Relaciones reflectividad-intensidad de precipitación (Z-R) resultantes de diferentes criterios teóricos y estadísticos de datos experimentales

La medición del espectro de tamaños de gotas de lluvia permite calcular la intensidad de precipitación (R) y la reflectividad (Z). Se estudiaron varias leyes potenciales que vinculan a estas variables. Se comprobó que tanto las relaciones Z-R teóricas como las obtenidas a partir de muestras en otras regiones (caso Marshall y Palmer) no son válidas al nivel de significancia del 5% para representar a R como una función de Z. Al obtener relaciones a partir de una muestra en una región de estudio -en este caso Buenos Aires- se comprobó que una relación Z-R apta es, como era de esperar, la que surge de hacer una regresión por cuadrados mínimos entre los logaritmos de las variables. Sin embargo, hay situaciones en que son otros los criterios que deben ser tenidos en cuenta. De los analizados, se destaca el que hace mínima la diferencia de los promedios y de las desviaciones standard (simultáneamente) de las series medida y calculada. Con este criterio se obtuvo una relación Z-R que produce una serie de intensidad de precipitación calculada cuyas características estadísticas son semejantes a las de la serie medida.Rainfall Rate (R) and Radar reflectivity (Z) can be calculated from the measurement of drop size spectra. Several potential laws which relate those variables were studied. Both theoretical Z-R relationships and other regions relationships (Marshall and Palmer for instance) fail to represent R as a function of Z at the 5% level of significance. When Z-R relationships were obtained with a sample in the study region, in this case Buenos Aires, it was confirmed that a suitable Z-R relationship is, as it was expected, the one obtained from a least squares regression line between the logarithms of the variables. However, there are situations in which other criteria must be taken into account. Among those analyzed, the one which minimizes the difference between the means and the standard deviations of the measured and calculated series, stands out. By this criterium a Z-R relationship which produces a rainfall rate calculated series with similar statistical characteristics to the measured series was obtained.Asociación Argentina de Geofísicos y Geodesta

Influencia de la propagación anómala en las observaciones de radar

En condiciones de super-refracción suelen presentarse anomalías en las pantallas de radar, debido a que la energía del radar se propaga siguiendo una trayectoria curva, la cual puede alcanzar o superar la curvatura de la superficie terrestre, ocasionando así la aparición de ecos de segundo barrido y aumento del tamaño de ecos, sean de terreno o nube. Se estudió la ocurrencia de estos casos (llamados "conductos") y su influencia en las tareas de observación del Programa Nacional de Lucha Antigranizo. Se usaron los datos de 546 sondeos de las últimas 5 temporadas, además de la Información de las películas de radar y datos de precipitación de la mesorred ubicada en la zona de estudio. No se consideraron conductos originados por descendente de tormenta. Se hallaron 40 casos de conducto, y su distribución durante el día presenta un mínimo en horas vespertinas. Los casos de conducto en capas bajas se presentan en los sondeos de la mañana y los elevados indistintamente a la mañana o a la tarde, lo cual es coherente con el origen de la Inversión que los produce: De radiación para los primeros y de subsidencia para los segundos. Se halló que la frecuencia de formación de conductos es 7,3%, de los cuales el 17,3% está asociado con eventos convectivos y que los ángulos máximos para los cuales existe conducto, son menores a 0,5 Las conclusiones principales son que estos conductos estarían asociados con tiempo bueno y que no Influyen en las observaciones de radar.In super-refraction conditions, anomalies often appear on the radar screens since the radar energy propagates following a curved trajectory which can equal or even surpass the earth's surface curvature producing In this way the appearance of second scan echoes and enlarging the echoes size (ground or cloud echoes). The occurrence of these cases, (called "ducts") and their effect on the observation tasks of the National Hall Suppression Programme was studied. Data from 546 soundings of the last 5 summer campaigns were used, as well as information from radar films and rain data from the mesonetwork - placed in the study area. Ducts due to storm downdraft were not taken into consideration. 40 duct occurrences were found and their day distribution presents a minimum In the evening. Low layer ducts are seen in the morning soundings and the elevated ones In the morning or the afternoon indistinctly, which agrees with the origin of the Inversion that produces them: Radiation for the first ones and subsidence for the second ones. It was found that the ducts formation frequency is 7,3% from which only 17,3% is associated with convective events, and that the maximum angles for which a duct exists are smaller than 0,5°. The principal conclusions are that ducts would be associated with fine weather and they do not influence radar observations.Asociación Argentina de Geofísicos y Geodesta

