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Andreas K. Goroch, C. W. Fairall, and K. L. Davidson

Abstract

Marine aerosol size distributions are fit to a gamma function using the technique of maximum likelihood. Aerosol measurements are separated into marine and continental components. The marine component is fit to the gamma function to provide a set of wind-speed-dependent coefficients. The results are in good agreement with earlier measurements.

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C. W. Fairall, K. L. Davidson, and G. E. Schacher

Abstract

Microthermal sensors contaminated by salt aerosol droplets are subject to erroneous temperature fluctuations caused by water vapor exchange in response to fluctuations in humidity. The effect was studied by comparing the values of mean-square temperature fluctuations indicated by contaminated and clean sensors. The effect was negligible for ambient relative humidities above 85%, primarily due to the lack of humidity fluctuations. The errors were significantly diminished by frequent washing of the sensors.

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J. L. McBride, N. E. Davidson, K. Puri, and G. C. Tyrell

Abstract

The evolution of the large-scale flow through the four-month intensive observing period of TOGA COARE is documented from large-scale numerical analyses and GMS cloud imagery produced by the Australian Bureau of Meteorology and transmitted to the field stations during the experiment. The evolution of the flow is dominated by the following phenomena:

1) the normal seasonal evolution of the tropical flow over this region, including a southward and eastward progression of the tropical convective heat source as the Southern Hemisphere monsoon developed and matured;

2) a more eastward than normal progression of this monsoon circulation, associated with a warm event of the ENSO phenomenon;

3) the existence of a major westerly–easterly–westerly cycle of the Madden–Julian low-frequency wave occurring during the latter half of the experimental period, and

4) the development and subsequent movement of tropical cyclones in both (northern and southern) hemispheres.

The Madden–Julian event consisted of two eastward progressions across the domain of satellite-observed cloud, south of the equator. The horizontal scale of the cloud regions is approximately 10° latitude × 40° longitude and the eastward phase speed is approximately 3.7 m s−1. Linear correlation studies substantiate the eastward movement of both cloud and zonal wind across the domain. The correlation analysis reveals a strong relationship between cloud and low-level zonal wind, with the cloud variations leading those in wind by approximately five days.

Time-longitude sections of relative vorticity show that the synoptic activity also progressed eastward with the cloud, and its structure is suggestive that the controlling dynamics (for the synoptic activity) may be the energy dispersion mechanism of Davidson and Hendon. The development of each westerly event was accompanied by a major change in the Southern Hemisphere deep-layer mean flow from easterly to westerly.

Examination of flow fields and satellite imagery for individual days shows that the peak of the first westerly event is associated with the flow patterns surrounding two Southern Hemisphere tropical cyclones. The subsequent rapid evolution to an easterly state occurs as the cyclones move eastward and southward, and the monsoon flow collapses in their wake. There is an accompanying ridging at low levels in the subtropics and the establishment of the Southern Hemisphere subtropical jet. The subsequent reestablishment of the monsoon (the second westerly event) occurs from west to east with the eastward moving cloud bands. There is also a suggestion that an equatorward extension of a Southern Hemisphere upper-level trough may have played a role.

Major active and break periods are identified over four tropical subdomains over the TOGA COARE region. These are most easily defined in the Southern Hemisphere subdomains. They are characterized by a slowly ,varying signal in the satellite-observed average cloud-top temperature. Superimposed on this is a rapid transition between the active and break states.

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Esther Chiew, Rachel A. Davidson, Joseph E. Trainor, Linda K. Nozick, and Jamie L. Kruse

Abstract

An increasing number of national, state, and local programs have offered grants or other monetary incentives to encourage homeowners to retrofit their homes to reduce damage from natural hazard events. Despite this fact, little is known about how these offerings influence a homeowner’s decision to carry out such structural retrofits. This paper studies the impact that different grant program designs in particular have on the decision to undertake different types of retrofits to mitigate against hurricane damage. Using data from a survey of homeowners in the eastern half of North Carolina, we implement a mixed logit model that allows for the combination of both revealed-preference and stated-preference data available from the survey. Our findings show that offering a grant results in households being, on average, 3 times as likely to retrofit as when a grant is not offered. In addition, both the percentage of retrofit cost and the maximum dollar amount covered by the grant have a substantial impact on the probability that households choose to retrofit. Living closer to the coastline also has a significant impact on the probability that households will choose to retrofit. Counter to some previous research, we find that households who have experienced two or more hurricanes are less likely to choose to retrofit their homes. From our research, we find that the percentage of retrofit cost covered by the grant and the total cost are both important factors when deciding on the best grant program configuration.

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K. L. Davidson, C. W. Fairall, P. Jones Boyle, and G. E. Schacher

Abstract

An evaluation of the ability of an integrated (slab) marine atmospheric boundary-layer (MABL) model to predict changes in the inversion and mixed-layer temperature and humidity using data from the Los Angeles-San Diego Basin is described. The model microphysics and initialization methods are evaluated separately. The Stage and Businger stratocumulus entrainment closure formulation is used. Standard radiative flux approximations (e.g., delta-Eddington) are employed with up-to-date cloud microphysical parameterizations. The assumption of well-mixed properties is relaxed to permit a constant vertical gradient that is a function of the surface flux and the entrainment rate. Initialization of the subsidence rate receives considerable attention and is analyzed using data from suitably spaced multiple stations and from a single station. Two cases, a cloud covered and a clear sky period, are examined. In both cases the island and shoreline data are from regularly reporting locations and from a research ship which moved around the region. In one case an instrumented aircraft also provided vertical temperature profiles.

Evaluation of the prediction for the cloudy sky case concentrates on the model physics. In this case, the external forcing (subsidence, surface wind and sea-surface temperature) is based on observations during the prediction period and updated every 6 h. Comparison of observations and model results illustrates the important role of both long- and shortwave radiation, and the validity of the Stage and Businger entrainment closure. The agreement is quite good for mixed-layer parameters, mixed-layer depth and the cloud base.

Evaluation of the prediction for the clear sky case emphasizes the initialization problem. Entrainment and radiative flux divergences are roughly an order of magnitude smaller in the cloud-free situation. External forcing for this case is based on data available prior to the prediction (versus updates during the prediction). Since subsidence was large during the period, the initialization was well-tested. A period when the subsidence rate was well-established from radar and acoustic remote sensing showed excellent agreement between observed and predicted values for more than 18 h of a 24 h forecast. Results during a period when subsidence was based on single-station-derived information showed reasonable agreement only during the first 12 h of a 24 h forecast. The influence of very near coastline effects is evident in the comparison of mixed-layer temperatures and humidities at the land stations.

It is concluded that existing integrated mixed-layer predictive models can, with caution, be applied to coastal prediction problems on the basis of multiple- or single-station data. Specification of the subsidence and the effects of near-coastal circulations are critical.

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