisture convergence.

Full access
Michael G. Bosilovich and Siegfried D. Schubert

Abstract

Numerous studies suggest that local feedback of surface evaporation on precipitation, known recycling, is a significant source of water for precipitation. Quantitative results on the exact amount of recycling have been difficult to obtain in view of the inherent limitations of diagnostic recycling calculations. The current study describes a calculation of the amount of local and remote geographic sources of surface evaporation for precipitation, based on the implementation of three-dimensional constituent tracers of regional water vapor sources [termed “water vapor tracers” (WVTs)] in a general circulation model. The major limitation on the accuracy of the recycling estimates is the veracity of the numerically simulated hydrological cycle, though it is noted that this approach also can be implemented within the context of a data assimilation system. In the WVT approach, each tracer is associated with an evaporative source region for a prognostic three-dimensional variable that represents a partial amount of the total atmospheric water vapor. The physical processes that act on a WVT are determined in proportion to those that act on the model's prognostic water vapor. In this way, the local and remote sources of water for precipitation can be predicted within the model simulation and validated against the model's prognostic water vapor. As a demonstration of the method, the regional hydrologic cycles for North America and India are evaluated for six summers (June, July, and August) of model simulation. More than 50% of the precipitation in the midwestern United States came from continental regional sources, and the local source was the largest of the regional tracers (14%). The Gulf of Mexico and Atlantic regions contributed 18% of the water for midwestern precipitation, but further analysis suggests that the greater region of the tropical Atlantic Ocean may also contribute significantly. In most North American continental regions, the local source of precipitation is correlated with total precipitation. There is a general positive correlation between local evaporation and local precipitation, but it can be weaker because large evaporation can occur when precipitation is inhibited. In India, the local source of precipitation is a small percentage of the precipitation, owing to the dominance of the atmospheric transport of oceanic water. The southern Indian Ocean provides a key source of water for both the Indian continent and the Sahelian region.

Full access
Randal D. Koster, Max J. Suarez, and Siegfried D. Schubert

Abstract

In an atmospheric general circulation model (AGCM), the physical bounds on soil moisture content and the nonlinear relationship between soil moisture and evaporation lead to distinct geographical patterns in key surface energy and water balance variables. In particular, simple hydrological considerations suggest—and extensive AGCM simulations confirm—that the variance and skew of seasonally averaged [June–August (JJA)] air temperature on the planet should be maximized in specific, and different, regions: a variance maximum should appear on the dry side of the soil moisture variance maximum, and a positive skew maximum should appear on the wet side of the temperature variance maximum. These ideas are tested with multidecade observational temperature data from the Global Historical Climatology Network (GHCN). In the United States, where sufficient data exist, the predicted patterns in the seasonal temperature moments show up where expected. These results suggest that hydrological considerations do indeed control the patterns of seasonal temperature variance and skew in nature.

Full access
Randal D. Koster, Siegfried D. Schubert, Anthony M. DeAngelis, Andrea M. Molod, and Sarith P. Mahanama

Abstract

Past studies have shown that accurate soil moisture initialization can contribute significant skill to near-surface air temperature (T2M) forecasts at subseasonal leads. The mechanisms by which soil moisture contributes such skill are examined here with a simple water balance–based model that captures the essence of soil moisture behavior in a state-of-the-art subseasonal-to-seasonal (S2S) forecasting system. The simple model successfully transforms initial soil moisture contents into average “forecast” evapotranspiration (ET) values at 16–30-day lead that agree well, during summer, with the values forecast by the full NASA GEOS S2S system, indicating that soil moisture initialization dominates over forecast meteorological conditions in determining ET fluxes at subseasonal leads. When the simple model’s ET anomalies are interpreted in terms of T2M anomalies, a similar conclusion is reached for T2M: soil moisture initialization explains much (about 50% in the eastern half of the continental United States) of the T2M anomaly values produced by the full GEOS S2S system at 16–30-day lead, and the T2M forecasts produced by the simple model capture about one-half of the skill attained by the full system. The simple model’s framework is particularly conducive to an analysis of uncertainty in forecasts. Drier soils are generally found to induce larger uncertainty in ET (and thus T2M) forecasts, a result linked to the functional form relating ET to soil moisture in the simple model and verified by an analysis of the ensemble spreads within the forecasts produced by the full GEOS S2S system.

