A. Rocha

A sensitivity study of the WRF model in wind simulation for an area of high wind energy

The performance of the Weather Research and Forecast (WRF) model in wind simulation was evaluated under different numerical and physical options for an area of Portugal, located in complex terrain and characterized by its significant wind energy resource. The grid nudging and integration time of the simulations were the tested numerical options. Since the goal is to simulate the near-surface wind, the physical parameterization schemes regarding the boundary layer were the ones under evaluation. Also, the influences of the local terrain complexity and simulation domain resolution on the model results were also studied. Data from three wind measuring stations located within the chosen area were compared with the model results, in terms of Root Mean Square Error, Standard Deviation Error and Bias. Wind speed histograms, occurrences and energy wind roses were also used for model evaluation. Globally, the model accurately reproduced the local wind regime, despite a significant underestimation of the wind speed. The wind direction is reasonably simulated by the model especially in wind regimes where there is a clear dominant sector, but in the presence of low wind speeds the characterization of the wind direction (observed and simulated) is very subjective and led to higher deviations between simulations and observations. Within the tested options, results show that the use of grid nudging in simulations that should not exceed an integration time of 2 days is the best numerical configuration, and the parameterization set composed by the physical schemes MM5-Yonsei University-Noah are the most suitable for this site. Results were poorer in sites with higher terrain complexity, mainly due to limitations of the terrain data supplied to the model. The increase of the simulation domain resolution alone is not enough to significantly improve the model performance. Results suggest that error minimization in the wind simulation can be achieved by testing and choosing a suitable numerical and physical configuration for the region of interest together with the use of high resolution terrain data, if available.

INTERANNUAL VARIABILITY OF SOUTH-EASTERN AFRICAN SUMMER RAINFALL. PART 1: RELATIONSHIPS WITH AIR-SEA INTERACTION PROCESSES

This paper investigates the role that air‐sea interaction processes may play in interannual variability of south-eastern African summer rainfall. The principal spatial modes of south-eastern African summer rainfall are first identified using principal component analysis. Four modes are retained. The most important mode of variability is found to represent rainfall variability over most of the domain, particularly in the regions to the south. The influence of ENSO (as measured by the SOI) on summer rainfall is investigated in detail for different SOI leads. The relationship is such that during the summer following the onset of an ENSO event, south-eastern Africa tends to experience dry conditions. Strongest relationships are found with the SOI leading rainfall by about 3 to 6 months. A second index, the Brandon‐Marion Index (BMI) which is indicative of changes in the pressure field over the Indian Ocean correlates with rainfall better than the SOI. Strongest correlations are found when this index leads rainfall by about 1 to 3 months. More importantly, a partial correlation analysis reveals that the BMI influences rainfall independently of ENSO. Both the SOI and the BMI are potential predictors of summer rainfall. An investigation of rainfall associations with global SST anomalies reveals areas in the tropical Indian and Pacific Oceans that are linked with rainfall changes over the subcontinent. The relationship is such that warm anomalies tend to be followed by dry conditions over much of south-eastern Africa. Strongest relationships are found when SSTs lead the rainfall season by about 1 to 3 months. Well-defined atmospheric anomalies are identified during dry south-eastern African summers. These include, amongst others, anomalously warm tropospheric temperatures and marked low-level cyclonic circulation anomalies over the central Indian Ocean, which generate abnormally weak easterly winds along much of the south-eastern coast of Africa. These perturbations to the low-level flow divert moisture from the continent and result in precipitation decreases. An important and related finding is the fact that the SST‐rainfall link over the Indian Ocean remains strong after the ENSO effects have been removed, suggesting that the atmospheric circulation anomalies observed over south-eastern Africa during dry summers, are linked mainly to SST anomalies over the Indian Ocean. This hypothesis will be tested in a companion paper through a series of GCM simulations.

Low-frequency variability of seasonal rainfall over the Iberian peninsula and ENSO

This study is concerned with the influence of El Nino-Southern Oscillation (ENSO) on seasonal precipitation over the Iberian Peninsula (IP), and the stability of this association over time. Principal Component Analysis (PCA) is applied to rainfall data in order to identify regions of coherent interannual variability. Two regions are identified in the IP, namely its south-eastern part, and the remaining region. Lag correlations have been applied to rainfall and the Southern Oscillation Index (SOI), the latter leading the former. For the entire period, 1900-1996, ENSO appears to be related to above-average autumn precipitation in both regions, and below-average spring rainfall over the south-eastern IP. For winter, the ENSO signal in the region is weak, and not statistically significant. Strongest correlations occur for leads of 3-6 months. However, when the ENSO-rainfall association is analysed over a 20-year moving window from 1900 to 1996, it becomes clear that it has changed substantially over time. For the last decades of the period, ENSO has been associated with below-average spring and winter rainfall over the south-eastern IP, and above-average autumn rainfall in the Peninsula as a whole. Over the south-eastern IP, the association for autumn has weakened considerably recently. The strength of the ENSO-rainfall association appears to be related to the frequency and/or intensity of ENSO events during the 1900-1996 period.

Wind resource modelling in complex terrain using different mesoscale–microscale coupling techniques

Wind resource evaluation in two sites located in Portugal was performed using the mesoscale modelling system Weather Research and Forecasting (WRF) and the wind resource analysis tool commonly used within the wind power industry, the Wind Atlas Analysis and Application Program (WAsP) microscale model. Wind measurement campaigns were conducted in the selected sites, allowing for a comparison between in situ measurements and simulated wind, in terms of flow characteristics and energy yields estimates.

