The role of subsidence in a weakly unstable marine boundary layer: a case study

The diurnal evolution of a cloud free, marine boundary layer is studied by means of experimental measurements and numerical simulations. Experimental data belong to an investigation of the mixing height over inner Danish waters. The mixed-layer height measured over the sea is generally nearly constant, and does not exhibit the diurnal cycle characteristic of boundary layers over land. A case study, during summer, showing an anomalous development of the mixed layer under unstable and nearly neutral atmospheric conditions, is selected in the campaign. Subsidence is identified as the main physical mechanism causing the sudden decrease in the mixing layer height. This is quantified by comparing radiosounding profiles with data from numerical simulations of a mesoscale model, and a large-eddy simulation model. Subsidence not only affects the mixing layer height, but also the turbulent fluctuations within it. By analyzing wind and scalar spectra, the role of subsidence is further investigated and a more complete interpretation of the experimental results emerges

Opažanja sjeveroistočno-jadranske bure pomoću satelita TerraSAR-X: rani rezultati

Some early results of the TerraSAR-X observations of the northeastern Adriatic bora wind are presented in this paper. TerraSAR-X is a German X-band radar satellite launched in 2007 that carries phased array X-band synthetic aperture radar (SAR) operating in different polarizations and providing multiple imaging modes. SAR backscatter can be used to derive wind fields at spatial resolution that no other instrument can provide. Terrain-induced jet and wake patterns are particularly conductive to the SAR-instrument examination. Bora, a cold and dry downslope wind blowing from north-easterly directions on the eastern side of the Adriatic Sea, exhibits such a response. Since bora is primarily winter wind and the town of Senj is known for frequent and severe bora episodes we focus on TerraSAR-X scenes collected in the winters of 2011 and 2012 over an area with Senj roughly in its center. Recently developed XMOD2 geophysical model function is used for wind magnitude derivation, whereas the WRF model was employed to estimate the wind direction. The selected TerraSAR-X scenes have captured representative bora events exhibiting rich details in bora-induced jet and wake patterns on the lee of the Dinaric Alps. The details registered in the normalized radar cross section response strongly suggest the need for still higher resolution numerical simulations in order to properly model the orographic impact on and the fine details in the surface wind field. Comparisons with both research and operational modeling results indicate that the currently used geophysical model function may benefit from enlarging the matchup data base with samples of severe winds.U radu su prikazani rani rezultati detekcije bure na sjeveroistočnoj strani Jadrana pomoću satelita TerraSAR-X. TerraSAR-X je njemački satelit lansiran 2007. godine koji nosi „phased- array“ radar sintetičke aperture (SAR) s mogućnošću rada uz različite polarizacije i uz više načina snimanja. Povratno zračenje instrumenta SAR može se iskoristiti za određivanje polja vjetra uz prostorno razlučivanje koje ne omogućuje ni jedan drugi instrument. Područja niskih mlaznih struja i zavjetrinske tišine, uzrokovana terenom, posebno su podatni za ispitivanja pomoću instrumenta SAR. Bura - hladan, suh i jak planinski vjetar koji puše iz sjeverno istočnih smjerova na istočnoj strani Jadranskog mora, izaziva spomenuti odziv. Kako je bura primarno zimski vjetar a senjsko područje poznato po čestim epizodama olujne bure, rad je fokusiran na TerraSAR-X scene registrirane tijekom zima 2011. i 2012. godine u širem području približno centriranom na Senj. Nedavno razvijena geofizička modelska funkcija XMOD2 korištena je za određivanje brzine vjetra a WRF model za procjenu njegova smjera. Odabrane TerraSAR scene pokrivaju reprezentativne epizode bure te pokazuju bogatstvo detalja u daljinskim zapisima niskih mlaznih struja i zavjetrinskih struktura koji nastaju na jadranskoj strani Dinarida. Detalji zabilježeni u odzivnom normaliziranom radarskom presjeku uvjerljivo sugeriraju potrebu još boljeg prostornog razlučivanja u numeričkim simulacijama da bi se ispravno modeliralo orografski utjecaj i detalje pripovršinskog polja vjetra. Usporedbe s rezultatima, kako istraživačkog tako i operativnog modeliranja, ukazuju da bi proširenje baždarne baze podataka uzorcima olujnog i orkanskog vjetra moglo poboljšati geofizičku modelsku funkciju

Data Assimilation in high resolution Numerical Weather Prediction models to improve forecast skill of extreme hydrometeorological events.

