Department of Geophysics http www gfz hr Impact

Department of Geophysics http: //www. gfz. hr/ Impact of the large-scale wind and mesoscale shallow flows on the development of cumulonimbus clouds over Istria Karmen Babić, Marko Kvakić & Maja Telišman Prtenjak 1

Department of Geophysics http: //www. gfz. hr/ CONTENTS: 1. Introduction, aim & motivation 2. WRF model 3. Numerical simulations 4. Summary and conclusions 2

Introduction Department of Geophysics http: //www. gfz. hr/ • The role of the SB circulation, including SB front in triggering convection has long been recognized • Convection initiation often takes place when 2 or more features (fronts and /or rolls, thunderstorm outflows) collide or merge. Holland Mc. Bride (1989) • Locations of the very extensive Cb and SB research, e. g: Florida (e. g. Pielke, 1974; Yuter and Houze, 1995; ) Australia & Indonesian archipelago (MCTEX; e. g. Saito et al. , 2001) Japan, Kanto plain (e. g. Sano and Tsuboki, 2006) Spain (e. g. Azorín-Molina et al. , 2009) …… 3

Climatology of convection in Croatia Department of Geophysics http: //www. gfz. hr/ Alps Istria: an area with the highest frequency of thunder in Croatia Italy Lightning data from LINET network the analysis of some spatial and temporal characteristics of lightning in 4 years warm period (2006 -2009) The mean annual number of days with thunderstorms (1948 -1966); Penzar et al. (2001) Mikuš et al. (2012) Arbitrary choice Convective day = a day with more than 10 strokes of the total discharge data (CG+IC) detected per hour across limited area 402 convective days 4

Above northeastern Adriatic weather types Department of Geophysics http: //www. gfz. hr/ Ø the most common summer weather types during convective days: Ø non-gradient (NG) pressure conditions & low pressure pattern (C, T) Ø the peak in daytime convective activity during NG weather type in July Ø in 82% of overall days with convective activity 3 dominant large-scale wind regimes SW, NE, NW Mikuš et al. (2012) wind regimes 5

SB climatology over Istria (1997– 2006) Pazin ∆T → prime peak in August, → secondary max in June Department of Geophysics http: //www. gfz. hr/ due to dominance of daytime convection SLB frequency → max in August up to 65%of all summer days (June-September) In average during summer At least every second day with SB hodograph Pula-airport Ø Ø CW rotation of wind vectors mean SB speed about 3. 5 m/s mean LB speed about 2 m/s low steadiness of SB between 10 -13 CET 6

Department of Geophysics http: //www. gfz. hr/ QUESTION: Sea breeze + large-scale wind Convective activity? ? ? ? ? Cb development? ? ? ? 7

Department of Geophysics http: //www. gfz. hr/ WRF-ARW model Setup of the model: Ø two-way nesting Ø Lambert conformal projection Ø Ø Ø 3 domains : x = 13. 5 km, 4. 5 km, 1. 5 km 81 vertical levels (terrain- following) Ø Initial and boundary data from ECMWF Ø Schemes: § MYJ scheme for the PBL; § RRTM for the longwave radiation; § Dudhia scheme for the shortwave radiation; § Lin microphysics scheme; § Eta surface layer scheme; § five-layer thermal diffusion scheme for the soil temperature; § Betts-Miller-Janjic cumulus parameterization two outer domain 8

Three selected cases simulated by WRF In the finest model domain Department of Geophysics http: //www. gfz. hr/ Similarities Ø the Cb cloud over Istria Ø weather type: almost non-gradient pressure conditions Ø SB at the Pula-airport site (tip of the Istria peninsula). date The main dissimilarity Ø wind regimes Dominant large-scale wind Cb duration onset – end (CET) SB duration onset – end (CET) Max SB speed Case A 09 July 2006 NE 10: 50 – 16: 00 9 - 19 5. 1 Case B 08 June 2003 SW 12: 40 - 16: 30 9 - 20 3. 6 Case C 08 August 2006 NW 11: 10 – 16: 30 9 - 13 4. 1 For Case A sensitivity test: without microphysics 9

MAX ECHO (DBZ) at 13 CET Department of Geophysics http: //www. gfz. hr/ CASE A = large scale wind NE -11 CET – formation of convergence zone ( 15 km inland, 75 km long ) - 12 CET – cloudiness and precipitatation 5 mm – 15 mm over Istria Convective activity LINET (CG+IC) data betweenof Cb CET - 13 CET – formation 14 -15 (Pazin) - 14 – 16 CET – disipation of Cb (moved to south) along convergence zone 10

WRF 10 -m surface wind at 14 CET Department of Geophysics http: //www. gfz. hr/ 20 km statistical indices at 14 CET Wspeed (m/s) Wdirection (°) Temp (°C) MAE 1. 20 45. 45 4. 08 RMSE 1. 54 64. 31 3. 39 IOA 0. 66 0. 89 Measured surface wind field 0. 65 11

CASE A = large scale wind NE Department of Geophysics http: //www. gfz. hr/ 17 CET Results - large-scale NE wind enhanced the SB at the southeastern Istrian coast - prevented deeper penetration of the dominant western SB over the peninsula - convergence zone is not moved to east too much 17 CET - indication for the superposition between SB front and outflow below Cb along convegence zone - Sb weaker after the storm; - Cb act destrucive on the air-sea temp. diff. Comparison between control A run and sensitivity test 12

CASE B = large scale wind SW Measured surface wind field WRF 10 -m surface wind at 14 CET Department of Geophysics http: //www. gfz. hr/ 30 km 11 CET – formation of convergence zone ( 30 km inland, 75 km long ) 11 – 12 CET – cloudiness and precipitation 10 mm – 15 mm Convective activity 13

MAX ECHO (DBZ) at 13: 50 CET Department of Geophysics http: //www. gfz. hr/ 13 CET – formation of Cb ( north part of Istria ) 14 – 16 CET – disipation of Cb Ø deeper penetration of SB inland Ø convergence zone is moved to east CG lightning data at 13: 50 CET 14

CASE C = large scale wind NW Measured surface wind field WRF 10 -m surface wind at 14 CET Department of Geophysics http: //www. gfz. hr/ 30 km -10 CET – formation of convergence zone (30 km, 50 km long – highly curved in space) - 11 CET – penetration of SB deeper over land (in the central part of peninsula ) -12 – cloudiness and precipitation 5 – 25 mm convective activity 15

CASE C = large scale wind NW MAX ECHO (DBZ) at 13: 50 CET Department of Geophysics http: //www. gfz. hr/ Ø 13 CET – Cb moved southward of Istria, Rijeka and Cres (cloudiness and 45 mm precipitation) Ø large-scale NW wind is superimposed on the western SB producing larger inland penetration and amplifying the magnitude of the SB speed Ø 14 – 17 CET – dissipation of Cb LINET (CG+IC) data between 14 -15 CET 16

Summary -The large-scale winds (LSW) influence the SB development and evolution, therefore creating the zone of convergence -The interaction SB - LSW reinforces the convergence of the flow field in the boundary layer and consequently the intensity of SB fronts and its updrafts - The certain amount of cloudiness and precipitation has been developed in the zone of the convergence between 12 CET and 13 CET - The ZC, cloudiness and percipitation depended on the type of the LSW 17

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