Maja Telišman Prtenjak

Atmospheric conditions during high ragweed pollen concentrations in Zagreb, Croatia

We examined the atmospheric conditions favourable to the occurrence of maximum concentrations of ragweed pollen with an extremely high risk of producing allergy. Over the 2002–2009 period, daily pollen data collected in Zagreb were used to identify two periods of high pollen concentration (> 600 grains/m3) for our analysis: period A (3–4 September 2002) and period B (6–7 September 2003). Synoptic conditions in both periods were very similar: Croatia was under the influence of a lower sector high pressure system moving slowly eastward over Eastern Europe. During the 2002–2009 period, this type of weather pattern (on ~ 70% of days), in conjunction with almost non-gradient surface pressure conditions in the area (on ~ 30% of days) characterised days when the daily pollen concentrations were higher than 400 grains/m3. Numerical experiments using a mesoscale model at fine resolution showed successful multi-day simulations reproducing the local topographic influence on wind flow and in reasonable agreement with available observations. According to the model, the relatively weak synoptic flow (predominantly from the eastern direction) allowed local thermal circulations to develop over Zagreb during both high pollen episodes. Two-hour pollen concentrations and 48-h back-trajectories indicated that regional-range transport of pollen grains from the central Pannonian Plain was the cause of the high pollen concentrations during period A. During period B, the north-westward regional-range transport in Zagreb was supplemented significantly by pronounced horizontal recirculation of pollen grains. This recirculation happened within the diurnal local circulation over the city, causing a late-evening increase in pollen concentration.

Idealised numerical simulations of diurnal sea breeze characteristics over a step change in roughness

A two-dimensional hydrostatic meso-scale model is used to study possible effects of a step change in roughness on the daytime sea breeze (SB). Two approaches are employed for the calm synoptic condition with a clear sky. First, idealised numerical simulations are made with the length of aerodynamical roughness (z0) ranging from 0.05m to 0.6 m and by an increase of 0.05 m. The magnitudes are changed in all ground surface grid points. The specific features examined are the maxima of the wind velocity field, the turbulent fluxes and the turbulent kinetic energy (TKE), the height of the mixed layer, the inland penetration of the SB and the slope of the SB front. Two z0 effects are pointed out: an increase in the turbulent mixing and a reduction in the wind speeds in the surface layer. Results show that a relationship between these two effects is crucial for the SB inland penetration. A moderate z0 change increases both turbulence intensity and a vertical extent of the boundary layer. An increase of both heights and magnitudes of the SB circulation, caused by a vertical extension of the turbulent field, what followed by a faster inland penetration of the SB. For larger z0 change, the effect of dissipation overcomes the effect of the SB strengthening and the inland penetration is slower. Second, an urban area is simulated only by a locally increased z0. The rough area intensifies both TKE and the SB during its growing stage. An increase of z0 also reduces the wind speed in the vicinity of the urban area and it augments the slope of the SB front.

Grid implementation of the weather research and forecasting model

Atmospheric science is advancing towards very complex phenomena at ever smaller temporal and spatial scales. One of the principal tools utilized in atmospheric science are weather prediction models. These models usually demand large execution times and resource allocation, such as CPU time and storage space. The main goal of our research is porting of the Weather Research and Forecasting model to the Grid infrastructure. Porting has been done through bash scripts that are using existing Grid tools for job and data management, authentification mechanisms, and other application level services produced within the SEEGRID project. In this paper, through a few model runs on the Grid we describe certain benefits not only in the overall execution time but also in the ability of performing concurrent runs of the same model especially for scientific purposes. During the execution, we have also faced some drawbacks in data bandwidth, unreliability of some Grid services and relatively hard control of the model execution flow. The final conclusion is that there is a big need and justification for porting the WRF model to the Grid, although it takes a lot of effort to be properly implemented.

Near-surface wind variability over the broader Adriatic region: insights from an ensemble of regional climate models

Over the past few decades the horizontal resolution of regional climate models (RCMs) has steadily increased, leading to a better representation of small-scale topographic features and more details in simulating dynamical aspects, especially in coastal regions and over complex terrain. Due to its complex terrain, the broader Adriatic region represents a major challenge to state-of-the-art RCMs in simulating local wind systems realistically. The objective of this study is to identify the added value in near-surface wind due to the refined grid spacing of RCMs. For this purpose, we use a multi-model ensemble composed of CORDEX regional climate simulations at 0.11° and 0.44° grid spacing, forced by the ERA-Interim reanalysis, a COSMO convection-parameterizing simulation at 0.11° and a COSMO convection-resolving simulation at 0.02° grid spacing. Surface station observations from this region and satellite QuikSCAT data over the Adriatic Sea have been compared against daily output obtained from the available simulations. Both day-to-day wind and its frequency distribution are examined. The results indicate that the 0.44° RCMs rarely outperform ERA-Interim reanalysis, while the performance of the high-resolution simulations surpasses that of ERA-Interim. We also disclose that refining the grid spacing to a few km is needed to properly capture the small-scale wind systems. Finally, we show that the simulations frequently yield the accurate angle of local wind regimes, such as for the Bora flow, but overestimate the associated wind magnitude. Finally, spectral analysis shows good agreement between measurements and simulations, indicating the correct temporal variability of the wind speed.

