Uzroci razvoja superćelije i tornadogeneze 30. kolovoza 2003.

A supercell storm with tornadogenesis in the afternoon hours of the 30th August 2003 in NW Croatia is studied. As was seen from radar and satellite material the cell developed in Slovenia and crossed into Croatia, where interacting with favorable wind patterns (indicated by high SREH and vertical wind shear) it evolved into a supercell. Macroscale forcing was found to be lacking. On the other hand, the pattern, characterized by a pronounced thermal ridge and a baroclinic boundary associated with it, was favorable for supercell generation as has been seen in several studies in the USA. Mesoscale forcing was also enhanced by the existence of the jet stream and outflow boundaries from previous convection associated with a night-time cold front passage. Several severe storm and stability indices were tested for this case and it has been found that they generally did not correspond to the severity of the convection that took place.U poslijepodnevnim satima 30. kolovoza 2003. u sjevero-zapadnom dijelu Hrvatske zabilježen je prolazak superćelije uz koji je bila vezana i pojava tornadogeneze. Iz radarskih je i satelitskih slika vidljivo da se ćelija formirala u Sloveniji te da se nakon prelaska u Hrvatsku uslijed povoljnog polja vjetra (velike vrijednosti SREH i vertikalnog smicanja vjetra) razvila u superćeliju. Procesi prisile bili su primarno mezoskalni. Temperaturni greben i uz njega vezana baroklina granica uočena na području Hrvatske i Slovenije prema istraživanjima provedenim u SAD-u karakteristični su za pojavu i jačanje superćelija. Dodatni izvori prisile bili su konvergencija vezana uz lijevi ulazni kvadrant mlazne struje i granicu izlaznog toka konvekcije vezane uz prolazak hladne fronte u noćnim satima. Nekoliko indeksa stabilnosti i indeksa olujnog vremena je testirano za ovaj slučaj konvekcije s izostankom makroskalne prisile te je nađeno da većinom nisu odgovarali stvarnom intenzitetu konvekcije

KATABATIC FLOW WITH CORIOLIS EFFECT

Abstract Katabatic flows on long glaciers in high latitudes experience the Coriolis effect deflecting the flow thus affecting turbulent transports in the boundary layer. Analytically katabatic flows have been best represented by Prandtl model. However, the classic Prandtl model does not take into account the effect of the Coriolis force. It is found that after a straightforward inclusion of this effect, the solution to the model is correct only up to a constant, and does not simultaneously satisfy all boundary conditions. Therefore, a modified analytic Prandtl model is examined. The modified approximate solution agrees with a numerical solution, and qualitatively, with the results of other related studies. Besides a theoretical significance, our approximate analytic solution can be useful for developing a better parameterization in climate models having a poor vertical resolution for such shallow but persistent flows

ALADIN/HR: TESTING THE NEW SUB-GRID SCALE OROGRAPHY REPRESENTATION ON BURA CASES

Especially important for the coastal area of Croatia is the wind forecast, particularly during the bura windstorm episodes. Current operational version of ALADIN model is rather satisfactory for the wind field forecast, although in many cases the wind speed is somewhat overestimated. A modified subgrid scale orography representation was introduced and tested. The envelope was removed and to compensate for the loss of volume, changes in gravity wave drag parametrisation were introduced. New orography representation resulted in slight enhancement of upstream and general decrease of downstream wind speed, as well as reduction of mountain wave amplitude

ALADIN/HR: TESTING THE NEW RADIATION AND CLOUDINESS PARAMETRIZATION

Unsatisfactory model forecasts of 2m temperature and cloudiness in inversion cases with fog and low stratus clouds have encouraged research of alternative parametrization for these processes that could be used in the operational version of the Aladin model. Few computationally cheap modifications are introduced and tested. Different cloud schemes and cloud overlap assumptions play more important role than the modifications in the radiation scheme

