Wind driven winter currents in Lake Kinneret, Israel

Transcription

1 Wind driven winter currents in Lake Kinneret, Israel Elad Shilo *, Yossi Ashkenazy ** Alon Rimmer *** Shmuel Assouline **** and Yitzhaq Mahrer * * Department of Soil and Water Sciences, Hebrew University of Jerusalem, Rehovot, Israel ** Department of Solar Energy & Environmental Physics, Ben Gurion University, Sede Boqer campus, Israel *** Yigal Alon Kinneret Limnological Laboratory, Oceanographic and Limnological Research, Migdal, Israel **** Dept. of Environmental Physics and Irrigation, Institute of Soils, Water and Environment Sciences A.R.O - Volcani Center, Bet Dagan, Israel.

2 Objective Study of wind driven circulation in a medium size barotropic lake.

3 Lake Kinneret Background Stratification period- during summer (April to December) Complete mixing period during winter (January-March) supply about 25% of the national water consumption Motivation: Most of the studies focused on the lake hydrodynamics during the stratification period. There is a lack of knowledge about the dynamic response of the well mixed (barotropic) lake.

4 Important processes during winter: (a) Algal bloom (Peridinium) which is an important process of the LK food chain develops during winter. (b) Events of strong inflow of the Jordan River (the main stream interacting with the lake) which result in a transport of large amounts of solutes and suspended particles into the lake

5 Overview of Lake Kinneret and its surrounding Lake Kinneret (-209 m) Topography: -209 up to 800 m.

6 Jordan River 32 o 54 Beit Tzeida Lake Kinneret 32 o 52 Tabgha 32 o 50 St. A 32 o o N o km 32 o 42 Zemach 35 o o o o o 31 z x Lake Kinneret bathymetric map. Altitude in meters below MSL

7 Tools Meteorological model RAMS Hydrodynamic model ROMS (Regional Ocean Model System) Meteorological and current observations

8 Characteristic winter wind regime: Wind storms (duration of h), accompanied by several days with calm winds. Storms occur due to the presence of two main synoptic weather systems: (a) The presence of a high pressure system centered above Turkey, usually resulting in easterly strong wind over the lake. (b) The passage of a low pressure system (Cyprus low) toward the east coast of the Mediterranean Sea.

9 (a) High pressure system over Turkey, 1 January, 2001.

10 Easterly wind storm above Lake Kinneret m sec ZE BZ Lake Kinneret Tabgha Jordan River Beit Tzeida 32 o o 52 Direction (deg) 0 1/1/2001 2/1/2001 3/1/2001 4/1/ Time 0 1/1/2001 2/1/2001 3/1/2001 4/1/2001 Time Mean hourly values of wind magnitude (top) and direction (bottom) at Zemah ( ) and Beit-Tzeida ( ) during 1-3, January N 0 3 km 35 o o o 35 St. A Zemach 35 o o o o o o o 42

11 Experiment I: ROMS was forced with a uniform easterly wind field, 10 m sec -1 magnitude for the first 12 h. Afterward, the wind was switched-off. 10 m sec h Time

12 Simulated depth-averaged currents calculated by ROMS 12 h 36 h 60 h 84 h Forced period Propagation of topographic wave Topographic gyre pattern topographic Rossby wave

13 Forced period: formation of two circulation cells topographic gyre pattern Cyclonic propagation of this pattern as a free wave - topographic wave (second class motions) Due to bottom friction, currents magnitude decay. Wave completes half cycle in 48 h (e.g. flow reversal at the lake center).

14 Experiment II: RAMS simulation for 1-January, Basic configuration: 3 nested grids Cell size: 16, 4, 1 km respectively Dry (no precipitation) Mahrer and Pielke radiation scheme

15 RAMS simulated wind fields, 1 January, h 12 h 16 h 24 h Wind fields above the lake surface at selected hours.

16 8 2-2 R 2 = R 2 = 0.33 m sec (a) 0:00 6:00 12:00 18:00 0:00 6:00 Beit-Zeida m sec :00 6:00 12:00 18:00 0:00 6:00 Hour of day Hour of day m sec R 2 = 0.46 Zemah m sec R 2 = :00 6:00 12:00 18:00 0:00 6:00 Hour of day -8 0:00 6:00 12:00 18:00 0:00 6:00 Hour of day u v Simulated and observed wind components for 30 h begin at 0200 LST, 1 January, 200.

