Julia Nogués-Paegle

Alternating Wet and Dry Conditions over South America during Summer

Abstract Time series of outgoing longwave radiation (OLR) fields and various gridded reanalysis products are used to identify and describe periods with abundant and deficient rainfall over South America during summer. Empirical orthogonal function analyses of OLR anomalies filtered to retain variations longer than 10 days reveal a meridional seesaw of dry and wet conditions over tropical and subtropical South America. It appears that intensification of the South Atlantic convergence zone (SACZ) is associated with rainfall deficits over the subtropical plains of South America. In contrast, when the SACZ weakens, precipitation over these plains is abundant. These results are in agreement with those of Kousky and Casarin. This seesaw pattern appears to be a regional component of a larger-scale system, possibly related to the 30–60-day oscillation in the Tropics, with the southward extension and strengthening of the SACZ found with enhanced tropical convection over the central and eastern Pacific and dry cond...

Dependence of Simulated Precipitation on Surface Evaporation during the 1993 United States Summer Floods

Abstract Regional summertime atmospheric conditions of 1993 are analyzed with the University of Utah Local Area Model (ULAM) by nudging boundary values and large internal scales of the local model toward values produced by the Nested Grid Model (NCEP/NOAA) initial analyses and forecasts archived at 6-h intervals. The approach allows the local ULAM to develop finer-scale structures in the precipitation and circulation forecasts than those resolved by the NGM. The study focuses on the influence of surface evaporation upon rainfall and low-level flow in regional simulations. Much of the rainfall simulated in the control experiment occurred from the late afternoon to early morning hours, with a pronounced midday minimum over the flood region. The moisture flux from the south due to the low-level jet (LLJ) provides much of the moisture source for the precipitation, and it is shown that the net moisture influx is significantly larger than the rainfall rate over the flood region. As a consequence, modifications ...

Physical Mechanisms of the 1993 Summer Floods

Abstract The physical mechanisms responsible for the onset and maintenance of the 1993 summer floods were examined using the localized Eliassen-Palm flux diagnostics and solutions of a single-level primitive equation model linearized about a meridionally varying basic state. The unusually long persistent summer pattern is linked with the marked transient eddy activity in late May and June. The feedback of eddies in the time mean flow caused a strengthening and eastward extension of the Pacific jet and a strengthening of the jet over North America. Results from the model suggest that the summer pattern may be interpreted as that of a lee trough forced by the Rocky Mountains in the presence of a strong westerly mean flow maintained by the eddies upstream. Composites from cases similar to that of the 1993 summer exhibit strong low-level southerly flow cast of the Rockies and suggest that the low-level jet may be an important mechanism to sustain the anomalous rainfall. It is concluded that the effect of the ...

WRF Model Sensitivity to Choice of Parameterization over South America: Validation against Surface Variables

Abstract The Weather and Research Forecast model is tested over South America in different configurations to identify the one that gives the best estimates of observed surface variables. Systematic, nonsystematic, and total errors are computed for 48-h forecasts initialized with the NCEP Global Data Assimilation System (GDAS). There is no unique model design that best fits all variables over the whole domain, and nonsystematic errors for all configurations differ little from one another; such differences are in most cases smaller than the observed day-to-day variability. An ensemble mean consisting of runs with different parameterizations gives the best skill for the whole domain. Surface variables are highly sensitive to the choice of land surface models. Surface temperature is well represented by the Noah land model, but dewpoint temperature is best estimated by the simplest land surface model considered here, which specifies soil moisture based on climatology. This underlines the need for better unders...

Wavelike Southern Hemisphere extratropical teleconnections

Abstract The dynamical basis of intraseasonal oscillations of the Southern Hemisphere summer and winter seasons is studied with a combination of observed diagnostics and simplified prognostic models. High-frequency oscillations, zonal mean variations, and seasonal and interannual variabilities are removed from the six-year dataset in an effort to reduce the effect of high-frequency dynamical instabilities and long-period forced fluctuations. The diagnoses focus upon those processes that have most frequently been explained in terms of Rossby-wave propagation through atmospheres with variable refractive indices. It is useful to study both winter and summer seasons simultaneously because of the large changes in the seasonally averaged state and large consequent changes in atmospheric waveguides between these seasons. A nonlinear shallow-water model slowly relaxed toward the time-averaged winter and summer observed mean fields is used to describe the characteristics of wave propagation in a horizontally varyi...

