Turbulence, Low-Level Jets, and Waves in the Tyrrhenian Coastal Zone as Shown by Sodar

Margarita Kallistratova,Stefania Argentini,Roberto Sozzi,Giampietro Casasanta,Simone Bucci,Igor Petenko

doi:10.3390/atmos11010028

Abstract

The characteristics of the vertical and temporal structure of the coastal atmospheric boundary layer are variable for different sites and are often not well known. Continuous monitoring of the atmospheric boundary layer was carried out close to the Tyrrhenian Sea, near Tarquinia (Italy), in 2015–2017. A ground-based remote sensing instrument (triaxial Doppler sodar) and in situ sensors (meteorological station, ultrasonic anemometer/thermometer, and net radiometer) were used to measure vertical wind velocity profiles, the thermal structure of the atmosphere, the height of the turbulent layer, turbulent heat and momentum fluxes in the surface layer, atmospheric radiation, and precipitation. Diurnal alternation of the atmospheric stability types governed by the solar cycle coupled with local sea/land breeze circulation processes is found to be variable and is classified into several main regimes. Low-level jets (LLJ) at heights of 100–300 m above the surface with maximum wind speed in the range of 5–18 m s−1 occur in land breezes, both during the night and early in the morning. Empirical relationships between the LLJ core wind speed characteristics and those near the surface are obtained. Two separated turbulent sub-layers, both below and above the LLJ core, are often observed, with the upper layer extending up to 400–600 m. Kelvin–Helmholtz billows associated with internal gravity–shear waves occurring in these layers present opposite slopes, in correspondence with the sign of vertical wind speed gradients. Our observational results provide a basis for the further development of theoretical and modelling approaches, taking into account the wave processes occurring in the atmospheric boundary layer at the land–sea interface.

Highlights

Local atmospheric circulation is an important meteorological phenomenon, affecting the weather in coastal regions and influencing atmospheric pollution transport and diffusion, convective thunderstorms, aviation safety, the propagation of forest fires, the energy industry, sports, tourism, services, etc
Results on coastal atmospheric boundary layer (ABL) morphology were presented to indicate the principal features generated by the complicated interactions between local sea/land breeze circulation with level jets (LLJ), turbulence, and wave processes
A ground-based remote sensing instrument and conventional in situ micro-meteorological sensors were used for continuous monitoring of wind velocity vertical profiles, the thermal structure of the atmosphere, the height of the turbulent layer, turbulent heat and momentum fluxes in the surface layer, atmospheric radiation, and precipitation

Summary

Introduction

Local atmospheric circulation is an important meteorological phenomenon, affecting the weather in coastal regions and influencing atmospheric pollution transport and diffusion, convective thunderstorms, aviation safety, the propagation of forest fires, the energy industry (wind turbines), sports, tourism, services, etc. (see, e.g., [1,2,3]). Two inherent components—land and sea breezes—arise from differential heating between land and water surfaces and produce a daily cycle of wind velocity variation Under real conditions, these flows are strongly influenced by zonal winds, synoptic pressure fields, sea currents and upwellings, the orientation and orography of coastlines, radiation conditions, Atmosphere 2020, 11, 28; doi:10.3390/atmos11010028 www.mdpi.com/journal/atmosphere. The wind speed and related atmospheric parameters have been measured either close to the land surface near the coastline or on low masts. Such measurements do not provide a complete picture of the breeze circulation. Employment of ground-based remote sensing provides more comprehensive knowledge of the vertical structure of the breezes [11]

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