The complex orography typical of the Mediterranean area supports the formation, mainly during the fall season, of the so-called back-building Mesoscale Convective Systems (MCS) producing torrential rainfall often resulting into flash floods. These events are hardly predictable from a hydrometeorological standpoint and may cause significant amount of fatalities and socio-economic damages. Liguria region is characterized by small catchments with very short hydrological response time, and it has been proven to be very exposed to back-building MCSs occurrence. Indeed this region between 2011 and 2014 has been hit by three intense back-building MCSs causing a total death toll of 20 people and several hundred million of euros of damages. Building on the existing relationship between significant lightning activity and deep convection and precipitation, the first part of this work assesses the performance of the Lightning Potential Index, as a measure of the potential for charge generation and separation that leads to lightning occurrence in clouds, for the back-building Mesoscale Convective System which hit Genoa city (Italy) in 2014. An ensemble of Weather Research and Forecasting simulations at cloud-permitting grid spacing (1 km) with different microphysical parameterizations is performed and compared to the available observational radar and lightning data. The results allow gaining a deeper understanding of the role of lightning phenomena in the predictability of back-building Mesoscale Convective Systems often producing flash flood over western Mediterranean complex topography areas. Despite these positive and promising outcomes for the understanding highly-impacting MCS, the main forecasting issue, namely the uncertainty in the correct reproduction of the convective field (location, timing, and intensity) for this kind of events still remains open. Thus, the second part of the work assesses the predictive capability, for a set of back-building Liguria MCS episodes (including Genoa 2014), of a hydro-meteorological forecasting chain composed by a km-scale cloud resolving WRF model, including a 6 hour cycling 3DVAR assimilation of radar reflectivity and conventional ground sensors data, by the Rainfall Filtered Autoregressive Model (RainFARM) and the fully distributed hydrological model Continuum. A rich portfolio of WRF 3DVAR direct and indirect reflectivity operators, has been explored to drive the meteorological component of the proposed forecasting chain. The results confirm the importance of rapidly refreshing and data intensive 3DVAR for improving first quantitative precipitation forecast, and, subsequently flash-floods occurrence prediction in case of back-building MCSs events. The third part of this work devoted the improvement of severe hydrometeorological events prediction has been undertaken in the framework of the European Space Agency (ESA) STEAM (SaTellite Earth observation for Atmospheric Modelling) project aiming at investigating, new areas of synergy between high-resolution numerical atmosphere models and data from spaceborne remote sensing sensors, with focus on Copernicus Sentinels 1, 2 and 3 satellites and Global Positioning System stations. In this context, the Copernicus Sentinel satellites represent an important source of data, because they provide a set of high-resolution observations of physical variables (e.g. soil moisture, land/sea surface temperature, wind speed, columnar water vapor) to be used in NWP models runs operated at cloud resolving grid spacing . For this project two different use cases are analyzed: the Livorno flash flood of 9 Sept 2017, with a death tool of 9 people, and the Silvi Marina flood of 15 November 2017. Overall the results show an improvement of the forecast accuracy by assimilating the Sentinel-1 derived wind and soil moisture products as well as the Zenith Total Delay assimilation both from GPS stations and SAR Interferometry technique applied to Sentinel-1 data

A Synergistic Use of a High-Resolution Numerical Weather Prediction Model and High-Resolution Earth Observation Products to Improve Precipitation Forecast

open20siThe Mediterranean region is frequently struck by severe rainfall events causing numerous casualties and several million euros of damages every year. Thus, improving the forecast accuracy is a fundamental goal to limit social and economic damages. Numerical Weather Prediction (NWP) models are currently able to produce forecasts at the km scale grid spacing but unreliable surface information and a poor knowledge of the initial state of the atmosphere may produce inaccurate simulations of weather phenomena. The STEAM (SaTellite Earth observation for Atmospheric Modelling) project aims to investigate whether Sentinel satellites constellation weather observation data, in combination with Global Navigation Satellite System (GNSS) observations, can be used to better understand and predict with a higher spatio-temporal resolution the atmospheric phenomena resulting in severe weather events. Two heavy rainfall events that occurred in Italy in the autumn of 2017 are studied—a localized and short-lived event and a long-lived one. By assimilating a wide range of Sentinel and GNSS observations in a state-of-the-art NWP model, it is found that the forecasts benefit the most when the model is provided with information on the wind field and/or the water vapor content.openLagasio, Martina; Parodi, Antonio; Pulvirenti, Luca; Meroni, Agostino N.; Boni, Giorgio; Pierdicca, Nazzareno; Marzano, Frank S.; Luini, Lorenzo; Venuti, Giovanna; Realini, Eugenio; Gatti, Andrea; Tagliaferro, Giulio; Barindelli, Stefano; Monti Guarnieri, Andrea; Goga, Klodiana; Terzo, Olivier; Rucci, Alessio; Passera, Emanuele; Kranzlmueller, Dieter; Rommen, BjornLagasio, Martina; Parodi, Antonio; Pulvirenti, Luca; Meroni, Agostino N.; Boni, Giorgio; Pierdicca, Nazzareno; Marzano, Frank S.; Luini, Lorenzo; Venuti, Giovanna; Realini, Eugenio; Gatti, Andrea; Tagliaferro, Giulio; Barindelli, Stefano; Monti Guarnieri, Andrea; Goga, Klodiana; Terzo, Olivier; Rucci, Alessio; Passera, Emanuele; Kranzlmueller, Dieter; Rommen, Bjor

