Interaction of a sea breeze and a moist convection over the northeastern Adriatic coast: an analysis of the sensitivity experiments using the high‐resolution mesoscale model 

Gabrijela Poljak1, Maja Telišman Prtenjak1, Marko Kvakić2, Kristina Šariri3 and Željko Večenaj1

1Andrija Mohorovičić Geophysical Institute, Department of Geophysics, Faculty of Science, University of Zagreb, Zagreb, Croatia2ILRI, Nairobi, Kenya

3Croatian Metrology Institute, Zagreb, Croatiaemail: poljakica555@yahoo.com & telisman@gfz.hr

ReferencesBabić K, Mikuš P, Prtenjak MT (2012): The relationship between shallow thermal circulation regimes and cumulonimbus clouds along northeastern Adriatic coast. Geofizika, 29,103-120.Mikuš P, Prtenjak MT, Strelec Mahović N (2012): Analysis of the convective activity and its synoptic background over Croatia. Atmos. Res., 104/105,139-153.Pielke RA (2002): Mesoscale meteorological modeling, 2nd Edition, Academic Press, USA.Poljak G, Prtenjak, MT, Kvakić M, Strelec Mahović N, Babić K (2014): Wind patterns associated with the development of daytime thunderstorms over Istria, Ann. Geophys., 32, 401–420. Sović I, Šariri K, Živčić M (2013): High frequency microseismic noise as possible earthquake precursor. Research in Geophysics, 3, 8-12.

Fig. 1: The satellite SST and ground temperaturedistribution in the area of interest (the northernAdriatic at 11 CET on 9 July 2006) and measuring sitesas white circles (Udine, San Pietro Capofiume (SPC),Portorož-Airport, Pazin, Poreč, Pula-airport).

Motivation & AimThe northeastern (NE) Adriatic, in particular, the Istrian peninsula, represents the most convective area in Croatia especially in the warm part of the year (Mikuš et al., 2012). In at least 51% of daytime Cb occurrences, sea breezes (SB) develops along the coast. (e.g. Babić et al., 2012; Poljak et al., 2014). The SB characteristics highly depend on location and local topographic characteristics.The influence of the size and temperature of the water surface and topography on the SB-Cb interplay at particular Adriatic area is still insufficiently known.

Model WRF-ARW, V3initial and boundary conditions (every 6 h)ECMWF analysis;vegetation and land-use data: USGS 24;3 domains (dx=13.5 km, 4.5 km, 1.5 km) & 2-way nesting on a Lambert conformal projection;top of the atmosphere = 50 hPa & 80 sigma levels;

Several physical options for all domains:Constant:

RRTM for the longwave radiation; Dudhia scheme for the shortwave radiation; a five-layer thermal diffusion scheme for the soil temp.;the Betts-Miller-Janjić cumulus parameterization in 2 outer domain

Varying:PBL schemes: YSU, MYJ and BLMicrophysics schemes: Kessler, Lin, WSM6

Sensitivity numerical tests:(i) a varying SST field provided by the MSG SEVIRI geostationary satellite data (at 5 km) every hour (SST L3C hourly data, Fig. 1);(ii) a modified topography.

Results -> Sensitivity testsImpact of the SST variability (9/July/2006 at 15 CET)

ECMWF SST satellite SST

Fig. 6: Distribution of 10-m wind, (wind vectors and speed in m/s)in (a,b), sensible heat flux, W/m2 (c,d) and 10-m wind with themaximum simulated radar reflectivity factor, dBZ (e,f). Morerealistic SST distribution caused a larger sea-land temperaturedifference, stronger SB with larger vertical speeds and strongerdeep convection.

SummaryThe application of the three methods in the evaluation of the model and the determination of a model setup did not give a specific conclusion. However a choice of MYJ and Lin for PBL and microphysics options gave satisfactory result in the most cases.Sensistivity tests have shown the impact of the SB-Cb interaction. It primarily takes place in the boundary layer due to SB modification and thus affects the convergence zone and the position and the lifespan of convective cells.

DataThree chosen cases from Poljak et al. (2004):

(i) 8/June/2003(ii) 9/July/2006(iii) 8/August/2006

Radiosounding (Udine & SPC) and near surface data (Fig. 1).

Fig. 2: Modifying of the topography was done by a simple cosine weight function which is zero at the boundary and one in the center, defined over a 100x100 point square area.

Results -> Evaluation of the modelIn-situ statistical evaluation: Taylor diagram for Pula-Airport

Fig. 3: Taylor diagram for temperature (°C) and mixing ratio (kg/kg) for threeselected cases. The acceptable mesoscale model skill could be done if Stdev_WRF~ Stdev_M and RMSD < Stdev_M (Pielke, 2002). This is mostly not valid forKessler microphysics option.

Fig. 4: Experiments biases for all experiments for speed (m/s) in Pula Airport.Biases computed as observed minus each experiment forecast. Regardless themodel setup, the WRF model always underestimated wind speed.

In-situ statistical evaluation: spectral analysis¸

Fig. 5: Observed versus modelled time series (top) and power spectra (bottom)for Portorož-Airport site for 8/June/2003; (a,f) the horizontal zonal wind (m/s),(b,g) zonal wind component (m/s), (c,h) merdional wind component (m/s), (d,i)temperature (°C), (e,j) mixing ratio (kg/kg). The observed time series andspectrum is presented by black line.

The image moment analysis, IMA (e.g. Sović et al., 2013)

8/Jun/2003

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Experiments biases {obs-fcst} for speed (m/s)PBL 8/Jun/2004 9/Jul/2007 8/Aug/2006YSU -0.87 -0.99 -0.85 -0.82 -1.12 -0.72 -0.90 -0.93 -1.08 -1.09 -1.06 -1.34MYJ -0.86 -1.05 -1.09 -1.01 -0.97 -0.38 -1.32 -1.10 -1.11 -0.83 -0.77 -1.18BL -0.80 -0.88 -0.81 -0.85 -1.09 -0.83 -0.92 -1.08 -1.13 -0.97 -0.99 -1.00

O Kess Lin WSM6 O Kess Lin WSM6 O Kess Lin WSM6Microphysics

<-0.8 -0.8->-0.9 -0.9->-1.0 -1.0->-1.1 -1.0->-1.1 >-1.2

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Fig. 5: The IMA method is used forthe analysis of convection comparingthe maximum simulated radarreflectivity factor (dBZ) with observedones (source: ARSO-www.meteo.si).The model is more successful when theEuclidean distance (ED) is smaller,which means that the model is closerto reality.

Impact of the modified topography (MT): 9 July 2006

Fig. 7: Diurnal evolution of the modeled 10-m wind (in m/s)associated with the maximum simulated equivalent radarreflectivity factor (dBZ). The field comparison shows: (i) deeperintrusion of the flows with eastern directions over the peninsula;(ii) later development, smaller humidity advection and slowerinland penetration of the western SB (eliminated slope winds); (iii)the mountain ridge control the onset and accelerated theconvection.

AcknowledgementsThis work has been supported by theCATURBO (HRZZ, No. 09/151) project.