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  • Author or Editor: S. Mark Leidner x
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Christopher Grassotti
,
S. Mark Leidner
,
Jean-François Louis
, and
Ross N. Hoffman

Abstract

The authors report on characteristics of a rain flag derived from collocation of visible and infrared image data with rain rates over the North Atlantic Ocean obtained from microwave imagery (SSM/I) during a 3-week period (15 October 1996–2 November 1996). The rain flag has been developed as part of an effort to provide an indication of contamination by heavy rainfall in NASA scatterometer datasets. The primary results of this analysis indicate 1) that a simple albedo/infrared brightness temperature threshold is capable of flagging most of the heavy rainfall, though with a fairly high rate of false alarms, and 2) that the small difference in optimal threshold between the Tropics and midlatitudes can probably be ignored. Use of the rain flag in 12 assimilation experiments during this period showed that the number of rain-flagged wind vector cells is generally less than 1% of the number of cells. Overall, the impact from using the rain-flagged data is generally less than 5 m s−1 and localized (less than 5° of latitude and longitude). However, in some cases, the effect of excluding just one to five rain-flagged points can change the resulting analysis significantly, because their placement is critical for defining the flow along a front or some other shear-dominated environment.

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David R. Stauffer
,
Nelson L. Seaman
,
Glenn K. Hunter
,
S. Mark Leidner
,
Annette Lario-Gibbs
, and
Saffet Tanrikulu

Abstract

This paper describes a new methodology developed to provide objective guidance for cost-effective siting of meteorological observations on the mesoscale for air quality applications. This field-coherence technique (FCT) is based on a statistical analysis of the mesoscale atmospheric structure defined by the spatial and temporal“coherence” in the meteorological fields. The coherence, as defined here, is a measure of the distance scale over which there is temporal consistency in the spatial structure within a variable field. It indicates how well a measurement taken at one location can be used to estimate the value of that field at another location at a given analysis time. The FCT postulates that, the larger the field coherence is, the fewer measurement sites are needed to resolve adequately the dominant characteristics of that field.

Proof of concept was demonstrated using real data from an extensive field-program database over the San Joaquin Valley in the summer of 1990. The FCT next was applied to numerical model results for the same period, which produced similar guidance. The transferability of the methodology from real data to numerical model results having been demonstrated, the FCT then was applied in a model-based study over California’s South Coast Air Basin to contribute in the design of a new field program, the Southern California Ozone Study (SCOS97). Interpretation of the FCT results mostly corroborated a preliminary field-program design produced by the design team and based on past experience, subjective evaluation of historical datasets, and other considerations. However, the FCT results also led the design team to make several changes, which were confirmed by experts familiar with the meteorological behavior of the region and were included in the final SCOS97 field-program plan.

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