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Y. C. Sud
and
G. K. Walker

Abstract

In all the atmospheric general circulation models (GCMs) at the Goddard Laboratory for Atmospheres (GLA), the influence of oceanic salinity on the saturation vapor pressure of seawater is ignored. Since the relative humidity in the oceanic boundary layer is generally high while the saturation vapor pressure of seawater is lowered by salinity, its neglect could have a nontrivial influence on the near-surface specific humidity gradient, a primary determinant of oceanic evaporation. Such an approximation might effect the simulated circulation and rainfall systematically. To evaluate this idea, we carried out a 5-yr-long salinity simulation (S) with the GLA GCM in which the influence of salinity on the saturation vapor pressure of seawater was included. Corresponding to this, a control simulation (C) with the GLA GCM in which the salinity effect was ignored was also available. Analyses of S-minus-C fields have shown some evidence of discernible systematic errors in the global evaporation, boundary layer specific humidity, and several key parameters that affect the onset of moist convection, for example, convective available potential energy. Several other systematic effects are also evident even though they remain small compared to the interannual variability of climate. The systematic interactions caused by the neglect of salinity are evidently spurious, and even though the final outcome is less dramatic than anticipated originally, several persistent systematic errors can be noted in the 5-yr mean fields. Based on these results, we infer that coupled ocean–atmosphere models that ignore the influence of salinity on ocean evaporation might also benefit from the salinity correction. Indeed, the correction is so trivial to include, its neglect in the modern state-of-the-art GCMs is unwarranted.

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Taikan Oki
and
Y. C. Sud

Abstract

As a first step toward designing a comprehensive model for validating land surface hydrology and river flow in Earth system models, a global river channel network has been prepared at 1° latitude × 1° longitude resolution. The end product is the Total Runoff Integrating Pathways (TRIP) network. The aim of TRIP is to provide information of lateral water movement over land following the paths of river channels. Flow directions were determined from vector data of river channels and river pathways available in two recent atlases; however, an automatic procedure using a digital elevation map of the corresponding horizontal resolution was used as a first guess. In this way, a template to convert the river discharge data into mean runoff per unit area of the basin has been obtained. One hundred eighty major rivers are identified and adequately resolved; they cover 63% of land, excluding Antarctica and Greenland. Most of the river basin sizes are well within a 20% difference of published values, with a root-mean-square error of approximately 10%. Furthermore, drainage areas for more than 400 gauging stations were delineated. Obviously, the stream lengths in TRIP are shorter than the natural lengths published as data. This is caused by the meandering of rivers in the real world. Meandering ratio (r M ), the ratio of actual (published) river length to the idealized river length, has been calculated. Averaged globally for all available data, r M is 1.4, although it is 1.3 for rivers with areas larger than 500,000 km2. The r M data will be useful in the design of the Scheme for Total Runoff Integrating Pathways (STRIP). In the current form, TRIP can be used as a template for producing a time series of river flow using a simple version of STRIP.

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David M. Mocko
and
Y. C. Sud

Abstract

Four different methods of estimating land surface evapotranspiration are compared by forcing each scheme with near-surface atmospheric and soil- and vegetation-type forcing data available through International Satellite Land Surface Climatology Project Initiative I for a 2-yr period (1987–88). The three classical energy balance methods by Penman, by Priestley–Taylor, and by Thornthwaite are chosen; however, the Thornthwaite method is combined with a Mintz formulation of the relationship between actual and potential evapotranspiration. The fourth method uses the Simplified Simple Biosphere Model (SSiB), which is currently used in the climate version of the Goddard Earth Observing System II GCM. The goal of this study is to determine the benefit of using SSiB as opposed to one of the energy balance schemes for accurate simulation of surface fluxes and hydrology. Direct comparison of sensible and latent fluxes and ground temperature is not possible because such datasets are not available. However, the schemes are intercompared. The Penman and Priestley–Taylor schemes produce higher evapotranspiration than SSiB, while the Mintz–Thornthwaite scheme produces lower evapotranspiration than SSiB. Comparisons of model-derived soil moisture with observations show SSiB performs well in Illinois but performs poorly in central Russia. This later problem has been identified to be emanating from errors in the calculation of snowmelt and its infiltration. Overall, runoff in the energy balance schemes show less of a seasonal cycle than does SSiB, partly because a larger contribution of snowmelt in SSiB goes directly into runoff. However, basin- and continental-scale runoff values from SSiB validate better with observations as compared to each of the three energy balance methods. This implies a better evapotranspiration and hydrologic cycle simulation by SSiB as compared to the energy balance methods.

