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Ernesto Hugo Berbery

Abstract

The regional circulations that contribute moisture to the large precipitation over northwestern Mexico, the core region of the North American monsoon, are investigated using three summer seasons (July–September 1995–97) of Eta Model mesoscale analyses and forecasts. Analyses are produced by the Eta Model’s own four-dimensional data assimilation system that includes a diverse mix of observations. Comparison of the forecast precipitation with satellite estimates and previous observational studies shows similarity in location, shape, and scale of the patterns over northwestern Mexico; the magnitude of the precipitation over the slopes of the Sierra Madre Occidental is also similar to that from climatologies based on rain gauge observations. Examination of the morning and evening forecast precipitation also reveals agreement with equivalent estimates from high-resolution satellites. Excessive model forecast precipitation is found over the Isthmus of Tehuantepec in eastern Mexico, which seems related, at least in part, to deficiencies in the convective parameterization scheme.

Special attention is given to the diurnal cycle that is needed to resolve the interactions between circulation and precipitation. The Gulf of California exhibits evaporation through the entire diurnal cycle. In contrast, moisture flux divergence has a marked diurnal cycle with the largest magnitude over the gulf during the afternoon;this divergence is associated with the afternoon sea and valley breezes that favor a net transport of moisture toward the western slopes of the Sierra Madre Occidental. At the same time, large convergence of moisture flux develops over the slopes of the Sierra Madre Occidental, and is followed by intense afternoon–evening precipitation. The reverse circulation during nighttime and early morning results in moisture flux convergence near the coastline and over water, where early morning precipitation develops.

Large divergence of moisture flux is found over the northern sector of the Gulf of California at all times, and it results almost equally from transients and the time mean flow. The time mean flow is characterized by a nighttime and predawn low-level jet whose intensity is weaker than the Great Plains counterpart, but still appears to transport a significant amount of moisture into the southwestern United States. Northward transport of moisture is also accomplished by the transient fluxes that include, but are not limited to, the episodic northward moist surges frequently discussed in the literature.

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Seung-Jae Lee and Ernesto Hugo Berbery

Abstract

Deforestation and replacement of natural pastures by agriculture have become a common practice in the La Plata River basin in South America. The changes in land cover imply changes in the biophysical properties of the land surface, with possible impacts on the basin’s hydroclimate. To help understand to what extent the climate could be affected, and through which processes, ensembles of seasonal simulations were prepared using the Weather Research and Forecasting Model for a control case and a scenario assuming an expansion of the agricultural activities to cover the entire basin. The La Plata River basin shows different climate responses to the land cover changes depending on the region. The northern part of the basin, where forests and savanna were replaced by crops, experiences an overall increase in albedo that leads to a reduction of sensible heat flux and near-surface temperature. A reduction of surface roughness length leads to stronger low-level winds that, in turn, favor a larger amount of moisture being advected out of the northern part of the basin. The result is a reduction of the vertically integrated moisture flux convergence (VIMFC) and, consequently, in precipitation. In the southern part of the basin, changes from grasslands to crops reduce the albedo and thus increase the near-surface temperature. The reduction in surface roughness length is not as large as in the northern sector, reducing the northerly moisture fluxes and resulting in a net increase of VIMFC and, thus, in precipitation. Notably, advective processes modify the downstream circulation and precipitation patterns over the South Atlantic Ocean.

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Ernesto Hugo Berbery and Vicente R. Barros

Abstract

The main components of the hydrologic cycle of the La Plata basin in southeastern South America are investigated using a combination of observations, satellite products, and National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) global reanalyses. La Plata basin is second only to the Amazon basin in South America in river discharge and size and plays a critical role in the economies of the region. It is a primary factor in energy production, water resources, transportation, agriculture, and livestock.

Of particular interest was the evaluation of the annual cycle of the hydrologic cycle components. The La Plata annual-mean river discharge is about 21 000 m3 s−1, and the amplitude of its mean annual cycle is small: it is slightly larger during late summer, but continues with large volumes even during winter. The reason for this is that different precipitation regimes over different locations contribute to the total river discharge. One regime is found toward the northern boundary, where precipitation peaks during summer in association with the southernmost extension of the monsoon system. A second one is found over the central part of the basin, where precipitation peaks at different times in the seasonal cycle. Further analysis of the main tributaries of La Plata (Paraná, Uruguay, and Paraguay) reveals that each has a well-defined annual cycle but with different phases that can be traced primarily to each basin's physiography and precipitation regime.

