Nocturnal winds in joining shallow valleys of different sizes - Observationaland numerical studies based on the field experiment KASCADE

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Nocturnal winds in joining shallow valleys of different sizes - Observationaland numerical studies based on the

field experiment KASCADE

G. Duine, T. Hedde, P. Roubin, P. Durand

To cite this version:

G. Duine, T. Hedde, P. Roubin, P. Durand. Nocturnal winds in joining shallow valleys of different

sizes - Observationaland numerical studies based on the field experiment KASCADE. ICAM 2015 -

33rd International Conference on Alpine Meteorology, Aug 2015, Innsbruck, Austria. �cea-02509660�

Nocturnal winds in joining shallow valleys of different sizes - Observational and numerical studies based on the field experiment KASCADE

Gert-Jan Duine ^1,2 , T. Hedde ² , P. Roubin ² and P. Durand ¹

1 Laboratoire d’A´erologie, University of Toulouse, CNRS, France ^† , ² Laboratoire de Mod´elisation des Transferts dans l’Environnement, CEA, France

e-mail: gert-jan.duine@aero.obs-mip.fr

Motivation

Cadarache is the largest research site of Commissariat `a l’Energie Atomiques et aux Energies Alternatives (CEA) and is situated in the prealps of southeastern France. This centre comprises several facilities whose operation requires impact assessment considering the emission of pollutants. The region is susceptible to stable stratification periods which, in combination with complex orography, affects the conditions for dispersion of pollutants. The understanding and characterization of the local down-valley flows is thus a major issue for local and regional dispersion studies at Cadarache. KASCADE (KAtabatic winds and Stability over CAdarache for Dispersion of Effluents) was focused on valley winds in two cross-oriented valleys (see Fig. 1) and has been conducted in the winter of 2013. The experiment revealed the existence and dominance of two down-valley winds [1]. The study presented here is based on both modeling and observations.

Study Area

Two connected valleys of different size are un- der investigation (Fig. 1 and Table 1), the Du- rance Valley (DV) and its tributary Cadarache Valley (CV). They both facilitate the onset of nocturnal down-valley winds during stable stratification periods.

Table 1. Valley scales.

Cadarache ValleyDurance Valley

Length [km] 6 60

Slope angle [ ^◦ ] 1 0.2

Depth [m] 100 200

Width [km] 1 - 2 5 - 8

Cadarache Valley (CV) SODAR

M30 GBA

Plateau de Valensole

Le Maladroit (394 m) Le Luberon (1129 m)

10km

France

30°

125°

Sainte Victoire (1011 m)

DV

34 km:

Sisteron

1 km

Durance Valley (DV)

Lower DV

Middle DV

La Vautubière (635 m) Clue de Mirabeau

CDV wind DDV wind

Figure 1: Study area with a focus on the Durance Valley (left) and a zoom on the Cadarache Valley (right), the measurements sites for KASCADE are given on the right figure. The Durance down-valley (DDV) and Cadarache down-valley (CDV) wind directions are indicated by the arrows.

KASCADE campaign

The KASCADE campaign has been conducted from mid-December 2012 to mid-March 2013 to reveal the local wind field during clear sky and weak synoptic forcing conditions. The ex- periment consisted out of continous measure- ments by a flux tower (M30, see Fig. 1), a wind profiler (SODAR) and standard atmo- spheric observations (GBA), and was comple- mented by 23 Intensive Observations Periods (IOPs) by means of tethered balloon sound- ings and regular radiosonde releases. All in- struments have been calibrated afterwards.

General characteristics

Stability forms easily in the region, and along the nights a stable boundary layer (SBL) regularly grows up to 300 m agl (Fig. 2). The two valleys of different sizes facilitate down-valley winds during weak synoptic forcing leading to valley flows of different scales: the Cadarache down-valley (CDV) wind and Durance down-valley (DDV) wind. Dur- ing winter of 2013, they were very dominant winds.