Intraseasonal interactions between the tropics and extratropics in the Southern Hemisphere

Abstract The mechanics of the interaction between tropical beating [estimated from outgoing longwave radiation (OLR)] and Southern Hemisphere (SH) subtropical and extratropical circulations on intraseasonal time scales are discussed. Base points are selected from teleconnectivity and teleconnection maps between OLR and zonal wind, heights, and meridional component of the divergent wind. Then, composites are formed for pentads with OLR anomalies at the base point greater in magnitude than one standard deviation. Enhanced convection over Indonesia is found to be associated with increases of both the southward component of the meridional divergent wind and of the westerly, zonal wind to the south of the heating region during the SH summer. The increased westerly wind gradients, resulting to a certain extent from strengthened northerly flow, together with increased values of the southward component of the divergent wind, lead to an enhancement of the Rossby wave source in the vorticity equation in the vicinit...

Predictability of the NCEP–NCAR Reanalysis Model during Austral Summer

Abstract Ten years of forecasts during Northern Hemisphere winter produced by the NCEP–NCAR reanalysis project are analyzed and their errors are documented with a focus on forecasts over South America. Previous studies have documented a seesaw pattern in the South Atlantic Convergence Zone (SACZ). Events associated with a strong SACZ are periods when the subtropical plains of South America exhibit precipitation deficits. When the SACZ weakens, precipitation in the plains is abundant. The forecast errors during these periods are examined separately. An error climatology is also obtained based on all available forecasts for January and February. It is found that cases with weak SACZ are characterized by relatively smaller forecast errors in the Americas and neighboring oceans during the first three days than are strong SACZ cases. The error growth for these weak SACZ cases is larger, and the forecast errors exceed those in the strong SACZ cases after about 5 days. Upper-tropospheric divergence is systematic...

Land-atmosphere interactions during a northwestern Argentina low event.

Abstract The impact of changes in soil moisture in subtropical Argentina in rainfall distribution and low-level circulation is studied with a state-of-the-art regional model in a downscaling mode, with different scenarios of soil moisture for a 10-day period. The selected case (starting 29 January 2003) was characterized by a northwestern Argentina low event associated with well-defined low-level northerly flow that extended east of the Andes over subtropical latitudes. Four tests were conducted at 40-km horizontal resolution with 31 sigma levels, decreasing and increasing the soil moisture initial condition by 50% over the entire domain, and imposing a 50% reduction over northwest Argentina and 50% increase over southeast South America. A control run with NCEP/Global Data Assimilation System (GDAS) initial conditions was used to assess the impact of the different soil moisture configurations. It was found that land surface interactions are stronger when soil moisture is decreased, with a coherent reducti...

Transient Response of the Southern Hemisphere Subtropical Jet to Tropical Forcing

Abstract The effect of tropical latent heat release in accelerations of the Southern Hemisphere subtropical jet is discussed based on a case study for 6–8 August 1979 and general circulation model simulations. This jet is a main feature of the winter time circulation extending over Australia and the western Pacific Ocean. An intensification of this jet was observed for 8 August, with peak values of 93 m s−1. Satellite pictures and daily precipitation amounts revealed that the 6–8 August period was characterized by extensive rainfall in the tropics, with precipitation diminishing from that day on. Level III-b data for the Global Weather Experiment produced by the Geophysical Fluid Dynamics Laboratory for these dates was projected into the normal modes of a primitive equation model linearized about a basic state at rest. The analysis shows that the jet accelerations were due both to external and internal Rossby modes, with somewhat stronger contributions from the external mode. An enhancement of the local m...

The Australian Subtropical Jet during the Second Observing Period of the Global Weather Experiment

Abstract The upper-level circulation of the Southern Hemisphere winter is characterized by two distinct zonal wind maxima: a subtropical jet found in the vicinity of Australia and the western Pacific Ocean, and a polar jet which maximizes in the Indian Ocean in the 45°–55°S latitudinal belt. This paper describes the global characteristics of the atmosphere for cases with strong subtropical jets during the 1979 Northern Hemisphere summer. Such cases are shown to co-exist with episodes of strong release of latent heat in the Northern Hemisphere at similar longitudes. Gridded analyses of the Global Weather Experiment produced by the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration are used to obtain composites of atmospheric motions prior to the onset and during active episodes of the Asian Monsoon. Projections of these motions into the normal modes of a linearized primitive equation model about a basic state at rest are presented to isolate observed global flow ch...