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K.-M. Lau, J. H. Kim, and Y. Sud

Results of an intercomparison study under the Atmospheric Model Intercomparison Project (AMIP) to assess the abilities of 29 global climate models (GCMs) in simulating various aspects of regional and hydrologic processes in response to observed sea surface temperature and sea ice boundary forcings are presented. The authors find that the models generally portray an earthlike climate to approximately 10%–20% of the global land surface temperature (= 14.8°C) and global precipitation (= 2.3 mm day−1) While a majority of the models have a reasonable global water budget, about a quarter of the models show significant errors in the total global water balance.

While the model frequency distributions of heavy precipitation associated with deep convection are in reasonable agreement with observations, a systematic underestimate of the frequency of occurrence of light precipitation events (< 1 mm day−1) is present in almost all the AMIP models, especially over continental desert regions and over tropical and subtropical oceanic regions contiguous to the west coasts of continents where low-level stratocumulus clouds tend to occur. This discrepancy is presumably related to the crude treatment of moist processes, especially those related to low clouds and nonconvective precipitation in the models. Another common problem in the global rainfall distribution is the presence of spectral rain or spurious gridpoint-scale heavy rain. The artificial anchoring of rainfall to topographic features in the Maritime Continent appears to be a generic problem in many GCMs. Models differ substantially in the magnitude of the rainfall amount over the eastern Pacific ITCZ for all seasons. The simulated boreal summer rainfall distributions have large variability over the Indian subcontinent and the Bay of Bengal. The northward migration of the monsoon convective zones are not well simulated. In particular, the East Asian monsoon rainband over the subtropical western Pacific is ill-defined or absent in all models.

On the interannual timescale, the models show reasonable skills in simulating the fluctuations of the Southern Oscillation and the eastward migration of the major equatorial precipitation zone during ENSO. Most models show useful rainfall prediction skill in the Tropics associated with ENSO-related SST forcing. However, the models do not show any useful skill for extratropical rainfall prediction from specified anomalous global SST forcing. Overall, the models depict a reasonably realistic annual cycle of water balance over regions where long-term local moisture balance is maintained—that is, (P–E) ≈ 0—over large interior land regions in the extratropics. In regions of strong dynamic control—that is, (P–E) >>0—such as the tropical western Pacific, monsoon regions, and the ITCZ, the intermodel variability is very large.

The simulated water balance over large river basins has been validated against hydrographic river discharge data using a river-routing model. Results show that while the model ensemble mean runoffs are consistent with the climatological observed river discharge for the Amazon and Mississippi, the intermodel variability is substantial. The models yield even more divergent results over other world river basins. These results suggest that while some GCMs may have moderate capability in capturing some aspects of the climatological variation of runoff, it is premature to use them for climate studies related to continental-scale water balance. A ranking of the AMIP models and some possible implications based on the above performance are also presented.

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Jae H. Kim, M. J. Newchurch, and Kunhee Han

Abstract

This work describes the first method to determine the tropical tropospheric ozone column directly from the Total Ozone Mapping Spectrometer (TOMS) space-borne instrument based on the physical differences in ozone-column detection as a function of its scan-angle geometry. Comparisons to tropical ozonesonde observations suggest the accuracy of these retrievals is ∼20%. Tropospheric ozone derived from this scan-angle method (SAM) exhibits a broad enhancement over South America, the southern Atlantic Ocean, and western South Africa and a minimum over the central Pacific Ocean in September–October. An ozone enhancement in equatorial North Africa is seen in March, the northern burning season. This ozone abundance is not detected by other retrieval methods. The magnitude of the ozone enhancement south of the equator is greater than the enhancement north of the equator. Abnormally high tropospheric ozone occurs over the western Pacific Ocean during the El Niño season when the ozone amounts are as high as those over Africa.

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C.-H. Ho, S.-J. Park, S.-J. Jeong, J. Kim, and J.-G. Jhun

Abstract

The impacts of harvested cropland in the double cropping region (DCR) of the northern China plains (NCP) on the regional climate are examined using surface meteorological data and the satellite-derived normalized difference vegetation index (NDVI) and land surface temperature (LST). The NDVI data are used to distinguish the DCR from the single cropping region (SCR) in the NCP. Notable increases in LST in the period May–June are found in the area identified as the DCR on the basis of the NDVI data. The difference between the mean daily maximum temperature averaged over the DCR and SCR stations peaks at 1.27°C in June. The specific humidity in the DCR is significantly smaller than in the SCR. These results suggest that the enhanced agricultural production by multiple cropping may amplify regional warming and aridity to further modify the regional climate in addition to the global climate change. Results in this study may also be used as a quantitative observed reference state of the crop/vegetation effects for future climate modeling studies.