Variability and changes of daily climate extremes over the core crop region of Argentina

Ponencia presentada en: XI Congreso de la Asociación Española de Climatología celebrado en Cartagena entre el 17 y el 19 de octubre de 2018.[ES]La variabilidad y los cambios en extremos climáticos afectan la región núcleo de cultivos de Argentina y pueden incrementar su vulnerabilidad ocasionando desastres sin precedentes. Este estudio investiga los cambios de largo período y la variabilidad interanual de los extremos climáticos diarios de precipitación y temperatura y evalúa en qué medida los reanálisis globales reproducen la variabilidad observada en el pasado reciente. Los datos incluyen observaciones con calidad controlada (1963-2013) y los reanálisis ERA-Interim y NCEP2 (1979-2011). Los extremos climáticos se caracterizan espacial y temporalmente con 11 índices de los propuestos por el Equipo de Expertos sobre Detección e Índices de Cambio Climático. Se aplicó un Análisis Espectral Singular para detectar los modos principales de las series temporales medias areales de los índices. Se ajustaron tendencias no-paramétricas lineales a las series temporales de cada índice para estimar la distribución espacial de los cambios medios. Los extremos de temperatura están cambiando hacia condiciones más cálidas. Los días cálidos han estado aumentando desde 1990 mientras que los días fríos han ido decreciendo. Las noches cálidas y frías muestran una señal de calentamiento significativa que parece estar estabilizándose en las últimas décadas. Los eventos de precipitación intensa aumentaron constantemente en la mayor parte de la región desde 1970. La cantidad máxima anual de precipitación en un día aumentó desde la década de 1970 hasta la del 2000, estabilizándose en años recientes. El reanálisis ERA-Interim puede reconocer los extremos de temperatura en tiempo y en espacio, mientras que el antiguo NCEP2 presenta errores sistemáticos. Ambos reanálisis reproducen la máxima precipitación anual en 5 días con grandes sesgos. Aunque se esperaría que los reanálisis agreguen información para extremos climáticos en áreas de observaciones escasas, aún deben usarse con mucha precaución y solo como complemento de las observaciones.[EN]Variability and changes in climate extremes affect the core crop region of Argentina and may increase its vulnerability leading to unprecedented disasters. This study investigates the long-term changes and interannual variability of daily temperature and precipitation climate extremes and assesses to what extent global reanalyses reproduce the observed variability in the recent past. Datasets include quality-controlled observations (1963-2013) and ERA-Interim and NCEP2 reanalyses (1979-2011). Climate extremes are characterized spatially and temporally by 11 indices proposed by the Expert Team on Climate Change Detection and Indices. A Singular Spectrum Analysis was applied to detect the leading modes of the area-averaged index time series. Nonparametric linear trends were fitted to each index time series to estimate the spatial distribution of mean changes. Temperature extremes are changing towards warmer conditions. Warm days has been increasing since 1990 while cold days has been decreasing. Warm and cold nights show a significant signal of warming that seems to be stabilizing in recent decades. Intense precipitation events in most of the region increased steadily since 1970. The annual maximum amount of 1-day precipitation events increased from the 1970s to the 2000s, stabilizing in recent years. The ERA-Interim reanalysis can recognize temperature extremes in time and space, while the older NCEP2 presents systematic biases. Both reanalyses reproduce the annual maximum 5-day precipitation with large biases. Although reanalyses would be expected to add information for climate extremes in areas of scarce observations, they still need to be used with great caution and only as a complement to observations.We appreciate the grant from the PRODACT 2018 of the Science and Technical Secretariat (FICH UNL). This research was carried out with support of Projects CRN3035 and CRN3095 of the Inter-American Institute for Global Change Research (IAI), which is supported by the US National Science Foundation. UNL Project C.A.I. + D. 2016 32/180 is also acknowledged

Global Meteorological Drought: A Synthesis of Current Understanding with a Focus on SST Drivers of Precipitation Deficits

