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- Author or Editor: Stjepan Ivatek-Šahdan x
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Abstract
The results of numerically modeled wind speed climate, a primary component of wind energy resource assessment in the complex terrain of Croatia, are given. For that purpose, dynamical downscaling of 10 yr (1992–2001) of the 40-yr ECMWF Re-Analysis (ERA-40) was performed to 8-km horizontal grid spacing with the use of a spectral, prognostic full-physics model Aire Limitée Adaptation Dynamique Développement International (ALADIN; the “ALHR” version). Then modeled data with a 60-min frequency were refined to 2-km horizontal grid spacing with a simplified and cost-effective model version, the so-called dynamical adaptation (DADA). The statistical verification of ERA-40-, ALHR-, and DADA-modeled wind speed on the basis of data from measurement stations representing different regions of Croatia suggests that downscaling was successful and that model accuracy generally improves as horizontal resolution is increased. The areas of the highest mean wind speeds correspond well to locations of frequent and strong bora flow as well as to the prominent mountain peaks. The best results are achieved with DADA and contain bias of 1% of the mean wind speed for eastern Croatia while reaching 10% for complex coastal terrain, mainly because of underestimation of the strongest winds. Root-mean-square errors for DADA are significantly smaller for flat terrain than for complex terrain, with relative values close to 12% of the mean wind speed regardless of the station location. Spectral analyses suggest that the shape of the kinetic energy spectra generally relaxes from k −3 at the upper troposphere to the shape of orographic spectra near the surface and shows no seasonal variability. Apart from the buildup of energy on smaller scales of motions, it is shown that mesoscale simulations contain a considerable amount of energy related to near-surface and mostly divergent meso-β-scale (20–200 km) motions. Spectral decomposition of measured and modeled data in temporal space indicates a reasonable performance of all model datasets in simulating the primary maximum of spectral power related to synoptic and larger-than-diurnal mesoscale motions, with somewhat increased accuracy of mesoscale model data. The primary improvement of dynamical adaptation was achieved for cross-mountain winds, whereas mixed results were found for along-mountain wind directions. Secondary diurnal and tertiary semidiurnal maxima are significantly better simulated with the mesoscale model for coastal stations but are somewhat more erroneous for the continental station. The mesoscale model data underestimate the spectral power of motions with less-than-semidiurnal periods.
Abstract
The results of numerically modeled wind speed climate, a primary component of wind energy resource assessment in the complex terrain of Croatia, are given. For that purpose, dynamical downscaling of 10 yr (1992–2001) of the 40-yr ECMWF Re-Analysis (ERA-40) was performed to 8-km horizontal grid spacing with the use of a spectral, prognostic full-physics model Aire Limitée Adaptation Dynamique Développement International (ALADIN; the “ALHR” version). Then modeled data with a 60-min frequency were refined to 2-km horizontal grid spacing with a simplified and cost-effective model version, the so-called dynamical adaptation (DADA). The statistical verification of ERA-40-, ALHR-, and DADA-modeled wind speed on the basis of data from measurement stations representing different regions of Croatia suggests that downscaling was successful and that model accuracy generally improves as horizontal resolution is increased. The areas of the highest mean wind speeds correspond well to locations of frequent and strong bora flow as well as to the prominent mountain peaks. The best results are achieved with DADA and contain bias of 1% of the mean wind speed for eastern Croatia while reaching 10% for complex coastal terrain, mainly because of underestimation of the strongest winds. Root-mean-square errors for DADA are significantly smaller for flat terrain than for complex terrain, with relative values close to 12% of the mean wind speed regardless of the station location. Spectral analyses suggest that the shape of the kinetic energy spectra generally relaxes from k −3 at the upper troposphere to the shape of orographic spectra near the surface and shows no seasonal variability. Apart from the buildup of energy on smaller scales of motions, it is shown that mesoscale simulations contain a considerable amount of energy related to near-surface and mostly divergent meso-β-scale (20–200 km) motions. Spectral decomposition of measured and modeled data in temporal space indicates a reasonable performance of all model datasets in simulating the primary maximum of spectral power related to synoptic and larger-than-diurnal mesoscale motions, with somewhat increased accuracy of mesoscale model data. The primary improvement of dynamical adaptation was achieved for cross-mountain winds, whereas mixed results were found for along-mountain wind directions. Secondary diurnal and tertiary semidiurnal maxima are significantly better simulated with the mesoscale model for coastal stations but are somewhat more erroneous for the continental station. The mesoscale model data underestimate the spectral power of motions with less-than-semidiurnal periods.
Abstract
Dynamical downscaling has been applied to global ensemble forecasts to assess its impact for four cases of severe weather (precipitation and wind) over various parts of Croatia. It was performed with the Croatian 12.2-km version of the Aire Limitée Adaptation Dynamique Développement International (ALADIN) limited-area model, nested in the ECMWF TL255 (approximately 80 km) global ensemble prediction system (EPS). The 3-hourly EPS output was used to force the ALADIN model over the central European/northern Mediterranean domain.