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David M. Mocko
and
Y. C. Sud

Abstract

Refinements to the snow-physics scheme of the Simplified Simple Biosphere Model (SSiB) are described and evaluated. The upgrades include a partial redesign of the conceptual architecture of snowpack to better simulate the diurnal temperature of the snow surface. For a deep snowpack, there are two separate prognostic temperature snow layers: the top layer responds to diurnal fluctuations in the surface forcing, while the deep layer exhibits a slowly varying response. In addition, the use of a very deep soil temperature and a treatment of snow aging with its influence on snow density is parameterized and evaluated. The upgraded snow scheme produces better timing of snowmelt in Global Soil Wetness Project (GSWP)-style simulations using International Satellite Land Surface Climatology Project (ISLSCP) Initiative I data for 1987–88 in the Russian Wheat Belt region.

To simulate more realistic runoff in regions with high orographic variability, additional improvements are made to SSiB's soil hydrology. These improvements include an orography-based surface runoff scheme as well as interaction with a water table below SSiB's three soil layers. The addition of these parameterizations further helps to simulate more realistic runoff and accompanying prognostic soil moisture fields in the GSWP-style simulations.

In intercomparisons of the performance of the new snow-physics SSiB with its earlier versions using an 18-yr single-site dataset from Valdai, Russia, the revised version of SSiB described in this paper again produces the earliest onset of snowmelt. Soil moisture and deep soil temperatures also compare favorably with observations.

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Y. C. Sud
,
G. K. Walker
,
V. M. Mehta
, and
William K-M. Lau

Abstract

A recent version of the Goddard Earth Observing System GCM which contains several upgrades to the model's prognostic cloud physics and microphysics as well as snow and ice hydrology, was used to isolate the influences of the annual cycles of solar irradiation and sea surface temperatures (SSTs) on the annual cycle of circulation and precipitation. Four 50-month-long integrations were produced with the GCM. The first integration, called the control simulation, C, was forced with daily interpolated SSTs from a 30-yr climatology of monthly SST data. In this simulation both SSTs and incoming solar irradiance had their normally prescribed annual cycles. The next two companion simulations were called S1, which used annual mean prescribed incoming solar irradiation, and S2, which used annual mean prescribed SST; everything else was kept similar to C in these two simulations. In the fourth simulation, called S3, both SSTs and incoming solar irradiation at the top of the atmosphere were prescribed to always maintain their annual mean values. This constraint virtually eliminated all the annual cycle forcings in the simulation. Nevertheless, all simulations had the diurnal cycle of solar irradiation. An intercomparison of these simulations revealed the following.

  1. The poleward excursions of the zonal mean ITCZ and precipitation are strongly modulated by the annual cycles of SSTs and solar forcings. For the majority of the regions, particularly in the subtropical monsoonal regions, for example, India, Southeast Asia, and Australia, the influence of the annual cycle of solar heating was found to be stronger than that of the annual cycle of SSTs.

  2. The precipitation and circulation patterns over the Kuroshio Current region off the east coast of Asia were most affected by the SSTs and were strongly linked to the annual cycle(s) of local SSTs.

  3. The annual mode of precipitation over Amazonia had two regimes: an equatorial regime with a maximum in the month of March in S1 and a corresponding maximum in the month of January in S2. Control C, which had both annual cycles, produced both the “month of January” and “month of March” modes of precipitation. This shows how solar and SST annual cycles jointly influence the simulated annual modes of precipitation over South America. Surprisingly, the annual modes of precipitation in C were roughly equal to the sum of the annual modes of precipitation of S1 and S2.

  4. Precipitation over Sahelian Africa is significantly reduced in simulations lacking the annual cycle of solar irradiation. The opposite kind of influence of the annual cycle of solar radiation was noted in almost all other monsoonal regions: India, Southeast Asia, as well as Australia. The only exception is the continental United States, where the solar annual cycle showed only a relatively minor influence on the annual mode of precipitation.

  5. The simulated tropical intraseasonal oscillations (TIOs) were reasonably robust in each of the four simulations. This suggests that TIOs are an outcome of the internal dynamics of the atmosphere that may in turn be forced by the interactions among the physical and dynamical processes of the atmosphere. This conclusion is consistent with the robustness of the observed TIO modes throughout the annual cycle and the significant dependence of TIOs on physical parameterization(s).

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