Drought affects virtually every region of the world, and potential shifts in its character in a changing climate are a major concern. This article presents a synthesis of current understanding of meteorological drought, with a focus on the large-scale controls on precipitation afforded by sea surface temperature (SST) anomalies, land surface feedbacks, and radiative forcings. The synthesis is primarily based on regionally focused articles submitted to the Global Drought Information System (GDIS) collection together with new results from a suite of atmospheric general circulation model experiments intended to integrate those studies into a coherent view of drought worldwide. On interannual time scales, the preeminence of ENSO as a driver of meteorological drought throughout much of the Americas, eastern Asia, Australia, and theMaritime Continent is now well established, whereas in other regions (e.g., Europe, Africa, and India), the response to ENSO is more ephemeral or nonexistent. Northern Eurasia, central Europe, and central and eastern Canada stand out as regions with few SST-forcedimpacts on precipitation oninterannual time scales.Decadal changesin SST appear to be a major factor in the occurrence of long-term drought, as highlighted by apparent impacts on precipitation of the late 1990s ‘‘climate shifts’’ in the Pacific and Atlantic SST. Key remaining research challenges include (i) better quantification of unforced and forced atmospheric variability as well as land–atmosphere feedbacks, (ii) better understanding of the physical basis for the leading modes of climate variability and their predictability, and (iii) quantification of the relative contributions of internal decadal SST variability and forced climate change to long-term drought

Domain choice in an experimental nested modeling prediction system for South America

The purposes of this paper are to evaluate the new version of the regional model, RegCM3, over South America for two test seasons, and to select a domain for use in an experimental nested prediction system, which incorporates RegCM3 and the European Community-Hamburg (ECHAM) general circulation model (GCM). To evaluate RegCM3, control experiments were completed with RegCM3 driven by both the NCEP/NCAR Reanalysis (NNRP) and ECHAM, using a small control domain (D-CTRL) and integration periods of January–March 1983 (El Niño) and January–March 1985 (La Niña). The new version of the regional model captures the primary circulation and rainfall differences between the two years over tropical and subtropical South America. Both the NNRP-driven and ECHAM-driven RegCM3 improve the simulation of the Atlantic intertropical convergence zone (ITCZ) compared to the GCM. However, there are some simulation errors. Irrespective of the driving fields, weak northeasterlies associated with reduced precipitation are observed over the Amazon. The simulation of the South Atlantic convergence zone is poor due to errors in the boundary condition forcing which appear to be amplified by the regional model. To select a domain for use in an experimental prediction system, sensitivity tests were performed for three domains, each of which includes important regional features and processes of the climate system. The domain sensitivity experiments were designed to determine how domain size and the location of the GCM boundary forcing affect the regional circulation, moisture transport, and rainfall in two years with different large scale conditions. First, the control domain was extended southward to include the exit region of the Andes low level jet (D-LLJ), then eastward to include the South Atlantic subtropical high (D-ATL), and finally westward to include the subsidence region of the South Pacific subtropical high and to permit the regional model more freedom to respond to the increased resolution of the Andes Mountains (D-PAC). In order to quantify differences between the domain experiments, measures of bias, root mean square error, and the spatial correlation pattern were calculated between the model results and the observed data for the seasonal average fields. The results show the GCM driving fields have remarkable control over the RegCM3 simulations. Although no single domain clearly outperforms the others in both seasons, the control domain, D-CTRL, compares most favorably with observations. Over the ITCZ region, the simulations were improved by including a large portion of the South Atlantic subtropical high (D-ATL). The methodology presented here provides a quantitative basis for evaluating domain choice in future studies

