S. Nickovic

A long Saharan dust event over the western Mediterranean: Lidar, Sun photometer observations, and regional dust modeling

A long Saharan dust event affected the western Mediterranean in the period 12– 28 June 2002. Dust was present mainly between 1- and 5-km height affecting most parts of the Iberian Peninsula and reaching western/central Europe. Intensive backscatter lidar observations over Barcelona (Spain) and Sun photometer data from two stations (El Arenosillo, Spain, and Avignon, France) are used to evaluate different configurations the Dust Regional Atmospheric Modeling (DREAM) system. DREAM currently operates dust forecasts over the Mediterranean region (http://www.bsc.es/projects/earthscience/ DREAM/) considering four particle size bins while only the first two are relevant for long-range transport analysis since their life time is larger than 12 hours. A more detailed bin method is implemented, and two different dust distributions at sources are compared to the operational version. Evaluations are performed at two wavelengths (532 and 1064 nm). The dust horizontal and vertical structure simulated by DREAM shows very good qualitative agreement when compared to SeaWIFS satellite images and lidar height-time displays over Barcelona. When evaluating the modeled aerosol optical depth (AOD) against Sun photometer data, significant improvements are achieved with the use of the new detailed bin method. In general, the model underpredicts the AOD for increasing A ° ngstro¨m exponents because of the influence of anthropogenic pollution in the boundary layer. In fact, the modeled AOD is highly anticorrelated with the observed A ° ngstro¨m exponents. Avignon shows higher influence of small anthropogenic aerosols which explains the better results of the model at the wavelength of 1064 nm over this location. The uncertainties of backscatter lidar inversions (20–30%) are in the same order of magnitude as the differences between the model experiments. Better model results are obtained when comparing to lidar because most of the anthropogenic effect is removed.

Development and evaluation of the BSC-DREAM8b dust regional model over Northern Africa, the Mediterranean and the Middle East

The BSC-DREAM8b model and its predecessor are analysed in terms of aerosol optical depth (AOD) for 2004 over Northern Africa, the Mediterranean and the Middle East. We discuss the model performance and we test and analyse its behaviour with new components. The results are evaluated using hourly data from 44 AERONET stations and seasonally averaged satellite observations. The operational versions strongly underestimate the winter AOD over the Sahel and overestimate the AOD over the Middle East and the Mediterranean achieving a low average annual correlation (~0.35). The use of a more detailed size distribution and a corrected wash-out ratio, together with a new dry deposition scheme, improves the transport over the Mediterranean, although underestimations remain over the Sahel and overestimations over the Middle East. The inclusion of a ‘preferential source’ mask improves the localisation of the main North African sources and consequently the dust transport towards Europe and the Atlantic. The use of a more physically based dust emission scheme and a new soil texture database leads to significant improvements in the representation of emissions and the transport over the Sahel, achieving an average annual correlation of 0.53. In this case, the use of a preferential source mask does not introduce significant improvements.

The storm of October 21–22, 1994, over Greece: Observations and model results

During October 21, 1994, a cold front passed over Greece. This frontal passage provoked catastrophic floods and there were many casualties. Eleven deaths were reported during this event, nine of them inside the Greater Athens Area. Significant damages occurred in transportation telecommunication and energy supply networks, especially in the eastern part of the country. This paper reports on the simulations of the observed storm conducted by two numerical models : the Colorado State University-Regional Atmospheric Modelling System (CSU-RAMS) and the η-cta/National Meteorological Center (ETA/NMC) model. The intercomparison of results between a regional research-oriented model (RAMS) with an operational model (ETA/NMC) penrmitted to explore the capabilities and limitations of each one of them. RAMS was operated in a nonhydrostatic mode using explicit microphysics and grid nesting (two nests with 40- and 10-km horizontal grid interval) and provided results which compare favorably with observations, suggesting that the model can adequately represent the mesoscale structure of the system. ETA/NMC is a hydrostatic limited-area model using parameterization of large-scale and convective precipitation. It was operated with 25-km horizontal resolution and it forecasted successfully the major characteristics of the system but failed in reproducing quantitatively the precipitation pattern at the mesoscale.

