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Giouli Mihalakakou, Helena A. Flocas, Manthaios Santamouris, and Costas G. Helmis

Abstract

The effect of the synoptic-scale atmospheric circulation on the urban heat island phenomenon over Athens, Greece, was investigated and quantified for a period of 2 yr, employing a neural network approach. A neural network model was appropriately designed and tested for the estimation of the heat island intensity at 23 stations during the examined period. The day-by-day synoptic-scale atmospheric circulation in the lower troposphere for the same period was classified into eight statistically distinct categories. The neural network model employed as an input the corresponding synoptic categories in conjunction with four meteorological parameters that are closely related to the urban heat island. It was found that the synoptic-scale circulation is a predominant input parameter, affecting considerably the heat island intensity. Also, it was demonstrated that the high pressure ridge mostly favors the heat island phenomenon and categories characterized by intense northerly component winds are responsible for its nonappearance or termination.

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Helena A. Flocas, Vasiliki D. Assimakopoulos, Costas G. Helmis, and Hans Güsten

Abstract

The horizontal and vertical distributions of volatile organic compounds (VOCs) and ozone (O3) concentrations within the lower troposphere over the greater Athens area, Greece, under sea-breeze conditions were studied. Furthermore, an attempt was made to explain the dynamic and chemical mechanisms responsible for the formation of these distributions. Measurements were collected using a specially instrumented Falcon 20-E5 research aircraft, ground-based meteorological instrumentation, and a network of air quality monitoring stations within the context of the Scientific Training and Access to Aircraft for Atmospheric Research Throughout Europe (STAAARTE) 1997 experimental campaign. Relatively high ozone values (55–100 ppbv) were identified within the first 300–400 m above ground, and significantly reduced values were found over the depth of the atmospheric boundary layer. High values of VOC concentrations [150–350 ppbCarbon (C)] were observed near the ground as well as within the first 300–400 m above ground. At higher altitudes, of 1400–1600 m, VOC concentrations remained relatively high (100–200 ppbC). It was demonstrated that the sea-breeze circulation plays a major role in the formation of the above-mentioned concentration levels and that chemical transformations explain specific characteristics.

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James Edson, Timothy Crawford, Jerry Crescenti, Tom Farrar, Nelson Frew, Greg Gerbi, Costas Helmis, Tihomir Hristov, Djamal Khelif, Andrew Jessup, Haf Jonsson, Ming Li, Larry Mahrt, Wade McGillis, Albert Plueddemann, Lian Shen, Eric Skyllingstad, Tim Stanton, Peter Sullivan, Jielun Sun, John Trowbridge, Dean Vickers, Shouping Wang, Qing Wang, Robert Weller, John Wilkin, Albert J. Williams III, D. K. P. Yue, and Chris Zappa

The Office of Naval Research's Coupled Boundary Layers and Air–Sea Transfer (CBLAST) program is being conducted to investigate the processes that couple the marine boundary layers and govern the exchange of heat, mass, and momentum across the air–sea interface. CBLAST-LOW was designed to investigate these processes at the low-wind extreme where the processes are often driven or strongly modulated by buoyant forcing. The focus was on conditions ranging from negligible wind stress, where buoyant forcing dominates, up to wind speeds where wave breaking and Langmuir circulations play a significant role in the exchange processes. The field program provided observations from a suite of platforms deployed in the coastal ocean south of Martha's Vineyard. Highlights from the measurement campaigns include direct measurement of the momentum and heat fluxes on both sides of the air–sea interface using a specially constructed Air–Sea Interaction Tower (ASIT), and quantification of regional oceanic variability over scales of O(1–104 mm) using a mesoscale mooring array, aircraft-borne remote sensors, drifters, and ship surveys. To our knowledge, the former represents the first successful attempt to directly and simultaneously measure the heat and momentum exchange on both sides of the air–sea interface. The latter provided a 3D picture of the oceanic boundary layer during the month-long main experiment. These observations have been combined with numerical models and direct numerical and large-eddy simulations to investigate the processes that couple the atmosphere and ocean under these conditions. For example, the oceanic measurements have been used in the Regional Ocean Modeling System (ROMS) to investigate the 3D evolution of regional ocean thermal stratification. The ultimate goal of these investigations is to incorporate improved parameterizations of these processes in coupled models such as the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) to improve marine forecasts of wind, waves, and currents.

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