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J. Egger, G. Meyers, and P. B. Wright

Abstract

Maps of regression coefficients of sea level pressure, surface wind and cloudiness on a Southern Oscillation Index are presented for Northern Hemisphere fall and winter. To cheek the significance of the fields obtained, an attempt is made to relate the wind fields to the pressure fields by assuming a balance of the pressure gradient force, the frictional force and the Coriolis force. It turns out that the most conspicuous features of both the wind and cloudiness fields can be derived from the pressure field. It is found that the Southern Oscillation Index is positively correlated with surface easterlies and downward motion near the dateline. Significant fluctuations of the meridional wind are found over the Coral Sea and in the vicinity of the ITCZ.

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Michael B. Meyer and G. Garland Lala

Abstract

We present a detailed investigation of the local radiation fog climatology, carried out in support of our ongoing field program to study radiation fog mechanisms at Albany, New York. At Albany, a distinct ”radiation fog season” is observed during September and October. We show that this late-summer/early-autumn maximum in radiation fog observations is primarily due to a sufficient period of nocturnal cooling coupled with an adequate moisture supply. Five critical surface synoptic patterns are responsible for initiating the radiation fog process. In addition, radiation fog life cycles are generally confined to a modest time window centered on sunrise. Key parameters necessary for forecasting the onset time of fog are shown to be the initial relative humidity and nocturnal cooling rate.

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Steven J. Meyer and Kenneth G. Hubbard

Not all weather data are collected by federal agencies. Fueled by the need for more specific meteorological data in real or near-real time, the number of automated weather stations (AWSs) and AWS networks has expanded to the state and private sector over the past decade. This study employed a survey to determine the spatial extent and disposition of these nonfederal AWSs and AWS networks in the United States and Canada, the type of measurements taken, the operating procedures (i.e., maintenance and data-retrieval techniques), and the uses of the data (e.g., research, public service, agency needs). The rapid growth and expansion in the number of AWSs and networks can be viewed as a positive step toward expanding data available for meteorological research and service. As AWS networks continue to grow and expand in the United States and Canada, it is recommended that an AWS climatic database be established. With proper logistical coordination and the cooperation of network operators, development of such a database can become reality.

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A. Schiller, S. E. Wijffels, and G. A. Meyers

Abstract

Experiments using OGCM output have been performed to assess sampling strategies for the Argo array in the Indian Ocean. The results suggest that spatial sampling is critical for resolving intraseasonal oscillations in the upper ocean, that is, about 500 km in the zonal and about 100 km in the equatorial meridional direction. Frequent temporal sampling becomes particularly important in dynamically active areas such as the western boundary current regime and the equatorial waveguide. High-frequency sampling is required in these areas to maintain an acceptable signal-to-noise ratio, suggesting a minimum sampling interval of 5 days for capturing intraseasonal oscillations in the upper Indian Ocean. Sampling of seasonal and longer-term variability down to 2000-m depth is less critical within the range of sampling options of Argo floats, as signal-to-noise ratios for sampling intervals up to about 20 days are almost always larger than one. However, these results are based on a single OGCM and are subject to model characteristics and errors. Based on a coordinated effort, results from various models could provide more robust estimates by minimizing the impact of individual model errors on sampling strategies.

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X. Lin, Kenneth G. Hubbard, and George E. Meyer

Abstract

The air temperature radiation shield is a key component in air temperature measurement in weather station networks; however, it is widely recognized that significant errors in the measured air temperature exist due to insufficient airflow past the air temperature sensor housed inside the shield. During the last several decades, the U.S. National Weather Service has employed a number of different shields in air temperature measurements. This paper focuses on the airflow characteristics inside air temperature shields including the Maximum–Minimum Temperature System (MMTS), the Gill shields, and the Cotton Region Shelter (CRS).

Average airspeed profiles and airflow efficiency inside the shields are investigated in this study under both windtable and field conditions using an omnidirectional hot-wire sensor. Results from the windtable measurements indicate that the average airspeeds inside the shields oscillated along the center line of the Gill and MMTS shields as the “windtable air” speed was changed from 1.03 to 2.62 m s−1; the MMTS airflow efficiency demonstrated a nearly constant value, but the Gill’s airflow efficiency increased. A linear transfer equation between the airspeed measured at the normal operating position for the temperature sensor inside the shield and the ambient wind speed was found under field conditions for all three nonaspirated air temperature radiation shields (CRS, Gill, and MMTS). Results indicate that the naturally ventilated temperature radiation shields are unable to provide sufficient ventilation when the ambient wind speed is less than 5 m s −1 at the radiation shield height.

