Stefania Argentini

A model‐aided investigation of winter thermally driven circulation on the Italian Tyrrhenian coast: A case study

[1] Data collected during a campaign carried out in and around Rome in February 1996 using sodars, tethered balloon, and surface data suggest the development of a winter sea breeze that could not be detected by conventional data and analyses. A model-aided study is performed to verify the development of thermally driven local circulation and to prove the onset of a winter sea breeze regime, propagating inland and interacting with the urban heat island (UHI). The area of interest for the study is the central part of Italy (latitude 41– 43N, longitude 10–16E). The MM5 mesoscale model was used at high resolution: three nested domains with up to 3 km of grid resolution were used in the area where the sea breeze was observed. To highlight the effect of the sea breeze, a simulation with a thick cloud layer in the innermost domain was performed. By removing the cloud layer the diurnal evolutionofthehorizontaltemperature gradientledtothedevelopmentoftheseabreezeand the UHI circulation, in agreement with the data. Sensitivity tests were carried out to evaluate the effect of land use resolution and of satellite-retrieved/climatological sea surface temperatureonthesimulationresults.Doppler-sodardatarecordedatthreesites,tethersonde profiles, and surface data were used to verify the results. As expected, using high-resolution land use and the daily sea surface temperature retrieved by satellite observations led to improvementsinthelocalcirculationproducedbythemodel.However,themodelfallsshort in reproducing some of the sea breeze characteristics. It finds that the sea breeze starts too early and lasts for a shorter time than the observed one does, and the wind direction shows a too strong northerly component. Also, it slightly underestimates the daily temperature pattern. The interaction of the urban heat islands of Rome and Ostia with the sea breeze flow is analyzed to explain the causes of these discrepancies. INDEX TERMS: 3307 Meteorology and Atmospheric Dynamics: Boundary layer processes; 3322 Meteorology and Atmospheric Dynamics: Land/ atmosphere interactions; 3329 Meteorology and Atmospheric Dynamics: Mesoscale meteorology; KEYWORDS: sea breeze, modeling, urban heat island Citation: Ferretti, R., G. Mastrantonio, S. Argentini, R. Santoleri, and A. Viola, A model-aided investigation of winter thermally driven circulation on the Italian Tyrrhenian coast: A case study, J. Geophys. Res., 108(D24), 4777, doi:10.1029/2003JD003424, 2003.

Observed and Modelled Convective Mixing-Layer Height at Dome C, Antarctica

The mixing-layer height is estimated using measurements from a high resolution surface-layer sodar run at the French-Italian station of Concordia at Dome C, Antarctica during the summer 2011–2012. The temporal and spatial resolution of the sodar allows the monitoring of the mixing-layer evolution during the whole diurnal cycle, i.e. a very shallow nocturnal boundary layer followed by a typical daytime growth. The behaviour of the summer mixing-layer height, variable between about 10- and 300 m, is analyzed as a function of the mean and turbulent structure of the boundary layer. Focusing on convective cases only, the retrieved values are compared with those calculated using a one-dimensional prognostic equation. The role of subsidence is examined and discussed. We show that the agreement between modelled and experimental values significantly increases if the subsidence is not kept fixed during the day. A simple diagnostic equation, which depends on the time-averaged integral of the near-surface turbulent heat flux, the background static stability and the buoyancy parameter, is proposed and evaluated. The diagnostic relation performance is comparable to that of the more sophisticated prognostic model.

