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[1] A unique combination of temporal and spatial measurements provides a description of an extraordinarily large upwelling event in Lake Tahoe, CA-NV. The 4 d event, which engulfed half of the lake's surface and had an amplitude of 500 m, was recorded with in situ and spaceborne instruments. The vertical mixing that ensued, was characterized by a large transfer of heat across the thermocline, resulting in the replacement of the distinct two-layer thermal structure by a diffuse, temperature gradient. Prior to the event, mixing energy due to the cooling flux at the surface was two orders of magnitude larger than the mixing energy associated with the wind. This dominance by cooling yielded the two-layer structure. During the event, wind energy was of similar magnitude to the cooling energy. The large bottom velocities that were produced at the end of the event were sufficient to re-suspend sediment into the water column.

Abstract. Convectively driven currents can arise in the littoral zones of lakes, as a result of either differential heating or differential cooling of the shallow water. The result of these flows is to produce a surface flow away from shore with a bottom return flow or a bottom flow away from shore with a surface return flow. Measurements taken in a shallow embayment of Clear Lake, California, USA, show the presence of both kinds of convectively driven flows under a large variety of summer conditions. The magnitude of these flows is sufficient to transport material a distance on the order of 0.5 km during such events. Through both advection and dispersion the net result of this process would be to reduce the accumulation of particles and particle-associated contaminants such as mercury in the littoral zone and to move them offshore where they are more prone to permanent burial or further transport.

Water current measurements based on the Doppler principle have become a standard approach in aquatic sciences. An acoustic Doppler current profiler (ADCP) estimates the water velocity in a series of depth-integrated bins by measuring the Doppler shift in frequency between a transmitted pulse of high-frequency sound and the echo that is reflected from scatterers in the water. If the measurements are performed on a moving boat, then a measure of the boat velocity is needed so the measured ADCP velocity can be corrected by subtracting the boat movement. In waters that are shallow enough for the instrument to locate the bottom, the ADCP's "bottom-tracking" capability provides a good estimate of the boat velocity for the correction. However, in deeper water, navigational information is needed to correct for the boat movement. Shipmounted ADCPs, combined with accurate navigation systems such as the Global Positioning System (GPS), have been used successfully in the past for recording velocity transects (Joyce et al. 1982). The Differential Global Positioning System (DGPS) has provided accurate positioning for many oceanic expeditions (Pierce et al. 1999;Trump and Marmorino 1997). Oceanographic research vessels typically move at 10 kt (5 m s -1 ), and commercial ships of opportunity that are used for oceanographic measurements move at even greater speeds (20 to 30 kt). GPS accuracy is sufficient for many of these cruises, particularly when velocities are averaged over hundreds or even thousands of meters.When an ADCP is used on a near-stationary platform to measure relatively slow currents, navigation errors can significantly degrade the current data. Even if the platform were completely stationary, the collected GPS data would yield a non-zero "velocity" of the platform. A 24-h dockside test (a motionless platform) of a DGPS system showed that 5-minaveraged position data, clustered about their mean values with a standard deviation of 7.7 m, had apparent eastward and northward velocity standard deviations of 1. 55 AbstractWhen boat-mounted acoustic Doppler current profiler (ADCP) velocity measurements are taken in deep water where bottom-tracking capability no longer exists, navigational data via the Differential Global Positioning System (DGPS) are commonly collected to correct for the boat's contribution to the recorded ADCP velocity. The boat's cruise speed and the desired averaging interval for the water velocity are often great enough for navigational errors to become insignificant contributors to the uncertainty of the resulting water velocity. When the boat or other measurement platform is nearly stationary, navigational errors become significant and lead to an incorrect water velocity when navigational corrections are applied. Furthermore, when the water current is on the same order of magnitude as the slowly moving boat, the boat movement may not be easily distinguished from the fluid velocity. Factors that affect the quality of DGPS navigational estimates include the number of available satellites and thei...

[1] The disruption of an arrested bottom Ekman layer on a lake boundary slope is initiated when buoyancy enters the balance between the Coriolis force and the pressure gradient. This process is shown to behave in a lake in a qualitatively similar manner to the ocean. A greater bottom slope and stronger stratification are found to dramatically shorten the shutdown timescale of the Ekman flux compared to typical values on the continental shelf and slope. Convergence of the flow at the density front, where the thermocline intersects the bottom, leads to a detrainment of bottom boundary layer water into the interior of the lake and thus facilitates the transport of mass and heat independent of turbulence generation in the boundary layer by internal wave interactions. Internal waves complicate the flow structure. The boundary water detrainment and gravitational instability due to Ekman layer dynamics and the critical reflection and vertical overturns associated with the internal waves combine in effective boundary mixing.