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H. K. Ha, A. K. Wåhlin, T. W. Kim, S. H. Lee, J. H. Lee, H. J. Lee, C. S. Hong, L. Arneborg, G. Björk, and O. Kalén

Abstract

The circulation pathways and subsurface cooling and freshening of warm deep water on the central Amundsen Sea shelf are deduced from hydrographic transects and four subsurface moorings. The Amundsen Sea continental shelf is intersected by the Dotson trough (DT), leading from the outer shelf to the deep basins on the inner shelf. During the measurement period, warm deep water was observed to flow southward on the eastern side of DT in approximate geostrophic balance. A northward outflow from the shelf was also observed along the bottom in the western side of DT. Estimates of the flow rate suggest that up to one-third of the inflowing warm deep water leaves the shelf area below the thermocline in this deep outflow. The deep current was 1.2°C colder and 0.3 psu fresher than the inflow, but still warm, salty, and dense compared to the overlying water mass. The temperature and salinity properties suggest that the cooling and freshening process is induced by subsurface melting of glacial ice, possibly from basal melting of Dotson and Getz ice shelves. New heat budgets are presented, with a southward oceanic heat transport of 3.3 TW on the eastern side of the DT, a northward oceanic heat transport of 0.5–1.6 TW on the western side, and an ocean-to-glacier heat flux of 0.9–2.53 TW, equivalent to melting glacial ice at the rate of 83–237 km3 yr−1. Recent satellite-based estimates of basal melt rates for the glaciers suggest comparable values for the Getz and Dotson ice shelves.

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Kingtse C. Mo, J. E. Schemm, H. Kim, and W. R. Higgins

Abstract

The impact of initial conditions on summer precipitation over North America for July–September was examined by comparing long-term simulations of the Atmospheric Model Intercomparison Project (AMIP) runs with the ensemble simulations (SIMUs) initialized at the end of June each year. Both types of simulations use the observed SSTs as boundary conditions, and hence, the differences are a result of the initial conditions. Experiments were performed using the NCEP Global Forecast System (GFS) T126L28 model with 28 vertical levels. Over the monsoon core region, the model has the correct relationship between evaporation (E) and soil moisture and the T126 model has enough horizontal resolution to simulate the moisture transport from the Gulf of California. Over the Great Plains, the model has dry and warm biases. These biases are larger in the AMIP runs in comparison with the SIMU. Two major model errors contribute to the biases: 1) the deep soil layer (10–200 cm) is far too dry and does not have enough variability; and 2) the model does not have the correct relationship between E and soil moisture at the top level 10 cm (SM10cm). The linear E–SM10cm relationship in the model has a large slope in spring and early summer. Evaporation tends to drop too sharply and too quickly during the model dry periods; it does increase during rainfall periods, but the model still has an overall deficit in E. The SIMU runs have better estimated E values supplied by the initial conditions and the surface conditions deteriorate slow enough to have better seasonal mean summer E and precipitation than the AMIP runs. The results here may be model dependent. For the GFS model, the initial conditions supply better estimates of soil moisture and surface fluxes at the beginning of a simulation, which compensates for model errors.

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H. J. Lee, M. O. Kwon, S.-W. Yeh, Y.-O. Kwon, W. Park, J.-H. Park, Y. H. Kim, and M. A. Alexander

Abstract

Arctic sea ice area (SIA) during late summer and early fall decreased substantially over the last four decades, and its decline accelerated beginning in the early 2000s. Statistical analyses of observations show that enhanced poleward moisture transport from the North Pacific to the Arctic Ocean contributed to the accelerated SIA decrease during the most recent period. As a consequence, specific humidity in the Arctic Pacific sector significantly increased along with an increase of downward longwave radiation beginning in 2002, which led to a significant acceleration in the decline of SIA in the Arctic Pacific sector. The resulting sea ice loss led to increased evaporation in the Arctic Ocean, resulting in a further increase of the specific humidity in mid-to-late fall, thus acting as a positive feedback to the sea ice loss. The overall set of processes is also found in a long control simulation of a coupled climate model.

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Y. C. Sud, W. K-M. Lau, G. K. Walker, J-H. Kim, G. E. Liston, and P. J. Sellers

Abstract

Two 3-year (1979–1982) integrations were carried out with a version of the GLA GCM that contains the Simple Biosphere Model (SiB) for simulating land-atmosphere interactions. The control case used the usual SiB vegetation cover (comprising 12 vegetation types), while its twin, the deforestation case, imposed a scenario in which all tropical rainforests were entirely replaced by grassland. Except for this difference, all other initial and prescribed boundary conditions were kept identical in both integrations.

An intercomparison of the integrations shows that tropical deforestation

• decreases evapotranspiration and increases land surface outgoing longwave radiation and sensible heat flux, thereby warming and drying the planetary boundary layer. This happens despite the reduced absorption of solar radiation due to higher surface albedo of the deforested land.

• produces significant and robust local as well as global climate changes. The local effect includes significant changes (mostly reductions) in precipitation and diabatic heating, while the large-scale effect is to weaken the Hadley circulation but invigorate the southern Ferrel cell, drawing larger air mass from the indirect polar cells.