Use of a High-Resolution Sodar to Study Surface-layer Turbulence at Night

Measurements in the atmospheric surface layer are generally made with point sensors located in the first few tens of metres. In most cases, however, these measurements are not representative of the whole surface layer. Standard Doppler sodars allow a continuous display of the turbulent thermal structure and wind profiles in the boundary layer up to 1000 m, with a few points, if any, in the surface layer. To overcome these limitations a new sodar configuration is proposed that allows for a higher resolution in the surface layer. Because of its capabilities (echo recording starting at 2 m, echo intensity vertical resolution of approximately 2 m, temporal resolution of 1 s) this sodar is called the surface-layer mini-sodar (SLM-sodar). Features and capabilities of the SLM-sodar are described and compared with the sodar. The comparison of the thermal vertical structure given by the SLM-sodar and the sodar provides evidence that, in most cases, the surface layer presents a level of complexity comparable to that of the entire boundary layer. Considering its high vertical resolution, the SLM-sodar is a promising system for the study of the nocturnal surface layer. The nocturnal SLM-sodar measurements have shown that, depending on wind speed, the structure of the surface layer may change substantially within a short time period. At night, when the wind speed is greater than 3 m s−1, mechanical mixing destroys the wavy structure present in the nocturnal layer. Sonic anemometer measurements have shown that, in such cases, also the sensible heat flux varies with height, reaching a peak in correspondence with the wind speed peak. Under these conditions the assumption of horizontal homogeneity of the surface layer and the choice of the averaging time need to be carefully treated.

Estimate of the Arctic Convective Boundary Layer Height from Lidar Observations: A Case Study

A new automated small size lidar system (microlidar or MULID) has been developed and employed to perform aerosol measurements since March 2010 at Ny Alesund (, ), Svalbard. The lidar observations have been used to estimate the PBL height by using the gradient method based on abrupt changes in the vertical aerosol profile and monitor its temporal evolution. The scope of the present study is to compare several approaches to estimate the PBL height, by using lidar observations, meteorological measurements by radio soundings, and a zero-order one-dimensional model based on a parameterization of the turbulent kinetic energy budget within the mixing layer, under the assumptions of horizontal homogeneity, and neglecting radiation and latent heat effects. A case study is presented here for a convective PBL, observed in June 2010 in order to verify whether the Gradient Method can be applied to lidar measurements in the Arctic region to obtain the PBL height. The results obtained are in good agreement with the PBL height estimated by the analysis of thermodynamic measurements obtained from radio sounding and with the model.

Wavy Vertical Motions in the ABL Observed by Sodar

Some characteristics of wavelike motions in the atmospheric boundary layer observed by sodar are considered. In an experiment carried out in February 1993 in Milan, Italy, Doppler sodar measurements were accompanied by in situ measurements of temperature and wind velocity vertical profiles using a tethered balloon up to 600 m. The oscillations of elevated wavy layers containing intense thermal turbulence, usually associated with temperature-inversion zones, were studied by using correlation and spectral analysis methods. The statistics of the occurrence of wavelike and temperature-inversion events are presented. The height distributions of Brunt–Vaisala frequency and wind shear and their correlation within elevated inversion layers were determined, with a strong correlation observed between the drift rate of the wavy layers and the vertical velocity measured by Doppler sodar inside these layers. Spectral analysis showed similarities regarding their frequency characteristics. The phase speed and propagation direction of waves were estimated from the time delay of the signals at three antennae to provide estimates of wavelength. Moreover, wavelengths were estimated from the intrinsic frequency obtained from sodar measurements of the Doppler vertical velocity and oscillations of wavy turbulent layers. The two wavelength estimates are in good agreement.

Observations of near surface wind speed, temperature and radiative budget at Dome C, Antarctic Plateau during 2005

Abstract The annual and diurnal behaviours of near surface wind speed, temperature, and the radiative budget at Concordia Station (Dome C) in different seasons are shown. The wind speed was lowest in summer when a daily cycle was also observed. The largest mean values were concurrent with boundary layer growth in the afternoon. In winter and spring the wind speed reached the highest mean values. Perturbations in the wind flow were due to warming events which occurred periodically at Dome C. The lowest temperatures were in April and at the end of August. The coreless winter behaviour was perturbed by warming events which in many cases produced an increase in temperature of c. 20°C. The average temperature profiles show permanent thermal inversion, with the exception of a few hours in the afternoons during the summer. The strongest ground-based thermal inversions were observed in the polar winter. The largest potential temperature gradients were limited to a 30–40 m deep layer close to the surface. The net radiation was negative almost all the time with the exception of the period from mid-December to mid-January.

