Automatic detection of fixed echoes in sodar facsimile records using the wavelet transform

A method to detect and remove fixed echoes in sodar facsimile records is described. This method is based on the capability of the wavelet transform to objectively detect singularities in signals, and it is adapted to two-tone facsimile records. At the present time, no other relevant method for facsimile record processing, except human observation, has been described in the literature. Automatic removal of fixed echoes is useful during computer processing of facsimile records with modern algorithms in order to prevent false layer detection when it is not possible to avoid or remove fixed echoes before recording. A test of the method on complex examples shows its success and limitations

Second‐order spectral local isotropy of the humidity and temperature fields in an urban area

A theoretical and experimental analysis of the humidity and temperature spatial differentials (δ/δxj,) in an urban area is presented. The local isotropy form of the spectra of the streamwise and vertical differentials of a scalar field is derived. The experimental results for the humidity and temperature fields in the lowest part of the urban boundary layer are in satisfactory agreement (at least in form under stable conditions) with the local isotropy model indicating that these fields can be locally isotropic. A strong anisotropy was observed at small wavenumbers of the vertical differentials spectra and it is probably connected with surface‐layer plumes. The analysis of the humidity and temperature differentials is extended to their co‐spectra and the spectra of the vertical differentials are compared to the streamwise ones. The comparison of the structure parameters of the temperature or the humidity field in the streamwise and the vertical direction, estimated using a spectral and a variance (structure function) method, showed no systematic departure from the isotropic prediction of equality. Copyright © 1995 Royal Meteorological Societ

Fast-response humidity measurements with the psychrometric method

A simple method for correcting the time series of a wet temperature sensor characterized by slow response is presented in this paper. A first-order linear approximation of the heat transfer equation is used for correcting the wet temperature measurements. The sensor cutoff frequency is found by an easy experimental method using the phase spectrum between the wet temperature sensor and a much faster platinum thermometer under low wind speeds (up to 3 m s-1) and relatively dry atmosphere (relative humidity 20-40%). If such a correction is applied, the wet sensor output along with the computed vapor pressure time series using the psychrometric equation are considerably recovered. -Author

Wind Stress Curl and Coastal Upwelling in the Area of Monterey Bay Observed during AOSN-II

The article of record as published may be located at http://dx.doi.org/10.1175/2010JPO4305.1Aircraft measurements obtained during the 2003–04 Autonomous Ocean Sampling Network (AOSN-II) project were used to study the effect of small-scale variations of near-surface wind stress on coastal upwelling in the area of Monterey Bay. Using 5-km-long measurement segments at 35 m above the sea surface, wind stress and its curl were calculated with estimated accuracy of 0.02–0.03 N m22 and 0.1–0.2 N m22 per 100 kilometers, respectively. The spatial distribution of wind speed, wind stress, stress curl, and sea surface temperature were analyzed for four general wind conditions: northerly or southerly wind along the coastline, onshore flow, and offshore flow. Wind stress and speed maxima frequently were found to be noncollocated as bulk parameterizations imply owing to significant stability and nonhomogeneity effects at cold SST pools. The analyses revealed that complicated processes with different time scales (wind stress field variation, ocean response and upwelling, sea surface currents, and heating by solar radiation) affect the coastal sea surface temperature. It was found that the stress-curl-induced coastal upwelling only dominates in events during which positive curl extended systematically over a significant area (scales larger than 20 km). These events included cases with a northerly wind, which resulted in an expansion fan downstream from Point An˜ o Nuevo (wind speed peaks greater than about 8–10 m s21), and cases with an offshore/onshore flow, which are characterized by weak background upwelling due to Ekman transport. However, in general, observations show that cold pools of sea surface temperature in the central area of Monterey Bay were advected by ocean surface currents from strong upwelling regions. Aircraft vertical soundings taken in the bay area showed that dominant effects of the lee wave sheltering of coastal mountains resulted in weak atmospheric turbulence and affected the development of the atmospheric boundary layer. This effect causes low wind stress that limits upwelling, especially at the northern part of Monterey Bay. The sea surface temperature is generally warm in this part of the bay because of the shallow oceanic surface layer and solar heating of the upper ocean.This work was supported by the Marine Meteorology and Atmospheric Effects program of the Office of Naval Research (ONR) (Awards N0001405WR20338, N0001406WR20081, and N0001407WR20229). S. R. Ramp’s work was supported by ONR Grant N0001403WR20002 (Aerial Surveys of the Ocean and Atmosphere off Central California)

Estimation of ABL parameters using the vertical velocity measurements of an acoustic sounder

The friction velocity, the surface heat flux and the height of the Atmospheric Boundary Layer (ABL) are important parameters. In this work, vertical velocity variance (σ(ω)2) and wind velocity structure parameter (C(υ)2) profiles estimated by acoustic sounder measurements are used, along with similarity relations, to estimate these parameters in the unstable Atmospheric Boundary Layer and the friction velocity in the stable one. The data were collected by two acoustic sounders with different height range and resolution under various atmospheric conditions (stability) and at two experimental sites in different terrain. The C(υ)2 profiles are estimated using gate difference of the vertical velocity measurements and the assumption of local isotropy. The vertical velocity data are corrected for the significant effects of noisy measurements and sampling volume averaging on the σ(ω)2 and C(υ)2 estimations using original techniques that are presented in this work. The results of the similarity method using acoustic sounder data are compared against estimates of the corresponding atmospheric parameters obtained from direct measurements. The comparison confirms the ability of the method to provide reasonably accurate estimates of these parameters especially in the middle of the day