17 Simulated depth-averaged hourly currents calculated by ROMS driven by RAMS wind fields. 12 h 36 h 60 h 84 h Location of the current meter (7.5 m depth)

18 Main features: Formation of a double-gyre pattern. Enhancement of the anti-cyclonic gyre. Free propagation of the double gyre fits the characteristics of the lowest mode topographic wave. Wave completes ¼ cycle in 48 h.

19 5 2.5 cm sec :00 8:00 16:00 0:00 8:00 16:00 0:00 Hour of day Mean hourly values of observed ( ) and simulated (----) east-west depth average current component near the lake shore at Fullya site, for 1-2, January cm sec :00 8:00 16:00 0:00 8:00 16:00 0:00 Hour of day Mean hourly values of observed ( ) and simulated (----) north-south depth average current component near the lake shore at Fullya site, for 1-2, January 2001.

20 Second case-south west wind storm 22-26, January, 2004: Low pressure system, 23 January, 2004.

21 Second case-south west wind storm January, 2004: m sec A ZE Lake Kinneret Tabgha Jordan River Beit Tzeida 32 o o /1/ /1/ /1/ /1/ /1/ /1/2004 St. A 32 o 50 Direction (deg) Time /1/ /1/ /1/ /1/ /1/ /1/2004 Time N 0 3 km 35 o o o Zemach 35 o o o o o o 42 Mean hourly values of wind magnitude (top) and direction (bottom) at Zemah ( ) and St. A ( ) during 22-26, January 2004.

22 Experiment III and IV: III. Wind fields from station A (at the lake center) IV. Wind fields simulated by RAMS. Duration of simulation 120 h.

23 RAMS configuration for 22 January 2004 Weather conditions: cloudy sky and precipitation. 4 nested grids. cell size: 27, 9, 3, 1 km respectively. microphysics (level 3). Harrington scheme for radiation. Kain-Frits scheme for convection.

24 RAMS simulated wind fields, 22 January, h 30 h 36 h 54 h Wind fields above the lake surface

25 32 h 48 h 72 h 96 h 120 h Depth average circulation using observed wind from St. A for January Red circle indicates location of the current meter. 32 h 48 h 72 h 96 h 120 h Depth average circulation using RAMS simulated wind fields for January 2004.

26 Current components near the lake center (deepest area): Measured Vs. simulated RAMS 4 cm sec cm sec A :00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0: :00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 Time Time East-West current component North-South current component Mean hourly values of observed and simulated depth average current near the lake center during 22-26, January Observed ( ), simulated using spatially uniform wind fields ( _ _ ) and simulated using wind fields derive from RAMS ( _ _ ).

27 Cyclonic rotation of the current vector: observed Vs. simulated using RAMS simulated wind fields 360 Direction (deg) RAMS observed 0 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 Hour of day Mean hourly values of observed ( ) and simulated ( _ _ ) depth average current direction near the lake center during 22-26, January 2004.

28 Results Using RAMS generated winds or uniform wind results in the formation of two circulation cells. Uniform wind forcing: the wave completes almost ¼ cycle in 48 h. RAMS wind forcing: the wave completes half cycle in 48 h. Good agreement between measured and simulated currents were only obtained when using RAMS simulated wind fields.

29 Summary and Conclusions: All the experiments demonstrated the formation of a double-gyre pattern. All the experiments show the propagation of this pattern as a free topographic wave (after the wind ceases).

30 The impact of wind field properties on the wave propagation: Impact of an easterly wind field Uniform wind wave completes half a cycle in 48 h. RAMS wind - wave completes 1/4 cycle in 48 h (negative wind curl). South-West wind: Spatially uniform (temporal) wind from St. A - wave completes 1/4 cycle in 48 h. RAMS wind - wave completes half cycle in 48 h (positive wind curl).

31 Daily average vertical vorticity of the wind field (sec -1 ) January 2001: -1.87*10-6 (clockwise rotation, reduces the frequency of the topographic wave) January 2004: +3.7*10-6 (counter clockwise rotation, enhance the frequency of the topographic wave)

32 In order to accurately describe the flow pattern in a lake located in a complex terrain region it is necessary to use at least a one-way coupled atmosphere-lake model. Further study is required to better describe the surface wind field in complex terrain areas.

33 Thank s for listening