Ann. Geophysicae 14, 1207Ð1220 (1996) ( EGS Ð Springer-Verlag 1996 Summertime winds and direct cyclonic circulation: observations from Lake Geneva U. Lemmin, N. D’Adamo1 Laboratoire de recherches hydrauliques, Ecole Polytechnique Fe«de«rale de Lausanne, CH-1015 Lausanne, Switzerland Received: 11 March 1996/Revised 7 June 1996/Accepted: 14 June 1996 Abstract. Records of wind, air temperature and air pres- been the subject of many limnological studies since, both sure from nine stations, situated along the shoreline of in the field (Mortimer, 1974; Fischer et al., 1979; Imberger Lake Geneva, Switzerland, were analyzed for the summer and Hamblin, 1982) and in the laboratory (Monismith, period May to September. At all stations the consistent 1983). Accurate knowledge of the strength and the spatial appearance of significant spectral peaks and changes and temporal distribution of winds is needed as input into in wind direction at the diurnal frequency indicates the numerical lake models. The assumption of a constant importance of lake-land breezes. It is shown that the wind stress is only rarely justified (Simons, 1980). As surrounding topography has a strong modifying e¤ect Mortimer (1979, p. 214) pointed out, ‘‘Wind stress and its (temporal and spatial) on the lake-land breeze. Superim- horizontal distribution over the whole water surface is the posed on this cyclic wind pattern, short episodes of strong critical variable and is usually the least well defined.’’ winds with long fetch over parts of Lake Geneva are It is recognized that the kinetic energy introduced at regularly observed. Both of these winds exert a spatially the water surface by way of wind shear stress is of prime variable wind stress over the lake surface on the same time importance in mixing the water vertically, accelerating the scale. Typical examples of the expected lake’s response are water horizontally in the form of barotropic flows and in presented, among them the seasonally persistent gyre in setting up a spatially variable horizontal density field, the central part of the lake. Evidence is provided that this hence inducing baroclinic adjustments of the density dominant circulation is part of a direct cyclonic circula- structure as the stratification relaxes after a forcing event. tion, generated by the curl of the diurnal wind field. It is The particular response of the lake depends on the charac- concluded that the mean circulation is caused by these teristics of the forcing event. winds and a¤ected by the topography of the surrounding This work presents the results of an analysis of a series land. of summer time winds from several stations situated around the shorelines of the Lake Geneva basin. Lake current data are scrutinized for evidence of the expected circulation patterns. We shall show that it is essential to 1 Introduction represent the wind field on the scale of the major sur- rounding topography if the observed mean lake circula- In this study we analyze and interpret wind data recorded tion is to be explained. around Lake Geneva and baroclinic motion in the lake obtained from moored current meter records. In order to 2 Background determine the type of circulation that will be established in the lake in response to a particular wind forcing a know- Wind field ledge of the characteristics of the wind field is important. This was already obvious to Forel (1895), who coined the The summer wind field over a lake is, in general, domi- term ‘limnology’, in his fundamental investigations of the nated by event-structured contributions on two rather movement of water and its relationship to water quality in di¤erent scales. The first type of wind is related to the stratified lakes using Lake Geneva as ‘field laboratory’. passage of synoptic scale pressure cells. The time and The influence of winds on the hydrodynamics of lakes has length scales of these wind events are typically much larger than the lake basin and in a first order approxima- 1 Present address: Environmental Protection Agency, Perth 6009, tion it can be assumed that these winds are homogeneous Australia over the lake. Topographic constraints may have some Correspondence to: U. Lemmin e¤ects. 1208 U. Lemmin, N. D’Adamo: Summertime winds and direct cyclonic circulation: observations from Lake Geneva The second type of wind, the land-lake breeze, is a ther- the metalimnion. Amplitudes of the vertical displacements mally induced wind. It is generated on scales which are of internal density surfaces are small. This type of water comparable to the lake basin itself. Since these winds act movement was termed ‘‘direct’’ circulation by Strub and simultaneously around the whole perimeter of the basin Powell (1986). and perpendicular to the shoreline, the wind field will not The important di¤erence between residual and