The Annual Behavior of the Semidiurnal and Diurnal Pressure Variations in East Antarctica

Abstract A 15-yr (1980–95) 3-hourly record of surface pressure from the Dome Concordia automatic weather station (74.30°S, 123.00°E; 3280 m MSL), located in East Antarctica, was analyzed to study the annual behavior of the semidiurnal and diurnal variation associated with atmospheric tides excited by heating due to insolation absorption by ozone and water vapor. The mean daily behavior of the pressure variation shows maxima around 0900 and 2100 LT. This variation is more intense during the austral winter. The time series of a Total Ozone Mapping Spectrometer (TOMS) were analyzed to study the correlation between the local pressure tidal oscillations and the annual behavior of total ozone over the globe. A clear correlation between the intensity of the pressure semidiurnal variation and global total ozone in both the annual and long-term trends was found. The mean annual behavior of the semidiurnal tides and global total ozone is very similar, with two maxima and one deep minimum. Maxima in semidiurnal tide...

Wavelike Structures in the Turbulent Layer During the Morning Development of Convection at Dome C, Antarctica

In the period January–February 2014, observations were made at the Concordia station, Dome C, Antarctica to study atmospheric turbulence in the boundary layer using a high-resolution sodar. The turbulence structure was observed beginning from the lowest height of about 2 m, with a vertical resolution of less than 2 m. Typical patterns of the diurnal evolution of the spatio-temporal structure of turbulence detected by the sodar are analyzed. Here, we focus on the wavelike processes observed within the transition period from stable to unstable stratification occurring in the morning hours. Thanks to the high-resolution sodar measurements during the development of the convection near the surface, clear undulations were detected in the overlying turbulent layer for a significant part of the time. The wavelike pattern exhibits a regular braid structure, with undulations associated with internal gravity waves attributed to Kelvin–Helmholtz shear instability. The main spatial and temporal scales of the wavelike structures were determined, with predominant periodicity of the observed wavy patterns estimated to be 40–50 s. The horizontal scales roughly estimated using Taylor’s frozen turbulence hypothesis are about 250–350 m.

First Observations of Energy Budget and Bulk Fluxes at Ny Ålesund (Svalbard) during a 2010 Transition Period as Analyzed with the BEAR Station

A small-size meteorological mast, BEAR (Budget of Energy for Arctic regions) has been developed as a part of a new autonomous buoy for monitoring the sea ice mass balance. BEAR complements observations of the thickness and thermodynamic properties of the ice/snow pack determined by the so-called Ice-T (Ice-Thickness) buoy, giving access to bulk fluxes and energy budget at the surface, using meteorological measurements. The BEAR mast has been tested with success during ten days in April-May 2010 at Ny Alesund, in the Svalbard archipelago (Norway) showing that meteorological data were close to measurements at the same level of the Italian Climate Change Tower (CCT) from the ISAC-CNR. A discussion is undertaken on bulk fluxes determination and uncertainties. Particularly, the strategy to systematically use different relevant fluxes parameterizations is pointed out to explore flux range uncertainty before to analyze energy budget. Net radiation, bulk fluxes and energy budget are estimated using as average 10 minutes, 24 hours and the ten days of the experiment. The observation period was very short, but we observe a spring transition when the net radiation begins to warm the surface while the very small turbulent heat flux cools the surface.

Erratum to: Observed and Modelled Convective Mixing-Layer Height at Dome C, Antarctica

We have discovered a calculation error in the evaluation of the history scale Q represented by Eq. 3 (Sect. 4.2). We apologise for any confusion or inconvenience that may have arisen. The miscalculation does not affect the general performance of the diagnostic relation proposed in the same section, so that the conclusions remain almost unchanged. After the correction, the values of α (Eq. 4) and of the determination coefficient (R2) retrieved by the linear regression showed in Fig. 8 become 11.20±0.30 and 0.86, respectively. The corrected values of the parameters listed in the last column (Diagnost relation) of Table 2 are mae = 33, rmse = 47, F B = 0.19, and I oA = 0.76. Figures7 and 8 with the correct calculation of Q are shown below.