Statistical investigations of flow structures in different regimes of the stable boundary layer

A combination of methods originating from non-stationary timeseries analysis is applied to two datasets of near surface turbulence in order to gain insights on the non-stationary enhancement mechanism of intermittent turbulence in the stable atmospheric boundary layer (SBL). We identify regimes of SBL turbulence for which the range of timescales of turbulence and submeso motions, and hence their scale separation (or lack of separation) differs. Ubiquitous flow structures, or events, are extracted from the turbulence data in each flow regime. We relate flow regimes characterised by very stable stratification but different scales activity to a signature of flow structures thought to be submeso motions.Comment: Accepted for publication in Boundary Layer Meteorolog

Formiranje povratnog zavjetrinskog strujanja nad sjeveroistočnim Jadranom tijekom bure

In-situ measurements indicate the existence of reversed near-surface airflow over the north-eastern Adriatic during the bora wind. Here we examine its structure and evolution by means of a numerical simulation. The studied phenomenon developed during typical summer frontal bora that is associated with a cold air advection from the north-east. It is characterized by a sudden increase in wind speed and a brief duration. Within a sheltered area near the town of Malinska on the island of Krk, onshore flow occurred as the bottom branch of the lee rotor-like circulation that was associated with the hydraulic jump. While the closed circulation somewhat resembles the hypothesized hydraulic-jump rotor, there are significant differences, particularly in the strength of the turbulence associated with the rotor formation.Površinska mjerenja upućuju na postojanje povratnog površinskog strujanja na sjeveroistočnom Jadranu tijekom bure. Stoga ovdje ispitujemo njegovu strukturu i razvoj pomoću numeričkih simulacija. Proučavana se pojava razvila tijekom jednog tipičnog događaja ljetne frontalne bure koju karakterizira iznenadno povećanje brzine vjetra. U blizini Malinske, unutar zaklonjenog dijela tijekom bure, strujanje prema obali je nastalo kao donja grana zavjetrinske cirkulacije slične rotoru koja je povezana s hidrauličkim skokom. Iako zatvorena cirkulacija sliči hipotetskom rotoru uslijed hidrauličkog skoka, postoje značajne razlike, osobito u snazi turbulencije koja je povezana s nastankom rotora

SEVERE ADRIATIC BURA EVENT FROM 14 TO 15 NOVEMBER 2004

We investigated the dynamics and structure of a severe bura event. Apart from the routine meteorological data, in this study, the wind data from three ultrasonic anemometers were used for the first time. The episode was modeled by the mesoscale model MM5. In the analyses of the model results, particular emphasis is given to establishment of the spatial structure of the potential vorticity (PV), and, its comparison with PV fields from previous bura studies. Due to well-known bura gustiness, also examined is the ability of the mesoscale model to predict the bura wind gusts

Climate projections of meteotsunami hazards

Global climate models, indispensable for projecting the human-driven climate change, have been improving for decades and are nowadays capable of reproducing multiple processes (e.g., aerosols, sea-ice, carbon cycle) at up to 25 km horizontal resolution. Meteotsunami events – tsunami waves generated by mesoscale atmospheric processes – are properly captured only by sub-kilometre-scale downscaling of these models. However, the computational cost of long-term high-resolution climate simulations providing accurate meteotsunami hazard assessments would be prohibitive. In this article, to overcome this deficiency, we present a new methodology allowing to project sub-kilometre-scale meteotsunami hazards and their climate uncertainties at any location in the world. Practically, the methodology uses (1) synoptic indices to preselect a substantial number of short-term meteotsunami episodes and (2) a suite of atmospheric and oceanic models to downscale them from an ensemble of global models to the sub-kilometre-scale. Such approach, using hundreds of events to build robust statistics, could allow for an objective assessment of the meteotsunami hazards at the climate scale which, on top of sea level rise and storm surge hazards, is crucial for building adaptation plans to protect coastal communities worldwide

FIRST MEASUREMENTS OF BURA WIND AT SENJ WITH A THREE-AXIS ANEMOMETER

Measurements of the bora wind at Senj (east Adriatic) with a three-axis anemometer showed that at the periods smaller than 1 min turbulent eddies propagating downstream prevail, in accordance with conventional wisdom. At the periods of 5-10 min bora pulsations were observed, with rotary spectral analysis pointing to hithertho unsuspected dynamics of these pulsations

DISAPPEARANCE OF PULSATIONS IN SEVERE DOWNSLOPE WINDSTORMS

Idealized numerical simulations of the bora-type flow have been performed. The intention was to find what causes the pulsations in the developed windstorm state to disappear. Four different incident vertical profiles were used and the results showed that the main effect originates from the low-level positive shear, and the uppertropospheric jet stream acts as a secondary effect