Wind Lidar and Radiosonde Measurements of Low-Level Jets in Coastal Areas of the German Bight

For wind energy, the knowledge of the available wind resource is essential. Therefore, specific wind phenomena at the altitude range of wind turbines are currently the focus of investigations. One such specific feature is the low-level jet (LLJ). The article analyses LLJ properties at two locations in the German Bight: A wind lidar system for measuring wind profiles at heights from 50 m to 500 m a.g.l. (above ground level) was first installed at the offshore island of Heligoland, Germany, and then at the coastal island of Norderney, Germany, for one year. The LLJ is defined here as a maximum horizontal wind speed in the vertical profile of horizontal wind speed followed by a minimum wind speed, independent of the mechanism or origin of the phenomenon. The two sites showed a similar annual and diurnal distribution of LLJ events with a maximum occurrence in spring and summer and during the night, and a most frequent jet core height of around 120 m a.g.l. Based on radiosondes launched from Norderney at midnight and noon, it is shown that LLJ events at noon are most frequent when atmospheric conditions are stable. A case study shows the horizontal extent of an LLJ event over at least 100 km by simultaneous wind lidar measurements at four sites in the German Bight and mesoscale simulations with the weather research and forecast (WRF) model

High-resolution regional modeling of urban moisture island: mechanisms and implications on thermal comfort

The urban moisture island (UMI) can aggravate the thermal stress due to the urban heat island (UHI) in subtropical and tropical cities. In this study, we investigated the spatiotemporal variation patterns of UMI in Hong Kong, a subtropical coastal city, using the fine-resolution mesoscale Weather Research and Forecasting (WRF) model by integrating local climate zone (LCZ) maps based on the World Urban Database and Access Portal Tools (WUDAPT). Our results show that at regional scale, the UMI phenomenon tends to occur in coastal areas, possibly owing to rich moisture sources from sea breeze and inhibited moisture penetration due to barrier effects of mountains. Specifically, an all-day UMI effect was found in coastal low-density low-rise areas (LCZ5&8&10), while a nocturnal UMI effect and a daytime urban dry island (UDI) effect were found in coastal high-density high-rise areas (LCZ1&2). The UDI effect at daytime can be attributed to strong vertical moisture convection associated with intensive surface sensible heat fluxes in a strongly mixed urban boundary layer (UBL). The UMI effect at night can be attributed to blocked ventilation aisle, inhibited dewfall due to UHI, and weakened upward motion in a stable UBL. On the other hand, UMI can increase regional heat risks with additional 37.5% neighbourhoods in Extreme caution level and additional 6.1% neighbourhoods in Danger level. In addition, the impact of UMI on human thermal stress was found to be dominant at daytime in coastal low-density low-rise areas (LCZ5&8&10) and at nighttime in coastal high-density high-rise areas (LCZ1&2)

Applications of satellite winds for the offshore wind farm site Anholt

Rapid growth in the offshore wind energy sector means more offshore wind farms are placed closer to each other and in the lee of large land masses. Synthetic aperture radar (SAR) offers maps of the wind speed offshore with high resolution over large areas. These can be used to detect horizontal wind speed gradients close to shore and wind farm wake effects. SAR observations have become much more available with the free and open-access data from European satellite missions through Copernicus. Examples of applications and tools for using large archives of SAR wind maps to aid offshore site assessment are few. The Anholt wind farm operated by the utility company Ørsted is located in coastal waters and experiences strong spatial variations in the mean wind speed. Wind speeds derived from the Supervisory Control And Data Acquisition (SCADA) system are available at the turbine locations for comparison with winds retrieved from SAR. The correlation is good, both for free-stream and waked conditions. Spatial wind speed variations along the rows of wind turbines derived from SAR wind maps prior to the wind farm construction agree well with information gathered by the SCADA system and a numerical weather prediction model. Wind farm wakes are detected by comparisons between images before and after the wind farm construction. SAR wind maps clearly show wakes for long and constant fetches but the wake effect is less pronounced for short and varying fetches. Our results suggest that SAR wind maps can support offshore wind energy site assessment by introducing observations in the early phases of wind farm projects.</p