INTERANNUAL VARIABILITY OF SOUTH-EASTERN AFRICAN SUMMER RAINFALL. PART II. MODELLING THE IMPACT OF SEA-SURFACE TEMPERATURES ON RAINFALL AND CIRCULATION

This study is concerned with the possible physical link between global sea-surface temperatures (SSTs) and south-eastern African summer rainfall. We have performed a series of general circulation model (GCM) experiments, where the model atmosphere has been forced with certain SST anomaly patterns. These have been identified by Rocha and Simmonds (in part I) to be related to drought conditions over the subcontinent. Results show that anomalously warm SSTs in the tropical Pacific and Indian Oceans, typical of ENSO events, can generate dry conditions over much of south-eastern Africa. However, those in the central Indian Ocean, which are partially independent of ENSO, dominate the rainfall response. Sea-surface temperatures in the Atlantic Ocean have little or no effect on rainfall. In the model, warming of the central Indian Ocean generates low-level cyclonic atmospheric anomalies there, which weaken the predominantly eastern flow across the eastern coast of Africa. As a result, less moisture enters the continent and reduced precipitation takes place. Cool SSTs in the south Indian Ocean further enhance this scenario. Warm surface waters in the central and eastern Pacific Ocean generate upper-level westerly wind anomalies, which extend eastwards across into the Indian Ocean. Such upper-level wind changes have been related previously to ENSO and southern African drought.

The Association of Australian Winter Climate with Ocean Temperatures to the West

Abstract Two general circulation model experiments have been conducted with a view toward determining the sensitivity of Australian winter circulation and precipitation to the imposition of idealized sea surface temperature anomalies around the western half of Australia. In the first, a positive anomaly was imposed to the northwest of the continent and a negative anomaly to the southwest, while in the second this pattern was reversed. In the former, major changes were simulated in the circulation over the Australian sector, and significant precipitation increases were induced over more than half of the continent. By contrast, the responses to the forcing in the latter experiment were considerably less, particularly for precipitation over the continent, which showed virtually no change of significance. The results, taken in concert with those of Simmonds, suggest the important role played by warm ocean temperatures to the northwest of Australia in influencing winter rainfall. The results suggest that the a...

Numerical Simulation of Meso-Meteorological Circulations in the Lisbon Region

During the last 5 years, the University of Aveiro has directed significant research towards the analysis of regional air quality and problems related to the transport of airborne pollutants over Portugal (Coutinho et al., 1989; Borrego et al., 1991). One of the main conclusions of these studies was the necessity to consider mesoscale atmospheric circulations in studying atmospheric dispersion patterns in Portugal. Mesoscale meteorological models seem to be the most adequate tool to represent those circulations through the mathematical simulation of atmospheric physical processes.

Towards Operational Modeling and Forecasting of the Iberian Shelves Ecosystem

There is a growing interest on physical and biogeochemical oceanic hindcasts and forecasts from a wide range of users and businesses. In this contribution we present an operational biogeochemical forecast system for the Portuguese and Galician oceanographic regions, where atmospheric, hydrodynamic and biogeochemical variables are integrated. The ocean model ROMS, with a horizontal resolution of 3 km, is forced by the atmospheric model WRF and includes a Nutrients-Phytoplankton-Zooplankton-Detritus biogeochemical module (NPZD). In addition to oceanographic variables, the system predicts the concentration of nitrate, phytoplankton, zooplankton and detritus (mmol N m−3). Model results are compared against radar currents and remote sensed SST and chlorophyll. Quantitative skill assessment during a summer upwelling period shows that our modelling system adequately represents the surface circulation over the shelf including the observed spatial variability and trends of temperature and chlorophyll concentration. Additionally, the skill assessment also shows some deficiencies like the overestimation of upwelling circulation and consequently, of the duration and intensity of the phytoplankton blooms. These and other departures from the observations are discussed, their origins identified and future improvements suggested. The forecast system is the first of its kind in the region and provides free online distribution of model input and output, as well as comparisons of model results with satellite imagery for qualitative operational assessment of model skill.

Using a Physical Reference Frame to Study Global Circulation Variability

Abstract The 3D structures of the free oscillations of an adiabatic and hydrostatic atmosphere around a basic state at rest were used as a physical filtering for atmospheric data. This filtering procedure allows for the consideration of the three primitive variables (u, υ, ϕ) over the whole atmosphere simultaneously. Accordingly, the computed statistics do not simply rely on the information provided by a single variable of circulation, such as the 500-hPa geopotential field. Using this method, two classical patterns were isolated in the barotropic component of the circulation, one resembling the Pacific–North America (PNA) pattern, the other similar to the North Atlantic Oscillation (NAO) pattern in summer. Associating the barotropic and the second baroclinic components, a coupling in variability was retrieved between the strength of the winter stratospheric polar vortex and the tropospheric circulation over the North Atlantic. Until now these modes had only been recovered by means of statistical analysis...

Nitrogen oxides and ozone in Portugal: trends and ozone estimation in an urban and a rural site.

This study provides an analysis of the spatial distribution and trends of NO, NO2 and O3 concentrations in Portugal between 1995 and 2010. Furthermore, an estimation model for daily ozone concentrations was developed for an urban and a rural site. NO concentration showed a significant decreasing trend in most urban stations. A decreasing trend in NO2 is only observed in the stations with less influence from emissions of primary NO2. Several stations showed a significant upward trend in O3 as a result of the decrease in the NO/NO2 ratio. In the northern rural region, ozone showed a strong correlation with wind direction, highlighting the importance of long-range transport. In the urban site, most of the variance is explained by the NO2/NOX ratio. The results obtained by the ozone estimation model in the urban site fit 2013 observed data. In the rural site, the estimated ozone during extreme events agrees with observed concentration.