Strong downslope winds and application of hydraulic-like theory

An experimental campaign was conducted at the lee side of a 1 km high steep mountain in order to study the development of strong downslope winds under favorable conditions, using combined remote and insitu instrumentation. The examination of the upstream atmospheric conditions reveals that the development of strong downslope winds is favored by a mean state critical layer or a significant decrease in static stability (such as at the top of a temperature inversion) at the proper height above the mountain top. Strong downslope winds could occur even for wind directions with a deviation of 60° off the axis perpendicular to the ridgeline, as long as the cross-mountain wind has a significant value (at least 7 ms-1). The developed disturbances are associated with intense downdrafts of the order of 4-5 ms-1 within the first 600 m above ground and characteristic vertical turbulent structures that were observed by sodar. The same phenomenon is observed not only during nighttime but also under unstable and neutral conditions of the atmospheric boundary layer (ABL) albeit with weaker intensity. Fourier analysis of the vertical velocity field demonstrated that the typical time period of intense disturbances was about 5 min. Further evidence is also provided for the application of the hydraulic-like theory under real atmospheric conditions. Copyright 2000 by the American Geophysical Union

Influence of background flow on evolution of saronic gulf sea breeze

Results from an experimental campaign at the coastline of the Saronic Gulf during the summer of 1992 are presented. The frontal intensity and the rotation of the wind hodograph at the shoreline during sea-breeze case:; are examined under different background flow conditions. The frontal intensity classification is based on the vertical velocities induced, as measured by a high resolution acoustic sounder. Three representative cases are presented. Conclusions are based on the analysis of all observed sea-breeze flows. Background off-shore or shore-parallel flows are more probable to create a strong or weak front, respectively. The development of frontal characteristics under background on-shore flow is attributed to off shore land features. The wind hodograph rotation is shown to be associated to the initial direction of the sea breeze, which is determined by the background flow direction. When the background flow possesses a westerly component the hodograph shows anticlockwise rotation, while an easterly background component causes the wind vector rotation to be clockwise. © 1995

Estimation of the atmospheric surface momentum and heat fluxes using a high resolution acoustic radar

The surface heat flux (Qo) and the friction velocity (u*) are important parameters of the Atmospheric Boundary Layer (ABL). Vertical velocity variance (σw)2 profiles have been used to estimate the parameters Qo and u* from Acoustic Sounder (AS) data according to the similarity theory. A comparison was made using data from a 12 m high mast equipped with humidity, and fast responding wind and temperature sensors and a high resolution monostatic AS. The comparison confirms the ability of the (σw)2 profile method to provide, even in a noisy environment (inhabitant area), estimates of these parameters with reasonable accuracy and on a continuous basis. The requirements for the method to be applied is that convective conditions or a minimum mechanical mixing exist and the level of acoustic noise is not too high

A layer detection and classification algorithm for sodar facsimile records

This paper describes an application of image processing and pattern recognition techniques for the detection and classification of the layer echo structure within a window of a facsimile record of a monostatic acoustic sounder (sodar). The algorithm identifies automatically the upper and lower boundary of every layer in a facsimile record, even if it is a complex pattern. Consequently, it uses the boundaries of the layers to extract the classification features. This technique provides quantitative information for a facsimile record and follows a different approach than previous classification schemes that classify the record manually according to predetermined patterns. The algorithm was developed and tested using data collected by two different types of sodar under various meteorological conditions. An overall skill better than 90 per cent of the cases was achieved. The algorithm experienced some difficulty identifying the thermal structure over transitional atmospheric conditions, during which there is no distinct layer structure. © 1995 Taylor & Francis Group, LLC

Inland propagation of sea breeze under opposing offshore wind

According to past experience, the nearly stagnant conditions by the presumed equilibrium between the Saronikos Gulf sea breeze and an opposing synoptic flow is identified as the principal mechanism leading to high pollution episodes in Athens during the summer. However, previous experimental work has not examined in detail the interaction of the sea breeze flow with the opposing background flow. In this context, recent experimental work covering the basic key-locations of the Athens Basin focused on the inland propagation of the southerly sea breeze from the coast to the northern part of the basin mainly under moderate northerly background wind. During this campaign, a network of four meteorological stations established along the Athens Basin and a high range acoustic sounder at the centre of Athens operated over a two months time period in the summer of 1993. In addition, tethered balloon flights in the centre of Athens and on a sea vessel about 15km offshore were employed during an experimental day with moderate opposing background wind. The results from this experimental campaign include the documentation of the sea breeze delay and its intensity as a function of a breeze index and features of the vertical structure of the sea breeze over land as well as over sea