Simulations of the 2004 North American Monsoon: NAMAP2

The second phase of the North American Monsoon Experiment (NAME) Model Assessment Project (NAMAP2) was carried out to provide a coordinated set of simulations from global and regional models of the 2004 warm season across the North American monsoon domain. This project follows an earlier assessment, called NAMAP, that preceded the 2004 field season of the North American Monsoon Experiment. Six global and four regional models are all forced with prescribed, time-varying ocean surface temperatures. Metrics for model simulation of warm season precipitation processes developed in NAMAP are examined that pertain to the seasonal progression and diurnal cycle of precipitation, monsoon onset, surface turbulent fluxes, and simulation of the low-level jet circulation over the Gulf of California. Assessment of the metrics is shown to be limited by continuing uncertainties in spatially averaged observations, demonstrating that modeling and observational analysis capabilities need to be developed concurrently. Simulations of the core subregion (CORE) of monsoonal precipitation in global models have improved since NAMAP, despite the lack of a proper low-level jet circulation in these simulations. Some regional models run at higher resolution still exhibit the tendency observed in NAMAP to overestimate precipitation in the CORE subregion; this is shown to involve both convective and resolved components of the total precipitation. The variability of precipitation in the Arizona/New Mexico (AZNM) subregion is simulated much better by the regional models compared with the global models, illustrating the importance of transient circulation anomalies (prescribed as lateral boundary conditions) for simulating precipitation in the northern part of the monsoon domain. This suggests that seasonal predictability derivable from lower boundary conditions may be limited in the AZNM subregion.open131

Seasonal dependence of surface-atmosphere interactions for subtropical South America

The present research seeks to identify the role of soil moisture anomalies on the seasonal characteristics and the interanual variability of the atmospheric component of the hydrological cycle over the subtropical region of South America. With this purpose, long term Eta model ensemble simulations with different soil moisture initial conditions are performed to diagnose the corresponding interaction mechanisms. The dependence on other large scale factors like the El Niño-Southern Oscillation (ENSO) cycle is also assessed, by investigating separately El Niño (EN), La Niña (LN) and Neutral (NEU) years. Sensitivity experiments of the model response to soil moisture changes suggest that soil over the subtropical region during the warm season seems to be close to saturation. Consequently, precipitation is not as sensitive to increases of soil moisture as it is to decreases. When the initial soil moisture is decreased, there is a corresponding increase of the sensible heat flux and reduction of the latent heat flux, thereby increasing the Bowen ratio and the Convective Inhibition (CIN), while reducing the Convective Available Potential Energy (CAPE). As a consequence of these changes, two basic mechanisms are affected. On the one hand the reduction of evaporation implies less moisture availability for precipitation; on the other hand, an elevated and drier boundary layer implies a reduction in the moisture supply by the South American Low-level Jet (SALLJ). The two mechanisms act concurrently to reduce precipitation..Pages: 999-100

Evaluation of WRF model forecasts and their use for hydroclimate monitoring over Southern South America

Weather forecasting and monitoring systems based on regional models are becoming increasingly relevant for decision support in agriculture and water management. This work evaluates the predictive and monitoring capabilities of a system based on WRF Model simulations at 15-km grid spacing over the La Plata basin (LPB) in southern South America, where agriculture and water resources are essential. The model's skill up to a lead time of 7 days is evaluated with daily precipitation and 2-m temperature in situ observations for the 2-yr period from 1 August 2012 to 31 July 2014. Results show high prediction performance with 7-day lead time throughout the domain and particularly over LPB, where about 70% of rain and no-rain days are correctly predicted. Also, the probability of detection of rain days is above 80% in humid regions. Temperature observations and forecasts are highly correlated (r > 0.80) while mean absolute errors, even at the maximum lead time, remain below 2.7°C for minimum and mean temperatures and below 3.7°C for maximum temperatures. The usefulness of WRF products for hydroclimate monitoring was tested for an unprecedented drought in southern Brazil and for a slightly above normal precipitation season in northeastern Argentina. In both cases the model products reproduce the observed precipitation conditions with consistent impacts on soil moisture, evapotranspiration, and runoff. This evaluation validates the model's usefulness for forecasting weather up to 1 week in advance and for monitoring climate conditions in real time. The scores suggest that the forecast lead time can be extended into a second week, while bias correction methods can reduce some of the systematic errors.Fil: Müller, Omar Vicente. Universidad Nacional del Litoral. Facultad de Ingeniería y Ciencias Hídricas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; ArgentinaFil: Lovino, Miguel Angel. Universidad Nacional del Litoral. Facultad de Ingeniería y Ciencias Hídricas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; ArgentinaFil: Berbery, Ernesto H.. University of Maryland. Department of Atmospheric and Oceanic Science; Estados Unido