Results indicate that the identical clustering algorithm may yield differing results when applied to either global or to downscaled ensembles. It is argued that this is linked to the fact that a downscaled, higher-resolution ensemble resolves more explicitly small-scale features, in particular those strongly influenced by orographic forcing. This result has important implications in limited-area ensemble prediction, since it implies that downscaling may affect the interpretation or relevance of the global ensemble forecasts; that is, it may not always be feasible to make a selection (or a subset) of global lower-resolution ensemble members that might be representative of all possible higher-resolution evolution scenarios.
Abstract
Dynamical downscaling has been applied to global ensemble forecasts to assess its impact for four cases of severe weather (precipitation and wind) over various parts of Croatia. It was performed with the Croatian 12.2-km version of the Aire Limitée Adaptation Dynamique Développement International (ALADIN) limited-area model, nested in the ECMWF TL255 (approximately 80 km) global ensemble prediction system (EPS). The 3-hourly EPS output was used to force the ALADIN model over the central European/northern Mediterranean domain.
Results indicate that the identical clustering algorithm may yield differing results when applied to either global or to downscaled ensembles. It is argued that this is linked to the fact that a downscaled, higher-resolution ensemble resolves more explicitly small-scale features, in particular those strongly influenced by orographic forcing. This result has important implications in limited-area ensemble prediction, since it implies that downscaling may affect the interpretation or relevance of the global ensemble forecasts; that is, it may not always be feasible to make a selection (or a subset) of global lower-resolution ensemble members that might be representative of all possible higher-resolution evolution scenarios.
Abstract
While statistical analyses and observations show that severe bora with maximum gusts exceeding 40 m s−1 can occur in all parts of the Adriatic, the bora research to date has been mainly focused on the dynamics and structure of severe bora in the northern Adriatic. Examined to a significantly lesser degree is a less predictable counterpart in the southern Adriatic, where the Dinaric Alps are higher, broader, and steeper, and where the upwind bora layer is generally less well defined. Identification of the main differences in the sequence of mesoscale and macroscale events leading to the onset of bora in the northern and southern parts of the eastern Adriatic is of fundamental importance for its forecasting. To this end, presented here is a comparative analysis of the evolution and structure of two typical severe cyclonic bora events—one “northern” (7–8 November 1999) and one “southern” (6–7 May 2005) event. The analysis utilizes airborne, radiosonde, and ground-based observations, as well as the hydrostatic Aire Limitée Adaptation Dynamique Developement International (ALADIN/HR) mesoscale model simulations.
It is shown that the development of a severe bora in both the northern and southern Adriatic is critically dependent on the synoptic setting to create an optimal set of environmental conditions. For severe bora in the northern Adriatic, these conditions include a strong forcing of the northeasterly low-level jet and pronounced discontinuities in the upstreamflow structure that promote layering, such as lower- to midtropospheric inversions and environmental critical levels. The development of severe bora in the southern Adriatic is crucially dependent on the establishment of a considerably deeper upstream layer that is able to overcome the strong blocking potential of the southern Dinaric Alps. While the upstream layering is less pronounced, it is closely tied to the presence of a cyclone in the southern Adriatic or over the southern Balkan peninsula.
The upstream atmospheric layering is shown to strongly modulate bora behavior, and different phases of severe bora, related to the presence or absence of upstream layering, are shown to occur within a single bora episode. Furthermore, the presence of a mountain-parallel upper-level jet aloft appears to impede severe bora development in both the northern and southern Adriatic.
Abstract
While statistical analyses and observations show that severe bora with maximum gusts exceeding 40 m s−1 can occur in all parts of the Adriatic, the bora research to date has been mainly focused on the dynamics and structure of severe bora in the northern Adriatic. Examined to a significantly lesser degree is a less predictable counterpart in the southern Adriatic, where the Dinaric Alps are higher, broader, and steeper, and where the upwind bora layer is generally less well defined. Identification of the main differences in the sequence of mesoscale and macroscale events leading to the onset of bora in the northern and southern parts of the eastern Adriatic is of fundamental importance for its forecasting. To this end, presented here is a comparative analysis of the evolution and structure of two typical severe cyclonic bora events—one “northern” (7–8 November 1999) and one “southern” (6–7 May 2005) event. The analysis utilizes airborne, radiosonde, and ground-based observations, as well as the hydrostatic Aire Limitée Adaptation Dynamique Developement International (ALADIN/HR) mesoscale model simulations.
It is shown that the development of a severe bora in both the northern and southern Adriatic is critically dependent on the synoptic setting to create an optimal set of environmental conditions. For severe bora in the northern Adriatic, these conditions include a strong forcing of the northeasterly low-level jet and pronounced discontinuities in the upstreamflow structure that promote layering, such as lower- to midtropospheric inversions and environmental critical levels. The development of severe bora in the southern Adriatic is crucially dependent on the establishment of a considerably deeper upstream layer that is able to overcome the strong blocking potential of the southern Dinaric Alps. While the upstream layering is less pronounced, it is closely tied to the presence of a cyclone in the southern Adriatic or over the southern Balkan peninsula.
The upstream atmospheric layering is shown to strongly modulate bora behavior, and different phases of severe bora, related to the presence or absence of upstream layering, are shown to occur within a single bora episode. Furthermore, the presence of a mountain-parallel upper-level jet aloft appears to impede severe bora development in both the northern and southern Adriatic.