Interannual and interdecadal variability of the basin's precipitation is amplified in the variability of streamflow by a factor of 2, implying a high sensitivity of the hydrologic system to climate changes like those observed in the last few decades. This becomes more important when considering the large variability of streamflow: for example, the historical maxima of river discharge during the year following the onset of El Niño can triple the typical mean river discharge.

A crucial component of the atmospheric water cycle, the low-level jet east of the Andes, supplies moisture from tropical South America to La Plata basin throughout the year. In lower latitudes, the jet has the greatest intensity during summer, but south of about 15°S there is a phase shift and the largest moisture fluxes are found during winter and spring. This is an uncommon feature not observed in other regions like the Great Plains of the United States, where the low-level jet develops only during the warm season.

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Anna A. Sörensson and Ernesto Hugo Berbery

Abstract

This work examines the evolution of soil moisture initialization biases and their effects on seasonal forecasts depending on the season and vegetation type for a regional model over the La Plata basin in South America. WRF–Noah simulations covering multiple cases during a 2-yr period are designed to emphasize the conceptual nature of the simulations at the expense of the statistical significance of the results. Analysis of the surface climate shows that the seasonal predictive skill is higher when the model is initialized during the wet season and the initial soil moisture differences are small. Large soil moisture biases introduce large surface temperature biases, particularly for savanna, grassland, and cropland vegetation covers at any time of the year, thus introducing uncertainty in the surface climate. Regions with evergreen broadleaf forest have roots that extend to the deep layer whose moisture content affects the surface temperature through changes in the partitioning of the surface fluxes. The uncertainties of monthly maximum temperature can reach several degrees Celsius during the dry season in cases when 1) the soil is much wetter in the reanalysis than in the WRF–Noah equilibrium soil moisture and 2) the memory of the initial value is long because of scarce rainfall and low temperatures. This study suggests that responses of the atmosphere to soil moisture initialization depend on how the initial wet and dry conditions are defined, stressing the need to take into account the characteristics of a particular region and season when defining soil moisture initialization experiments.

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Katherine E. Lukens and Ernesto Hugo Berbery

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This article examines to what extent the NCEP Climate Forecast System (CFS) weeks 3–4 reforecasts reproduce the CFS Reanalysis (CFSR) storm-track properties, and if so, whether the storm-track behavior can contribute to the prediction of related winter weather in North America. The storm tracks are described by objectively tracking isentropic potential vorticity (PV) anomalies for two periods (base, 1983–2002; validation, 2003–10) to assess their value in a more realistic forecast mode. Statistically significant positive PV biases are found in the storm-track reforecasts. Removal of systematic errors is found to improve general storm-track features. CFSR and Reforecast (CFSRR) reproduces well the observed intensity and spatial distributions of storm-track-related near-surface winds, with small yet significant biases found in the storm-track regions. Removal of the mean wind bias further reduces the error on average by 12%. The spatial distributions of the reforecast precipitation correspond well with the reanalysis, although significant positive biases are found across the contiguous United States. Removal of the precipitation bias reduces the error on average by 25%. The bias-corrected fields better depict the observed variability and exhibit additional improvements in the representation of winter weather associated with strong-storm tracks (the storms with more intense PV). Additionally, the reforecasts reproduce the characteristic intensity and frequency of hazardous strong-storm winds. The findings suggest a potential use of storm-track statistics in the advancement of subseasonal-to-seasonal weather prediction in North America.

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Emily J. Becker and Ernesto Hugo Berbery

Abstract

The structure of the diurnal cycle of warm-season precipitation and its associated fields during the North American monsoon are examined for the core monsoon region and for the southwestern United States, using a diverse set of observations, analyses, and forecasts from the North American Monsoon Experiment field campaign of 2004. Included are rain gauge and satellite estimates of precipitation, Eta Model forecasts, and the North American Regional Reanalysis (NARR). Daily rain rates are of about the same magnitude in all datasets with the exception of the Climate Prediction Center (CPC) Morphing (CMORPH) technique, which exhibits markedly higher precipitation values.