2680 270 272 274 276 278 280

50 100 150 200 250 300

Temperature [K]

Height [m agl]

IOP 15

19 Feb 2013 05:15 UTC

SBLCV neutral layer

SBL_DV SBL depth

0 1 2 3 4 5 6 7 8

0 50 100 150 200 250 300

Wind speed [m s⁻¹]

Height [m agl]

N DDV E CDV S W N

Wind direction

IOP 15

19 Feb 2013 05:15 UTC

DDV wind

CDV wind

Figure 2: Tethersonde profile for temperature (left) and wind speed and direction (right) during IOP 15 at M30 site (see Fig. 1).

The CDV wind has a southeasterly direction and can grow up to valley depth (Fig. 2). Its speed is typically 1 - 4 m s ⁻¹ , and is strongly time dependent (Fig. 3).

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Time after sunset [h]

Wind direction

M30: 30m

−3 0 3 6 9 12 15

N E CDV S W N

0 0.2 0.4 0.6 0.8 1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

5%

10%

15%

W E

S N

<1 1 − 2 2 − 4 4 − 8 8 − 12 M30 : 30 m

up−valley wind

Cadarache down−valley wind

Figure 3: Sunset referenced time series of wind direction occurrence at M30 at 30 m agl (left) and wind rose (color scale in m s ⁻¹ ) at the same location (right). The Cadarache down-valley (CDV)

direction is southeasterly. Data is from 13th December 2012 to 19th March 2013.

The DDV wind has a north-northeasterly orientation (30 ^◦ ) and acts on larger scales. The valley is deeper, leading to thicker valley winds. The jet has typically a maximum speed around 200 m agl. The DDV wind is the strongest around sunrise just before convectively driven wind starts, and attains 4 - 8 m s ⁻¹ (Fig. 4). It takes around 4 hours after sunrise for the DDV wind to cease.

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Time after sunset [h]

Wind direction

GBA: 110m

−3 0 3 6 9 12 15

N DDV E S W N

0 0.2 0.4 0.6 0.8 1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

5%

10%

15%

W E

S N

<1 1 − 2 2 − 4 4 − 8

>=8 SODAR :175 m

up−valley wind

Durance

down−valley wind

Mistral

Figure 4: Sunset referenced time series of wind direction at GBA at 110 m agl (left) and wind rose (color scale in m s ⁻¹ ) at SODAR location at 175 m agl (right). The Durance down-valley (DDV)

direction is north-northeasterly. Data is from 13th December 2012 to 19th March 2013.

Acknowledgements

This work has been funded by the CEA in the form of a PhD-grant and the support for the experimental campaign.

LPCA (Dunkerque, France) is acknowledged for the Sodar observations.

Durance down-valley wind

Observations

The DDV wind onset at the observation site is highly vari- able (Fig. 5) due to the multiple origin (flow channeling and thermally driven flow) and the large DV fetch.

2 4 6 8 10 12 14

0 2 4 6 8

Number of occurrences

Time after sunset [h]

DDV wind onset at 225 m agl

Figure 5: Timing of the DDV wind onset for the 3-month period of SODAR observations.

DDV wind was observed up to 400 m agl (Fig. 6). It gets to a mature state from 6 to 9 hours after sunset.

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Height agl [m]

S W N DDV E S

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S W N DDV E S

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Height agl [m]

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0 100 200 300 400

Observations

Figure 6: Occurrences of wind direction observations from the SODAR for 23 IOPs. The top (bottom) four figures are sunset (sunrise) related. On the x-axis the downslope direction for Durance Valley

(DDV) is indicated.

Simulations

The WRF model has been used on a 1 km horizontal reso- lution to simulate the 23 IOPs. The Corine Land Cover was incorporated, and SRTM orography was used. The model was optimized for one IOP [2], and evaluated against ob- servations for all 23 IOPs.

In the optimized configuration, WRF is capable to simu- late the DDV wind to a good extent, critical for dispersion- related studies. The 23 IOPs simulated (Fig. 7), correspond well to observations. However, the DDV wind sets in and ceases too early.