Impact of mesoscale meteorological processes on anomalous radar propagation conditions over the northern Adriatic area

The impact of mesoscale structures on the occurrence of anomalous propagation (AP) conditions for radio waves, including ducts, super refractive and subrefractive conditions were studied. The chosen meteorological situations are the bora wind and the sporadic sea/land breeze (SB/LB) during three selected cases over a large portion of the northern Adriatic. For this purpose, we used available radiosoundings and numerical mesoscale model simulations (of real cases and their sensitivity tests) at a horizontal resolution of 1.5 km and 81 vertical levels. The model simulated the occurrences of AP conditions satisfactorily, although their intensities and frequency were underestimated at times. Certain difficulties appeared in reproducing the vertical profile of the modified refractive index, which is mainly dependent on the accuracy of the modeled humidity. The spatial distributions of summer AP conditions reveal that the surface layer above the sea (roughly between 30 and 100 m asl) is often covered by super refractive conditions and ducts. The SB is highly associated with the formations of AP conditions: (i) in the first 100 m asl, where trapping and super refractive conditions form because of the advection of cold and moist air, and (ii) inside the transition layer between the SB body and the elevated return flow in the form of subrefractive conditions. When deep convection occurs, all three types of AP conditions are caused by the downdraft beneath the cumulonimbus cloud base in its mature phase that creates smaller but marked pools of cold and dry air. The bora wind usually creates a pattern of AP conditions associated with the hydraulic jump and influences distribution of AP conditions over the sea surface.

Interaction of sea breeze and deep convection over the northeastern Adriatic coast: an analysis of sensitivity experiments using a high-resolution mesoscale model

This study investigates the sensitivity of a high-resolution mesoscale atmospheric model in the model reproduction of thermally induced local wind (i.e., sea breezes, SB) on the development of deep convection (Cb). The three chosen cases are simulated by the Weather and Research Forecasting (WRF-ARW) model at three (nested) model domains, whereas the area of the interest is Istria (peninsula in the northeastern Adriatic). The sensitivity tests are accomplished by modifying (1) the model setup, (2) the model topography and (3) the sea surface temperature (SST) distribution. The first set of simulations (over the three 1.5-day periods during summer) is conducted by modifying the model setup, i.e., microphysics and the boundary layer parameterizations. The same events are simulated with the modified topography where the mountain heights in Istria are reduced to 30% of their initial height. The SST distribution has two representations in the model: a constant SST field from the ECMWF skin temperature analysis and a varying SST field, which is provided by hourly geostationary satellite data. A comprehensive set of numerical experiments is statistically analyzed through several different approaches (i.e., the standard statistical measures, the spectral method and the image moment analysis). The overall model evaluation of each model setup revealed certain advantages of one model setup over the others. The numerical tests with the modified topography showed the influence of reducing the mountains heights on the pre-thunderstorm characteristics due to: (1) decrease of sensible heat flux and mid-tropospheric moisture and (2) change of slope-SB wind system. They consequently affect the evolution and dimensions of SBs and the features of the thunderstorm itself: timing, location and intensity (weaker storm). The implementation of the varying SST field in the model have an impact on the characteristics and dynamics of the SB and finally on the accuracy of Cb evolution, duration and the intensity. SST variations emphasized the importance of the phase matching in both daytime cycles of SB and Cb due to their extremely strong nonlinear relationship.

A multi-year study of radio-wave refractivity profiles above the Adriatic Sea up to an altitude of 40 km

Radio-wave refractivity conditions in the atmosphere are determined by the vertical gradient of the refractive index, which can be computed from regular aerological balloon probe measurements. This study is focused on the refractivity conditions over the Adriatic Sea using a multiannual series of data from four aerological stations. According to the refractive prof les computed from the aerological probes, two models are proposed: a polynomial model for both the troposphere and the stratosphere up to an altitude of 40 km and a much simpler linear model for the stratosphere alone. By conf rming a discontinuity in the refractive conditions between the troposphere and stratosphere, both models exhibited a good f t and superior characteristics over a range of heights from the surface up to 40 km compared with the widely used simple exponential model. New recommended regular refractive prof les can be used as a guide for any type of long-range radio system design.

Idealised Numerical Simulations of Aerodynamic Roughness Length Effects on Sea Breeze Characteristics

A two-dimensional hydrostatic meso-γ-scale model was used to study possible effects of a step change in roughness on the daytime sea breeze. Idealised numerical simulations with the length of aerodynamical roughness (z 0 ) ranging from 0.05 m to 1.0 m and by an increase of 0.05 m in all land grid points were made. Two z 0 effects could be pointed out: an increase in the mixing process and a reduction in the wind speeds in the surface layer. An increase of both heights and magnitudes of the sea breeze circulation, caused by a vertical extension of the turbulent field, followed a faster inland penetration of the sea breeze. The second effect resulting from a successive increase in z 0 values was a retardation of the sea breeze front near the ground, visible in its slope. This effect allowed the already faster inland penetration of the marine air high above the land.

Erratum to: Atmospheric conditions during high ragweed pollen concentrations in Zagreb, Croatia

V. MadžarevicAndrija Mohorovicic Geophysical Institute,DepartmentofGeophysics,FacultyofScience,UniversityofZagreb,Horvatovac 95,10000 Zagreb, Croatiae-mail: telisman@gfz.hrL. SrnecMeteorological and Hydrological Service of Croatia, Zagreb,Croatia,Gric 3,10000 Zagreb, CroatiaR. PeternelUniversity of Applied sciences Velika Gorica,Velika Gorica, CroatiaI. Hrga