The diurnal cycle of precipitation within the core region occurs earlier in the day at higher topographic elevations, evolving with a westward shift of the maximum. This shift appears in the observations, reanalysis, and, while less pronounced, in the model forecasts. Examination of some of the fields associated with this cycle, including convective available potential energy (CAPE), convective inhibition (CIN), and moisture flux convergence (MFC), reveals that the westward shift appears in all of them, but more prominently in the latter.

In general, warm-season precipitation in southern Arizona and parts of New Mexico shows a strong effect due to northward moisture surges from the Gulf of California. A reported positive bias in the NARR northward winds over the Gulf of California limits their use with confidence for studies of the moist surges along the Gulf; thus, the analysis is complemented with operational analysis and the Eta Model short-term simulations. The nonsurge diurnal cycle of precipitation lags the CAPE maximum by 6 h and is simultaneous with a minimum of CIN, while the moisture flux remains divergent throughout the day. During surges, CAPE and CIN have modifications only to the amplitude of their cycles, but the moisture flux becomes strongly convergent about 6 h before the precipitation maximum, suggesting a stronger role in the development of precipitation.

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Eli J. Dennis and Ernesto Hugo Berbery

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Soil hydraulic properties are critical in estimating surface and subsurface processes, including surface fluxes, the distribution of soil moisture, and the extraction of water by root systems. In most numerical weather and climate models, those properties are assigned using maps of soil texture complemented by look-up tables. Comparison of two widely used soil texture databases, the USDA State Soil Geographic database (STATSGO) and Beijing Normal University’s soil texture database (GSDE), reveals that differences are widespread and can be spatially coherent over large areas that can eventually lead to regional climate differences. For instance, over the U.S. Great Plains, GSDE stipulates finer soil grains than STATSGO, while the opposite is true over central Mexico. In this study, we employ the WRF/CLM4 modeling suite to investigate the sensitivity of the simulated regional climate to changes in the prescribed soil maps. Wherever GSDE has finer grains than STATSGO (e.g., over the U.S. Great Plains), the soil retains water more strongly, as evidenced by smaller latent heat flux (−20 W m−2), larger sensible heat flux (+20 W m−2), and correspondingly, a decrease in the 2-m humidity (−1 g kg−1) and an increase in 2-m temperature (+1.5 K). The opposite behavior is found over areas of coarser grains in GSDE (e.g., over central Mexico). Further, the changes in surface fluxes via soil texture lead to differences in the thermodynamic structure of the PBL. Results suggest that neither soil hydraulic properties nor soil moisture solely dictate the strength of surface fluxes, but in combination they alter the land–atmosphere coupling in nontrivial ways.

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Ernesto Hugo Berbery and Michael S. Fox-Rabinovitz

Abstract

The onset and evolution of the North American monsoon system during the summer of 1993 were examined from regional to large scales using the National Aeronautics and Space Administration (NASA) Goddard Earth Observing System (GEOS) stretched-grid GCM. The model's grid spacing for the dynamical core ranges from 0.4° × 0.5° in latitude–longitude over the United States to about 2.5° × 3.5° at the antipode, and the physical package is solved on an intermediate 1° × 1° uniform grid. A diagnostic analysis of the monsoon's onset reveals the development of a positive potential temperature (θ) anomaly at the surface that favors a lower-level cyclonic circulation, while a negative potential vorticity (PV) anomaly below the tropopause induces an upper-level anticyclonic circulation. Ignoring diabatic effects, this pattern is consistent with the superimposition of idealized PV and θ anomalies as previously discussed in the literature. The inclusion of the smaller-scale features of the core monsoon in the model simulation helps represent the continental out-of-phase relationship between the monsoon and the southern Great Plains precipitation, giving additional support to earlier results that highlight the strong nature of the link. A pattern of increased precipitation over the core monsoon is consistently associated with increases of moisture flux convergence and ascending motions, and the development of upper-level wind divergence. On the other hand, the southern Great Plains have a simultaneous decrease of precipitation associated with a change from convergence to divergence of moisture flux, decreased ascending motions, and a development of upper-level wind convergence.