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Steven J. Meyer, Kenneth G. Hubbard, and Donald A. Wilhite

Abstract

In a recent survey conducted by the University of Nebrask's Center for Agricultural Meteorology and Climatology of Agricultural Network (AGNET) users, the results of potential evapotranspiration (ET p) projections (calculated using the Blaney-Criddle approach, which employs “normal” climatic data to project ET p, estimates up to three days into the future) were labeled “unrealistic”. To improve these projection National Weather Service (NWS) forecast variables were used as input into the Blancy-Criddle and Penman equations. ET p, projections calculated according to the Penman equation, with data measured by automated weather stations as input, were assumed to represent the “best” attainable. ET p projections calculated using NWS forecasted values were compared with the “best” projections for the summer of 1985. Increased accuracy in ET p, projections due to increased accuracy in the individual forecasted input variables was evaluated.

Overall, daily ET p, projections made with the Blancy-Criddle equation were substantially improved using the NWS forecasted temperature in place of normal temperature; over a growing season, however, accurate estimates resulted from using normal temperatures. The use of NWS-forecasted variables as input into the Penman equation offers the greatest potential for improving ET p, projections. “Over” forecasting of all variables (relative to the estimation of ET p,) limited the ability of the Penman equation in this study. For greatest improvement in ET p, projections using the Penman equation, efforts should be concentrated on improving forecasts of relative humidity and solar radiation.

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Michael B. Meyer, G. Garland Lala, and James E. Jiusto

The Cloud Physics Section of the Atmospheric Sciences Research Center-State University of New York at Albany conducted a cooperative field study (FOG-82) during the autumn of 1982 as part of an ongoing radiation-fog research program. A computer-controlled data-acquisition system consisting of sophisticated soil, surface, and boundary-layer sensors, as well as contemporary aerosol and droplet probes was developed. These data are being used to address a variety of critical problems related to radiation-fog evolution.

Scientists from 10 universities and research laboratories participated in portions of FOG-82. Research objectives included studies of fog mesoscale meteorology, radiation studies, low-level water budget, vertical fog structure, fog supersaturation, condensation nuclei, and fog-water chemistry, as well as radiation-fog life cycles. A comprehensive description of the FOG-82 program and objectives is presented.

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Michael B. Meyer, James E. Jiusto, and G. Garland Lala

Abstract

An extensive boundary-layer field program was conducted which included simultaneous measurements of visibility and particle size distributions during fog and haze. Several empirical expressions relating changes in visibility to characteristics of the aerosol (droplet) size spectrum and relative humidity are presented and evaluated. Detailed analysis of one evolving dense fog revealed several points of interest regarding the behavior of drop size spectra, including a scheme for approximating fog supersaturation.

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S. D. Meyers, B. G. Kelly, and J. J. O'Brien

Abstract

Wavelet analysis is a relatively new technique that is an important addition to standard signal analysis methods. Unlike Fourier analysis that yields an average amplitude and phase for each harmonic in a dataset, the wavelet transform produces an “instantaneous” estimate or local value for the amplitude and phase of each harmonic. This allows detailed study of nonstationary spatial or time-dependent signal characteristics.

The wavelet transform is discussed, examples are given, and some methods for preprocessing data for wavelet analysis are compared. By studying the dispersion of Yanai waves in a reduced gravity equatorial model, the usefulness of the transform is demonstrated. The group velocity is measured directly over a finite range of wavenumbers by examining the time evolution of the transform. The results agree well with linear theory at higher wavenumber but the measured group velocity is reduced at lower wavenumber, possibly due to interaction with the basin boundaries.

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X. Lin, K. G. Hubbard, E. A. Walter-Shea, J. R. Brandle, and G. E. Meyer

Abstract

Air temperature measurement has inherent biases associated with the particular radiation shield and sensor deployed. The replacement of the Cotton Region Shelter (CRS) with the Maximum–Minimum Temperature System (MMTS) and the introduction of Automated Surface Observing System (ASOS) air temperature observing systems during the NWS modernization introduced bias shifts in federal networks that required quantification. In rapidly developing nonfederal networks, the Gill shield temperature systems are widely used. All of these systems house an air temperature sensor in a radiation shield to prevent radiation loading on the sensors; a side effect is that the air temperature entering a shield is modified by interior solar radiation, infrared radiation, airspeed, and heat conduction to or from the sensor so that the shield forms its own interior microclimate. The objectives of this study are to develop an energy balance model to evaluate the microclimate inside the ASOS, MMTS, Gill, and CRS shields, including the interior solar radiation, infrared radiation, and airspeed effects on air (sensor) temperature under day and night conditions. For all radiation shields, the model air temperature for shield effects was in good agreement between shields while the uncorrected “normal operating” temperatures were more variable from shield to shield. The solar radiation loading ratio was dramatically increased with a corresponding increase in the solar elevation angle for all shields except the ASOS shield, and are ranked as Gill > MMTS ≈ CRS > ASOS. The daytime infrared radiation effects on air temperature were ranked as ASOS > Gill > MMTS > CRS, but the nighttime infrared radiation effects were not so large and were uniformly distributed among negative and positive effects on air temperatures. For the nonaspirated radiation shields (MMTS, Gill, and CRS), increasing ambient wind speed improved the accuracy of air temperatures, but it was impossible to reach the accuracy claimed by manufacturers when the in situ measurements were taken under lower ambient wind speed (<4 ∼ 5 m s−1).

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