No abstract

A scalable datalogging system for micrometeorological, fast-response, in situ, and remote sensing applications is presented. The system is based on a standard x86 MINI-ITX computer and the open-source operating system Linux. Real-time access for debugging and remote system control is implemented via a network interface. A 160-GB, 2.5-in. hard disk drive provides extended local storage. The recorded data can alternately be stored at a remote location using the Network File System (NFS) included in Linux. Accurate time stamping of collected data points is implemented using the open-source software Network Time Protocol (NTP) and a global positioning system (GPS) receiver. The operational capability of the system is demonstrated over a period of several weeks with data from seven ultrasonic anemometer-thermometers and a barometer.

The frequency spectrum of angle-of-arrival (AOA) fluctuations of optical waves propagating through atmospheric turbulence carries information of wind speed transverse to the propagation path. We present the retrievals of the transverse wind speed, upsilon b, from the AOA spectra measured with a Schmidt-Cassegrain telescope equipped with a CCD camera by estimating the "knee frequency," the intersection of two power laws of the AOA spectrum. The rms difference between 30 s estimates of upsilon b retrieved from the measured AOA spectra and 30s averages of the transverse horizontal wind speed measured with an ultrasonic anemometer was 11 cm s(-1) for a 1 h period, during which the transverse horizontal wind speed varied between 0 and 80 cm s(-1). Potential and limitations of angle-of-arrival anemometry are discussed.

Near-ground observations of an atmospheric temperature interface in the stable nocturnal boundary layer are reported. The thermal effect of the interface passage was a 5-K decrease in temperature during a 5-min period in which changes in wind speed and wind direction were also observed. The interface is of unknown origin and horizontal and vertical scale. A 32-Hz time series of temperature measured with a sonic anemometer-thermometer at 1.5 m above ground level revealed a sharp, nonintermittent decrease in temperature (ϳ1 K s Ϫ1 ) nested within a more gradual, intermittent decrease (ϳ1 K min Ϫ1 ). A first attempt to extricate this subtle feature with regard to the relevant physics of generalized density currents is presented. It is shown that a very similar feature has been observed in the literature, but not interpreted for its physical significance. The contribution of this paper is its unique interpretation of the high-resolution temperature time series as it corresponds to the finescale physical character of an interface of density or temperature, and the motion of such an interface along the ground. Parameters of interest, including a spatial temperature gradient normal to the interface (ϳ1 K m Ϫ1 ), an interfacial thickness (2.8 m), and a thermal boundary layer for the interface (288 m), are calculated. Additionally, theoretical and empirical arguments are presented for a viscous suppression of turbulent mixing in the immediate vicinity of an interface of density or temperature.