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5 3 to 0 hours before sunset

Height agl [m]

S W N DDV E S

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S W N DDV E S

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Simulations

Figure 7: Same as Fig. 6 but for WRF simulations output.

Nowcast of the Cadarache down-valley wind

While permanent observations of wind are lacking in the area of the CDV wind, its detection is crucial for crisis man- agement. GBA however provides, among others, tempera- ture measurements at 2 and 110 m which can be used to nowcast this wind through a simple method developed to define a threshold from GBA observations, based on a di- chotomous forecasting verification principle [3].

Wind observations in CV CDV wind No CDV wind Nowcast

from GBA

Satisfied a: hit b: false alarm Not

satisfied

c: missed classification

d: correct rejection

𝑃𝐶 = 𝑎 + 𝑑 𝑎 + 𝑏 + 𝑐 + 𝑑 𝑏𝑖𝑎𝑠 = 𝑎 + 𝑏

𝑎 + 𝑐

^{0 −4 −2 0 1.52 4 6 8 10}

0.2 0.4 0.6 0.8 1.0

∆T

110m−2m GBA [°C]

PC [−]

PC 10 m: 0.91

bias 10 m: 1.03

0 1 2

bias [−]

PC 10 m Bias 10 m

Figure 8: Criterion to optimize a threshold for ∆T _110m−2m at GBA to nowcast the CDV wind.

The threshold ∆T _110m−2m of 1.5 ^◦ C results in an efficient dis- tinction of the CDV wind and a non-CDV wind (see Fig. 9) with a proportion correct (P C ) of 0.91. However, it fails during weak wind situations around sunset and sunrise tran- sitions, and for high wind speeds during night-time.

CDV wind nowcast ∆ T>1.5

^°

C

10 m (inside valley) wind direction

110 m (above valley) wind direction

N DDV E S W N

N E CDV S W N

110 m (above valley) wind direction No CDV wind nowcast ∆ T<1.5

^°

C

sunrise transition

night transition

sunset transition

day transition

Hours [UTC]

N DDV E S W N

N E CDV S W N

0−1 1−2 2−3 3−4 4−5 5−6 6−7 7−8 8−9 9−10 10−11 11−12 12−13 13−14 14−15 15−16 16−17 17−18 18−19 19−20 20−21 21−22 22−23 23−24

Figure 9: CDV wind nowcast after threshold application: expected CDV wind (left) and no CDV wind expected (right). White background is a correct classification, gray background is a wrong one. Color

scale indicates hours in UTC.

Conclusions & perspectives

The CDV and DDV winds are both dominant winds. The DDV wind sets in irregularly after sunset, but can be sim- ulated to a good extent on 1 km resolution. Thermally driven flow and forced channeling are recognized as be- ing the dominant drivers for DDV wind. The CDV wind is primarily thermally driven. The CV is too small to be re- solved by operational forecasting models, but the wind can be nowcasted from remote measurements.

In a next step, impact of dispersion on local and re- gional scale will be investigated by means of the WRF- FLEXPART model in a follow-up PhD-project.

The KASCADE dataset will be freely available on http://kascade.sedoo.fr soon. Until then, interested scien- tists can contact Dr. Thierry Hedde (thierry.hedde@cea.fr).

References

[1] Gert-Jan Duine, Thierry Hedde, Pierre Roubin, Pierre Durand, Marie Lothon, Fa- bienne Lohou, Patrick Augustin, and Marc Fourmentin. Valley-driven flows in stable stratification - observations in a complex orography area during the kascade field experiment. submitted to Q. J. Roy. Meteor. Soc., 2015.

[2] Peter Kalverla. Evaluation of the weather research and forecasting model for con- trasting diurnal cycles in the durance valley complex terrain during the kascade field campaign. MSc Thesis Wageningen University, 2014.

[3] Daniel S Wilks. Statistical methods in the atmospheric sciences, volume 100.

Academic press, 2011.

† Address for correspondence: Laboratoire d’A´erologie, 14 Av. Edouard Belin, 31400 Toulouse, France