Forecast errors in dust vertical distributions over Rome (Italy): Multiple particle size representation and cloud contributions

[1] In this study, forecast errors in dust vertical distributions were analyzed. This was carried out by using quantitative comparisons between dust vertical profiles retrieved from lidar measurements over Rome, Italy, performed from 2001 to 2003, and those predicted by models. Three models were used: the four-particle-size Dust Regional Atmospheric Model (DREAM), the older one-particle-size version of the SKIRON model from the University of Athens (UOA), and the pre-2006 one-particle-size Tel Aviv University (TAU) model. SKIRON and DREAM are initialized on a daily basis using the dust concentration from the previous forecast cycle, while the TAU model initialization is based on the Total Ozone Mapping Spectrometer aerosol index (TOMS AI). The quantitative comparison shows that (1) the use of four-particle-size bins in the dust modeling instead of only one-particle-size bins improves dust forecasts; (2) cloud presence could contribute to noticeable dust forecast errors in SKIRON and DREAM; and (3) as far as the TAU model is concerned, its forecast errors were mainly caused by technical problems with TOMS measurements from the Earth Probe satellite. As a result, dust forecast errors in the TAU model could be significant even under cloudless conditions. The DREAM versus lidar quantitative comparisons at different altitudes show that the model predictions are more accurate in the middle part of dust layers than in the top and bottom parts of dust layers.

Dust modelling and forecasting in the Barcelona Supercomputing Center: Activities and developments

The Barcelona Supercomputing Center (BSC) is the National Supercomputer Facility in Spain, hosting MareNostrum, one of the most powerful Supercomputers in Europe. The Earth Sciences Department of BSC operates daily regional dust and air quality forecasts and conducts intensive modelling research for short-term operational prediction. This contribution summarizes the latest developments and current activities in the field of sand and dust storm modelling and forecasting.

Sea-Salt Aerosol Forecasts Compared with Wave Height and Sea-Salt Measurements in the Open Sea

Sea-salt aerosol (SSA) could influence the Earth’s weather and climate acting as cloud condensation nuclei. In spite of the importance of SSA effects on the Earth’s climate and weather, there were no measurements of sea-salt aerosols in the open sea. At Tel-Aviv University, the DREAM-Salt prediction system has been producing daily forecasts of 3-D distribution of sea-salt aerosol concentrations over the Mediterranean model domain 20 W–45E, 15 N–50 N (http://wind.tau.ac.il/salt-ina/salt.html). In order to evaluate the model performance in the open sea, daily modeled sea-salt aerosol concentrations were compared directly with sea-salt ground-based measurements taken at the tiny island of Lampedusa, in the Central Mediterranean. In order to further test the robustness of the model, the model performance over the open sea was indirectly verified by comparing modeled SSA concentrations with wave height measurements collected by the ODAS Italia 1 buoy. Model-vs.-measurement comparisons show that the model is capable of producing realistic SSA concentrations and their day-to-day variations over the open sea, in accordance with observed wave height and wind speed.

Short-term changes in the northwest African Upwelling System induced by Saharan dust deposition events

During the last 7-year period (2000-2006) atmosphere circulation changes show strong influences on the dust storm deposition dynamics and, as a result, on the primary production dynamics of the northwest African Upwelling System. From 2000 to 2006, the annual mean sea level pressure became higher ranging from 1014 to 1015 mb. Mean annual zonal wind intensity became higher (from 1.1 to 1.8 m s-1), while the mean annual meridional was reduced from 6.2 to 5.3 m s-1 at the north of the Canary Islands. Mean annual satellite-derived AVHRR/NOAA SST recorded in the northwest African Upwelling became warmer in both locations, from 18.3°C to 18.8°C in Cape Ghir and from 19.5°C to 20.3°C north Canary Islands waters. CHL records from the SeaWiFS/OV-2 showed a different pattern trend. Mean annual CHL levels increased at Cape Ghir from 0.65 mg m-3 to 0.9 mg m-3 and significantly reduced from 0.59 mg m-3 to 0.31 mg m-3 at the north of the Canary Islands. Changes observed in the role of CHL during the last 7-years period could be associated to intensive dust deposition and exceptional weather warming observed in this area since 2000. However, this study focused on a 7-year period and conclusions on possible links between dust deposition and marine biochemistry activity cannot be generalized.

Chapter 1.5 Assessment of dust forecast errors by using lidar measurements over Rome

Abstract In this study, forecast errors in dust vertical distributions were analyzed. This was carried out by using quantitative comparisons between dust vertical profiles retrieved from lidar measurements over Rome, Italy, and those predicted by models. Three models were used: the four-particle-size Dust Regional Atmospheric Model (DREAM), the older one-particle-size version of the SKIRON model from the University of Athens (UOA), and the pre-2006 one-particle-size Tel Aviv University (TAU) model. SKIRON and DREAM are initialized on a daily basis using the dust concentration from the previous forecast cycle, while the TAU model initialization is based on the Total Ozone Mapping Spectrometer aerosol index (TOMS AI). The quantitative comparison shows that (1) the use of four-particle-size bins in the dust modeling instead of only one-size bin improves dust forecasts, (2) cloud presence could contribute to additional dust forecast errors in SKIRON and DREAM, (3) as far as the TAU model is concerned, its forecast errors were mainly caused by technical problems with TOMS measurements from the Earth Probe satellite. As a result, dust forecast errors in the TAU model could be significant even under cloudless conditions.