direct be homogeneous over the lake. Due to the way it is circulation resides in the transfer of energy from the wind generated locally this wind field is also more sensitive to into the circulation: in residual circulation the wind en- local topographic e¤ects which may cause a curl. ergy is first transformed into available potential energy in The first type of wind field often produces much higher the slanted internal density surfaces and then transformed wind velocities than those typical for the second type of further into the kinetic energy associated with the gyres. wind field. As a consequence it can be expected that the In direct circulation it is more e¦ciently transferred dir- lake will respond di¤erently to the two types of forcing. In ectly to the rotational currents (Strub and Powell, 1986). order to characterize a particular wind field the typical Verification of the Lake Tahoe simulation result by field spatial and temporal scales and strengths have to be observations is not presently available. Here we conclude established in the data analysis. This will be done here by that the lake response to inhomogeneous and variable statistical analysis, particularly spectral analysis. winds often observed over lakes requires further study. We will therefore first analyze the wind field for the presence of a curl. The current data will then be tested for Lake circulation aspects of the direct circulation. The dynamic response of a stratified lake depends upon the intensity and the fetch length of the wind, the ambient 3 Lake Geneva stratification and the geometry of the lake. Here we will limit ourselves to lakes with length scales between 10 and The site 100 km where the internal Rossby radius of deformation is less than the width of the basin (a typical value for Lake Lake Geneva is situated in the southwest of Switzerland Geneva is O(5) km). Consequently the Coriolis force will (Fig. 1). It is curved in shape, and it is composed of two a¤ect the lake’s response pattern to the wind forcing. main basins: a relatively small section in the west called For homogeneous, high wind speeds related to the the Petit Lac (‘small lake’; maximum depth 70 m) and passage of large-scale synoptic pressure cells upwelling, a larger section in the east called the Grand Lac (‘big lake’; downwelling and rapid vertical entrainment are predicted. maximum depth 300 m). The lake has a maximum length The baroclinic motion which results from the pressure and width of 72.3 and 13.8 km, respectively. Important to force caused by the internal surface slanting due to upwell- the wind-related hydrodynamics of the lake are the topo- ing and downwelling, a¤ected by the Coriolis force, will graphical features which surround the shores. Around the create cyclonic circulation. Interface tilting leads to inter- eastern and southern parts of the lake are the Alps, with nal Kelvin waves (Mortimer, 1953), traveling cyclonically elevations of more than 1000 m above the lake level and around the basin. The instantaneous currents associated with individual peaks of that height within 2 km from with the passage of these waves are at times much larger shore. The eastern half of the lake is sheltered by these than the long term mean values averaged over several high mountains from most of the winds that originate wave periods. As a result Kelvin waves induce only a weak outside of the lake confines. To the northwest are the Jura mean circulation. This type of lake response has been mountains. The central and western part of the lake make termed ‘‘residual’’ circulation (Cheng and Casulli, 1982). part of a valley some 50 km wide which separates these Numerical simulations (Simons, 1975; Huang and Sloss, mountain formations to the north and east of the lake. 1981) as well as observations (Pickett and Richards, 1975) The curved shape of the lake and the surrounding topo- from the Great Lakes, North America, show that a con- graphy causes winds over the lake to exert a spatially stant wind stress causes a long-lasting gyral circulation. If variable stress. Local fishermen characterize a total of 22 the wind is strong a two-gyre pattern is initially forced di¤erent winds by specific names (LeFranc, 1923). which relaxes to a one-gyre pattern over most of the lake when the wind decreases. For the case of a wind field with a curl over a stratified Past studies lake, most likely to occur with the lake-land breeze wind regime, Strub and Powell (1986) have shown through Past studies of the wind field at Cointrin (hereinafter CO; numerical simulation for Lake Tahoe, USA, that the cur- Graf and Prost, 1979) have shown that there exists at least rent structure generated in a stratified lake di¤ers greatly two separate identifiable directions (indicated in Fig. 1) of from that generated by a constant wind stress.