ALADIN/HR: TESTING THE NEW SUB-GRID SCALE OROGRAPHY REPRESENTATION ON BURA CASES

Especially important for the coastal area of Croatia is the wind forecast, particularly during the bura windstorm episodes. Current operational version of ALADIN model is rather satisfactory for the wind field forecast, although in many cases the wind speed is somewhat overestimated. A modified subgrid scale orography representation was introduced and tested. The envelope was removed and to compensate for the loss of volume, changes in gravity wave drag parametrisation were introduced. New orography representation resulted in slight enhancement of upstream and general decrease of downstream wind speed, as well as reduction of mountain wave amplitude

ALADIN/HR: TESTING THE NEW SEMI-LAGRANGIAN HORIZONTAL DIFFUSION SCHEME

Main role of the horizontal diffusion schemes in numerical models nowadays is to remove the information without forecast value and the energy accumulated due to finite truncation of a model spectrum acting as a numerical filter. However, during intensive cyclogenesis, especially over steep surfaces, the physical horizontal diffusion should not be neglected. A stable and efficient non-linear horizontal diffusion, based on the control of the degree of interpolation needed for the Semi-Lagrangian advection scheme, has been implemented in ALADIN. The results of several numerical experiments show better simulation of a mesoscale Adriatic cyclones, upper troposphere cyclones and beneficial impact on forecast of low cloudiness in anticyclones

The Community Foehn Classification Experiment

Strong winds crossing elevated terrain and descending to its lee occur over mountainous areas worldwide. Winds fulfilling these two criteria are called “foehn” in this paper although different names exist depending on region, sign of temperature change at onset, and depth of overflowing layer. They affect local weather and climate and impact society. Classification is difficult because other wind systems might be superimposed on them or share some characteristics. Additionally, no unanimously agreed-upon name, definition nor indications for such winds exist. The most trusted classifications have been performed by human experts. A classification experiment for different foehn locations in the Alps and different classifier groups addressed hitherto unanswered questions about the uncertainty of these classifications, their reproducibility and dependence on the level of expertise. One group consisted of mountain meteorology experts, the other two of Masters degree students who had taken mountain meteorology courses, and a further two of objective algorithms. Sixty periods of 48 hours were classified for foehn/no foehn at five Alpine foehn locations. The intra-human-classifier detection varies by about 10 percentage points (interquartile range). Experts and students are nearly indistinguishable. The algorithms are in the range of human classifications. One difficult case appeared twice in order to examine reproducibility of classified foehn duration, which turned out to be 50% or less. The classification dataset can now serve as a testbed for automatic classification algorithms, which - if successful - eliminate the drawbacks of manual classifications: lack of scalability and reproducibility

A scale-dependent model to represent changing aerodynamic roughness of ablating glacier ice based on repeat topographic surveys

Turbulent fluxes make a substantial and growing contribution to the energy balance of ice surfaces globally, but are poorly constrained owing to challenges in estimating the aerodynamic roughness length (z0). Here, we used structure from motion (SfM) photogrammetry and terrestrial laser scanning (TLS) surveys to make plot-scale 2-D and 3-D microtopographic estimations of z0 and upscale these to map z0 across an ablating mountain glacier. At plot scales, we found spatial variability in z0 estimates of over two orders of magnitude with unpredictable z0 trajectories, even when classified into ice surface types. TLS-derived surface roughness exhibited strong relationships with plot-scale SfM z0 estimates. At the glacier scale, a consistent increase in z0 of ∼0.1 mm d−1 was observed. Space-for-time substitution based on time since surface ice was exposed by snow melt confirmed this gradual increase in z0 over 60 d. These measurements permit us to propose a scale-dependent temporal z0 evolution model where unpredictable variability at the plot scale gives way to more predictable changes of z0 at the glacier scale. This model provides a critical step towards deriving spatially and temporally distributed representations of z0 that are currently lacking in the parameterisation of distributed glacier surface energy balance models