Restricted access
Randal D. Koster, Rolf H. Reichle, Siegfried D. Schubert, and Sarith P. Mahanama

Abstract

Hydrological variability at a given location is characterized in part by a horizontal length scale—a measure of how far one can travel from that location and still see similar time variations of a hydrological variable of interest. Here, using Level-2 soil moisture retrievals produced by the NASA Soil Moisture Active Passive (SMAP) mission, we compute global distributions of these length scales for the Northern Hemisphere warm and cold seasons (May–September and November–March, respectively). The length scales show significant spatial and seasonal variability, with, as expected, much larger values (e-folding scales of greater than 500 km) often seen in the cold season, when convective rainfall is less prominent. The SMAP-derived length scales are found to be largely consistent with those derived directly, where possible, from precipitation measurements. This suggests a unique value of the retrievals: outside of well-instrumented areas, satellite-based soil moisture datasets have the potential to provide otherwise unattainable estimates of the horizontal length scales of hydrological variability.

Full access
Eric F. Wood, Siegfried D. Schubert, Andrew W. Wood, Christa D. Peters-Lidard, Kingtse C. Mo, Annarita Mariotti, and Roger S. Pulwarty

Abstract

This paper summarizes and synthesizes the research carried out under the NOAA Drought Task Force (DTF) and submitted in this special collection. The DTF is organized and supported by NOAA’s Climate Program Office with the National Integrated Drought Information System (NIDIS) and involves scientists from across NOAA, academia, and other agencies. The synthesis includes an assessment of successes and remaining challenges in monitoring and prediction capabilities, as well as a perspective of the current understanding of North American drought and key research gaps. Results from the DTF papers indicate that key successes for drought monitoring include the application of modern land surface hydrological models that can be used for objective drought analysis, including extended retrospective forcing datasets to support hydrologic reanalyses, and the expansion of near-real-time satellite-based monitoring and analyses, particularly those describing vegetation and evapotranspiration. In the area of drought prediction, successes highlighted in the papers include the development of the North American Multimodel Ensemble (NMME) suite of seasonal model forecasts, an established basis for the importance of La Niña in drought events over the southern Great Plains, and an appreciation of the role of internal atmospheric variability related to drought events. Despite such progress, there are still important limitations in our ability to predict various aspects of drought, including onset, duration, severity, and recovery. Critical challenges include (i) the development of objective, science-based integration approaches for merging multiple information sources; (ii) long, consistent hydrometeorological records to better characterize drought; and (iii) extending skillful precipitation forecasts beyond a 1-month lead time.

Full access
Myong-In Lee, Siegfried D. Schubert, Max J. Suarez, Isaac M. Held, Ngar-Cheung Lau, Jeffrey J. Ploshay, Arun Kumar, Hyun-Kyung Kim, and Jae-Kyung E. Schemm

Abstract

The diurnal cycle of warm-season rainfall over the continental United States and northern Mexico is analyzed in three global atmospheric general circulation models (AGCMs) from NCEP, GFDL, and the NASA Global Modeling Assimilation Office (GMAO). The results for each model are based on an ensemble of five summer simulations forced with climatological sea surface temperatures.

Although the overall patterns of time-mean (summer) rainfall and low-level winds are reasonably well simulated, all three models exhibit substantial regional deficiencies that appear to be related to problems with the diurnal cycle. Especially prominent are the discrepancies in the diurnal cycle of precipitation over the eastern slopes of the Rocky Mountains and adjacent Great Plains, including the failure to adequately capture the observed nocturnal peak. Moreover, the observed late afternoon–early evening eastward propagation of convection from the mountains into the Great Plains is not adequately simulated, contributing to the deficiencies in the diurnal cycle in the Great Plains. In the southeast United States, the models show a general tendency to rain in the early afternoon—several hours earlier than observed. Over the North American monsoon region in the southwest United States and northern Mexico, the phase of the broad-scale diurnal convection appears to be reasonably well simulated, though the coarse resolution of the runs precludes the simulation of key regional phenomena.

All three models employ deep convection schemes that assume fundamentally the same buoyancy closure based on simplified versions of the Arakawa–Schubert scheme. Nevertheless, substantial differences between the models in the diurnal cycle of convection highlight the important differences in their implementations and interactions with the boundary layer scheme. An analysis of local diurnal variations of convective available potential energy (CAPE) shows an overall tendency for an afternoon peak—a feature well simulated by the models. The simulated diurnal cycle of rainfall is in phase with the local CAPE variation over the southeast United States and the Rocky Mountains where the local surface boundary forcing is important in regulating the diurnal cycle of convection. On the other hand, the simulated diurnal cycle of rainfall tends to be too strongly tied to CAPE over the Great Plains, where the observed precipitation and CAPE are out of phase, implying that free atmospheric large-scale forcing plays a more important role than surface heat fluxes in initiating or inhibiting convection.

Full access