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

Abstract

This study examines the characteristics of cold-season (November–March) daily precipitation over the contiguous United States during active periods of the Madden–Julian oscillation (MJO). A large response in the precipitation rate anomaly is found over the eastern United States when MJO-related enhanced tropical convection is moving through the far western to central Pacific (conventionally known as phases 5, 6, and 7 of the MJO). Positive anomalies occur in the region of the eastern Mississippi River basin, and negative anomalies occur in the Southeast. The relative stability of this pattern throughout the three phases suggests that they can be considered together. During phases 5–7, the central United States has a daily precipitation rate between 110% and 150% of normal, while the precipitation rate over much of Florida is less than 70% of normal. Much of the lower Mississippi River basin region receives somewhat more frequent daily precipitation during MJO phases 5–7, but a greater increase is found in the daily precipitation intensity, suggesting more intense storms. On the other hand, Florida has substantially fewer daily precipitation events, with a smaller decrease in the intensity.

To understand the atmospheric mechanisms related to the above shifts in daily precipitation, elements of the atmospheric circulation were examined. Positive moisture flux convergence anomalies, which have been linked to increased precipitation rate and intensity, are found in the region of increased precipitation rate during MJO phases 5–7. During those phases, the North American jet stream is shifted northward, likely leading to a higher incidence of storms over the lower Mississippi River basin and fewer storms over Florida. This is supported by the fact that the storm track also shows increased activity over the central United States during MJO phases 5–7.

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

Abstract

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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Katherine E. Lukens, Ernesto Hugo Berbery, and Kevin I. Hodges

Abstract

Northern Hemisphere winter storm tracks and their relation to winter weather are investigated using NCEP CFSR data. Storm tracks are described by isentropic PV maxima within a Lagrangian framework; these correspond well with those described in previous studies. The current diagnostics focus on strong-storm tracks, which comprise storms that achieve a maximum PV exceeding the mean value by one standard deviation. Large increases in diabatic heating related to deep convection occur where the storm tracks are most intense. The cyclogenesis pattern shows that strong storms generally develop on the upstream sectors of the tracks. Intensification happens toward the eastern North Pacific and all across the North Atlantic Ocean, where enhanced storm-track-related weather is found. In this study, the relation of storm tracks to near-surface winds and precipitation is evaluated. The largest increases in storm-track-related winds are found where strong storms tend to develop and intensify, while storm precipitation is enhanced in areas where the storm tracks have their highest intensity. Strong storms represent about 16% of all storms but contribute 30%–50% of the storm precipitation in the storm-track regions. Both strong-storm-related winds and precipitation are prone to cause storm-related losses in the eastern U.S. and North American coasts. Over the oceans, maritime operations are expected to be most vulnerable to damage offshore of the U.S. coasts. Despite making up a small fraction of all storms, the strong-storm tracks have a significant imprint on winter weather in North America potentially leading to structural and economic loss.

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Omar V. Müller, Ernesto Hugo Berbery, Domingo Alcaraz-Segura, and Michael B. Ek

Abstract

This work discusses the land surface–atmosphere interactions during the severe drought of 2008 in southern South America, which was among the most severe in the last 50 years in terms of both intensity and extent. Once precipitation returned to normal values, it took about two months for the soil moisture content and vegetation to recover. The land surface effects were examined by contrasting long-term simulations using a consistent set of satellite-derived annually varying land surface biophysical properties against simulations using the conventional land-cover types in the Weather Research and Forecasting Model–Noah land surface model (WRF–Noah). The new land-cover dataset is based on ecosystem functional properties that capture changes in vegetation status due to climate anomalies and land-use changes.

The results show that the use of realistic information of vegetation states enhances the model performance, reducing the precipitation biases over the drought region and over areas of excessive precipitation. The precipitation bias reductions are attributed to the corresponding changes in greenness fraction, leaf area index, stomatal resistance, and surface roughness. The temperature simulation shows a generalized increase, which is attributable to a lower vegetation greenness and a doubling of the stomatal resistance that reduces the evapotranspiration rate. The increase of temperature has a beneficial effect toward the eastern part of the domain with a notable reduction of the bias, but not over the central region where the bias is increased. The overall results suggest that an improved representation of the surface processes may contribute to improving the predictive skill of the model system.

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Omar V. Müller, Ernesto Hugo Berbery, Domingo Alcaraz-Segura, and Michael B. Ek
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