• decreases the surface stress (drag force) owing to reduced surface roughness of deforested land, which in turn intensifies winds in the planetary boundary layer, thereby affecting the dynamic structure of moisture convergence. The simulated surface winds are about 70% stronger and are accompanied by significant changes in the power spectrum of the annual cycle of surface and PBL winds and precipitation.

• Our results broadly confirm several findings of recent tropical deforestation simulation experiments. In addition, some global-scale climatic influences of deforestation not identified in earlier studies are delineated.

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A. K. Wåhlin, O. Kalén, L. Arneborg, G. Björk, G. K. Carvajal, H. K. Ha, T. W. Kim, S. H. Lee, J. H. Lee, and C. Stranne

Abstract

The ice shelves in the Amundsen Sea are thinning rapidly, and the main reason for their decline appears to be warm ocean currents circulating below the ice shelves and melting these from below. Ocean currents transport warm dense water onto the shelf, channeled by bathymetric troughs leading to the deep inner basins. A hydrographic mooring equipped with an upward-looking ADCP has been placed in one of these troughs on the central Amundsen shelf. The two years (2010/11) of mooring data are here used to characterize the inflow of warm deep water to the deep shelf basins. During both years, the warm layer thickness and temperature peaked in austral fall. The along-trough velocity is dominated by strong fluctuations that do not vary in the vertical. These fluctuations are correlated with the local wind, with eastward wind over the shelf and shelf break giving flow toward the ice shelves. In addition, there is a persistent flow of dense lower Circumpolar Deep Water (CDW) toward the ice shelves in the bottom layer. This bottom-intensified flow appears to be driven by buoyancy forces rather than the shelfbreak wind. The years of 2010 and 2011 were characterized by a comparatively stationary Amundsen Sea low, and hence there were no strong eastward winds during winter that could drive an upwelling of warm water along the shelf break. Regardless of this, there was a persistent flow of lower CDW in the bottom layer during the two years. The average heat transport toward the ice shelves in the trough was estimated from the mooring data to be 0.95 TW.

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J. M. Warnock, T. E. VanZandt, W. L. Clark, S. J. Franke, H. S. Kim, G. D. Nastrom, and P. E. Johnston

Abstract

An experimental field campaign to measure synoptic-scale vertical velocities was conducted from 5 to 11 January 1991 in the Urbana-Champaign, Illinois, region, which is in very flat terrain far from mountains. Both the Flatland and the Urbana wind-profiling radars, which are separated by 23.1 km, participated in the campaign. Meteorological sounding balloons were also launched from the Flatland Observatory site. In this study, lime averages are compared of the vertical wind velocity measured directly by both radars in order to help verify the capability of wind-profiling radars to measure synoptic-scale vertical velocities. This comparison, of course, also provides an opportunity to evaluate the performance of both radars.

The variance of the vertical velocity observed by the Flatland radar has been previously shown to be dominated by short-period fluctuations with most of the variance occurring at periods less than 6 h. Also, since March 1987 when the Flatland radar began operating nearly continuously, the vertical velocity measurements showed a nearly constant downward mean value of several centimeters per second in the troposphere. After bandpass filtering, the time-series measurements of vertical velocity to obtain 6-b and 1-day means, the filtered signal is compared to similar measurements made by the newly constructed Urbana radar. Both the 6-b and 1-day time averages of vertical velocity measured by the radars displayed large variations in time and height. Variations of 1.0–1.5 cm s−1 occurred frequently, which are considerably larger than the expected measurement error. Good to excellent agreement is generally found in the shape of height profiles measured by the two radars. These results suggest that wind-profiling radars located in very flat terrain are capable of measuring synoptic-scale vertical velocity profiles with useful precision.

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Steven R. Hanna, Michael J. Brown, Fernando E. Camelli, Stevens T. Chan, William J. Coirier, Olav R. Hansen, Alan H. Huber, Sura Kim, and R. Michael Reynolds

Computational fluid dynamics (CFD) model simulations of urban boundary layers have improved in speed and accuracy so that they are useful in assisting in planning emergency response activities related to releases of chemical or biological agents into the atmosphere in large cities such as New York, New York. In this paper, five CFD models [CFD-Urban, Finite Element Flow (FEFLO), Finite Element Model in 3D and Massively-Parallel version (FEM3MP), FLACS, and FLUENT–Environmental Protection Agency (FLUENT-EPA)] have been applied to the same 3D building data and geographic domain in Manhattan, using approximately the same wind input conditions. Wind flow observations are available from the Madison Square Garden 2005 (MSG05) field experiment. Plots of the CFD models' simulations and the observations of near-surface wind fields lead to the qualitative conclusion that the models generally agree with each other and with field observations over most parts of the computational domain, within typical atmospheric uncertainties of a factor of 2. The results are useful to emergency responders, suggesting, for example, that transport of a release at street level in a large city could extend for a few blocks in the upwind and crosswind directions. There are still key differences among the models for certain parts of the domain. Further examination of the differences among the models and the observations are necessary in order to understand the causal relationships.

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