The Gulf of California low-level jet (LLJ) was inspected with a multitaper method spectral analysis, showing significant peaks for both the diurnal cycle and synoptic-scale modes, the latter resulting from the recurrent passage of Gulf surges. Those modes were then separated with a singular spectrum analysis decomposition. Compared with the Great Plains LLJ, the Gulf of California LLJ has a weaker diurnal cycle amplitude and a smaller ratio of diurnal cycle to synoptic-scale amplitudes. Additionally, the 1993 southwestern U.S. monsoon was analyzed by constructing composites of surge and no-surge cases. Given the particular characteristics of 1993 that include the effect of Hurricane Hilary, the extension of these results to other years needs to be assessed. Surges are associated with a strong Gulf of California LLJ and increased moisture flux from the Gulf into Arizona, and they accounted for 80%–100% of the simulated precipitation over Arizona, western New Mexico, and southern Utah. As distance from the Gulf is increased, there is a rapid decay of this percentage so that northern Utah and eastern New Mexico precipitation is almost unrelated to the surges. The results from this research show that the model's regional downscaling results in a realistic representation of the monsoon-related circulations at multiple scales.

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Emily J. Becker, Ernesto Hugo Berbery, and R. Wayne Higgins

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This study examines the seasonal characteristics of daily precipitation over the United States using the North American Regional Reanalysis (NARR). To help understand the physical mechanisms that contribute to changes in the characteristics of daily precipitation, vertically integrated moisture flux convergence (MFC) and precipitable water were included in the study. First, an analysis of the NARR precipitation was carried out because while observed precipitation is indirectly assimilated in the system, differences exist. The NARR mean seasonal amount is very close to that of observations throughout the year, although NARR exhibits a slight systematic bias toward more-frequent, lighter precipitation. Particularly during summer, the precipitation intensity and the probability distribution function (PDF) indicate that NARR somewhat underestimates extremes and overestimates lighter events in the eastern half of the United States. The intensity and PDF of moisture flux convergence exhibit a strong similarity to those of precipitation, suggesting a link between strong MFC and precipitation extremes. On the other hand, the relationship between the precipitable water and precipitation PDFs is weaker, based on the lack of agreement of their gamma distribution parameters.

The dependence of the precipitation PDF on the lower-frequency modulation of ENSO was examined. During El Niño winters, the Southwest and central United States, Gulf of Mexico region, and southeastern coast have greater precipitation intensity and extremes than during La Niña, and the Ohio River and Red River basins have lower intensity and fewer extreme events. During summer, the northern Rocky Mountains receive higher intensity precipitation with more extreme events. Most areas where the change in the daily mean precipitation between ENSO phases is large have greater shifts in the extreme tail of the PDF. The ENSO-related response of moisture flux convergence is similar to that of precipitation. ENSO-related shifts in the precipitation PDF do not appear to have a strong relationship to the shifts in precipitable water.

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Carolina S. Vera, Paula K. Vigliarolo, and Ernesto Hugo Berbery

Abstract

The most active winter synoptic-scale wave patterns over South America are identified using an extended empirical orthogonal function (EEOF) technique and are physically diagnosed using composite methods. Results show that the leading modes of short timescale variability propagate along two main paths: over the subtropical jet latitudes (∼30°S) and over the subpolar jet latitudes (∼60°S). This research focuses on the subtropical mode and its evolution over South America.

The observed structure of the systems associated with the subtropical mode resembles that of midlatitude baroclinic waves. Both cyclonic and anticyclonic perturbations display significant modifications in their three-dimensional structure as they evolve over extratropical and subtropical South America. While the upper-level perturbations are mostly unaffected when moving eastward, the lower-level perturbations advance following the shape of the Andes Mountains and exhibit an abrupt equatorward migration at the lee side of the mountains. As a result of such detachment, smaller eddy heat fluxes are observed in the vicinity of the orography and consequently a weaker eddy baroclinic growth is observed. Once the upper-level system is on the lee side, the perturbations acquire a more typical baroclinic wave structure and low-level intensification of the system occurs. The latter is largest around 1000 km east of the orography, where enhanced moisture transports from tropical latitudes along the eastern portion of the low-level cyclone favor precipitation occurrence over southeastern South America. Those precipitation processes seem to provide a diabatic source of energy that further contributes to the strengthening of the low-level cyclone. In addition, an intensification of the cyclone once over the ocean was found in 60% of the situations considered, which is consistent with previous research suggesting an additional source of moisture and heat flux due to the warm waters of the Brazil Current.

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