Coastal front formation at the Llobregat delta.

Preliminary study

David Pino1,2 & Jordi Mazón1

1Applied Physics Department (UPC)2Institut d’Estudis Espacials de Catalunya (IEEC/CSIC)

Red MM5, Aveiro, 26-27 Abril 2007

1. Rain ranges anomaly. Climatology2. Coastal front hypothesis3. 2-3 September 2000 episode4. MM5 simulation setup5. Results6. Conclusions

Overline

Work done to obtain the Diploma Thesis of a 3 year technical studies (academic use of the MM5 model)

C

GV

F

Rain ranges anomaly

70 km

70 kmWest Mediterranean coastLlobregat deltaOrography influence

Mediterraneansea

300400500600700800900

300 400 500 600 700 800 900 1000

Precipitació Gavà (mm)

Pre

cip

itació

Pantà

de F

oix

(mm

)

300400500600700800900

1000

300 400 500 600 700 800 900 1000

Precipitacó Gavà (mm)

Pre

cip

itació

Vila

franca

(mm

)

300400500600700800900

300 400 500 600 700 800 900

Precipitació Vilafranca(mm)P

recip

itació

Pantà

de F

oix

(mm

)

300400500600700800900

1000

300 400 500 600 700 800 900 1000

Precipitació Gavà (mm)

Pre

cip

itació

Cubelle

s (

mm

)

G

F

C

V

F

V

Annual precipitation (1978-2005)

G

G

Larger precipitation at the delta station (G)Similar precipitation in the other stationsDriving mechanism?

0

100

200

300

400

0 50 100 150 200 250 300 350

P hivern Vilafranca(mm)

0

100

200

300

400

0 50 100 150 200 250 300 350

P hivern Gavà (mm)

0

100

200

300

400

0 50 100 150 200 250 300 350

P hivern Gavà (mm)

C

F

V

V

Winter (1978-2005)

G G

Larger precipitation at the delta station (G)Similar precipitation in the other stationsDriving mechanism?

050

100150200250300350

0 50 100 150 200 250 300 350

P primavera vilafranca (mm)

050

100150200250300350

0 50 100 150 200 250 300 350

P primavera vilafranca (mm)

050

100150200250300350

0 50 100 150 200 250 300 350

P primavera Gavà (mm)

050

100150200250300350

0 50 100 150 200 250 300 350

P primavera Gavà (mm)

V

V

C

F

C

V

Spring (1978-2005)

G

G

Associated to frontal systems related to low pressure centers located north of the area.Large precipitation at the northern station (V)

050

100150200250300350

0 50 100 150 200 250 300 350

P estiu Gava (mm)

0

100

200

300

400

0 100 200 300 400

P estiu Gava (mm)

050

100150200250300350

0 50 100 150 200 250 300 350

P. estiu Vilafranca (mm)

050

100150

200250

0 50 100 150 200 250 300

P estiu Gava (mm) V

C

V

F

F

Summer (1978-2005)

G G

G

Similar precipitation in all the stationsMainly produced by thunderstorms

0

100

200

300

400

500

0 100 200 300 400 500

P tardor Gavà (mm)

P ta

rdor

Pan

tà d

e F

oix

(mm

)

0

100

200

300

400

500

0 100 200 300 400 500

P tardor Gavà (mm)

P ta

rdor

Vila

fran

ca (

mm

)

0100200300400500600

0 100 200 300 400 500 600

P tardor Gavà (mm)

P ta

rdor

Cub

elle

s (m

m)

0100200300400500600

0 100 200 300 400 500 600

P tardor Vilafranca (mm)P

tard

or C

ubel

les

(mm

)V

F

C

V

C

Fall (1978-2005)

G

G

G

Large precipitation at the delta station (G)Similar precipitation in the other stationsProduced only by easterly warm air intrusion?

01020304050607080

Mesos

Pre

cipi

taci

ó (m

m)

mensual

nocturna (00 h -09 h)

• Large spatial variability. Concentrated near the delta.• During late summer/fall. • Nighttime precipitation.

Summary

C

GV

F

Annual Spring Summer

Fall Winter

Cubelles 503 106 97 195 105

Foix (pantà)

560 128 109 213 110

Gavà 645 140 108 261 136

Vilafranca

581 182 98 197 104

• Nocturnal land breeze:•Sea surface temperature around 26 ºC• Long nights, important land cooling (land breeze)

• Drainage winds along the Llobregat delta and small torrents of the surrounding hills.• Enlarging of the offshore cool flux at the cost.• The front is limited south and north by two surrounding small mountains.

Coastal fronts hypothesis

September 2-3 2000 episode

Low pressure at center EuropeLight surface winds850 hPa light westerly winds

Local observations: Gava-Viladecans

E

N

W

2 3

Change in the wind direction during the night.Winds coming mainly from the north-east

Radar images (from 0 to 6UTC every 10 min)

Domain Resolution (km) Number of points

1 27 40x40

2 9 37x34

3 3 52x52

4 1 70x70

Viladecans (B)

Simulation characteristics

• Moisture scheme: Simple ice

• Convection scheme: Kuo, Grell, none, none.

• PBL: MRF• Atmospheric radiation:

cloud27 vertical levels

6. Results (Domain 3)Air convergence at the Llobregat delta 0-6UTC (03/09)

Wind pattern and accumulated precipitation 0-6UTC (03/09)

6. Results (Domain 4)

Air convergence at the Llobregat delta 0-6UTC (03/09)

Wind pattern and accumulated precipitation 0-6UTC (03/09)

Vertical cross sections

Wind field at 06 UTC on 3rd September.

V component

MM5 vs observations (Viladecans weather station)

Temperature and winds are well reproduced.Rainfall rates are clearly underestimated.

Conclusions

• Climatology shows that precipitation rates have a great spatial variation.

• Most of precipitation episodes of the delta area occur during fall and nighttime.

• This fact can be associated to by the development of a coastal front that is restricted to the delta area. Fundamental mechanisms are related to land breeze, drainage winds and high sea surface temperature.

• An episode produced by this type of front has been studied using observations and MM5 simulations.

• MM5 simulation reproduce precipitation rates but not the spatial distribution.

• The MM5 simulation is able to reproduce the drainage winds along the valley and the coastal front.

Additional episodes will be studied.