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Susan K. Avery
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Susan K. Avery
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Susan K. Avery
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Susan K. Avery
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Susan K. Avery
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Anne K. Smith
and
Susan K. Avery

Abstract

A simple numerical model of the stratosphere has been used to examine the possibility that a resonant growth of wave 2 was responsible for the 1979 major sudden warning. The model solves for linear steady state solutions to the quasi-geographic wave equation in the presence of realistic damping. The basic state is taken from observations (NMC and LIMS), and the frequency of the wave forcing is varied over a wide range. The model results show that in the days during the initial observed amplification of wave 2 (14–15 February), a clear resonant mode existed. The maximum response is for a wave moving eastward with a period of 12–16 days. Another peak at very low frequency (period greater than 100 days) occurs on 22 February. Other days during the period 12–24 February show weaker, but nevertheless significant peaks for particular frequencies. The frequency of the maximum is lower for later days and is nearly stationary at the height of the warming around 21 February. This frequency shift found in the model corresponds closely to the observed wave behavior.

Although the details of the results vary with changes in the model resolution or lower boundary position, the resonant wave does not disappear. However, when the wave forcing is applied at the earth&'s surface rather than in the tropopause region, no resonance occurs. To test the effect of the lower boundary, the troposphere-stratosphere model was run with an internal vorticity forcing similar to the structure of the observed wave 2 in the troposphere. In this case the frequency dependence of the amplitude within the stratosphere was similar to that of the model with a tropopause boundary, although the magnitude was considerably smaller. This suggests that for resonance to have occurred, a planetary scale disturbance that did not propagate from the surface must have been maintained in the upper troposphere. The two well-developed blocking ridges present in the troposphere during this period may have contributed enough to planetary wave 2 to provide the necessary boundary conditions.

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Wesley Berg
and
Susan K. Avery

Abstract

Estimates of monthly rainfall have been computed over the tropical Pacific using passive microwave satellite observations from the Special Sensor Microwave/Imager (SSM/I) for the period from July 1987 through December 1991. The monthly estimates were calibrated using measurements from a network of Pacific atoll rain gauges and compared to other satellite-based rainfall estimation techniques. Based on these monthly estimates, an analysis of the variability of large-scale features over intraseasonal to interannual timescales has been performed. While the major precipitation features as well as the seasonal variability of the rainfall distributions show good agreement with expected values, the presence of a moderately intense El Niño during 1986–87 and an intense La Niña during 1988–89 highlights this time period.

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James G. Ferriday
and
Susan K. Avery

Abstract

A physically based algorithm sensitive to emission and scattering is used to estimate rainfall using the Special Sensor Microwave/Imager (SSM/I). The algorithm is derived from radiative transfer calculations through an atmospheric cloud model specifying vertical distributions of ice and liquid hydrometeors as a function of rain rate. The algorithm is structured in two parts: SSM/I brightness temperatures are screened to detect rainfall and are then used in a rain-rate calculation. The screening process distinguishes between nonraining background conditions and emission and scattering associated with hydrometeors. Thermometric temperature and polarization thresholds determined from the radiative transfer calculations are used to detect rain, whereas the rain-rate calculation is based on a linear function fit to a linear combination of channels. Separate calculations for ocean and land account for different background conditions. The rain-rate calculation is constructed to respond to both emission and scattering, reduce extraneous atmospheric and surface effects, and to correct for beam filling. The resulting SSM/I rain-rate estimates are compared to three precipitation radars as well as to a dynamically simulated rainfall event. Global estimates from the SSM/I algorithm are also compared to continental and shipboard measurements over a 4-month period. The algorithm is found to accurately describe both localized instantaneous rainfall events and global monthly patterns over both land and ocean. Over land the 4-month mean difference between SSM/I and the Global Precipitation Climatology Center continental rain gauge database is less than 10%. Over the ocean, the mean difference between SSM/I and the Legates and Willmott global shipboard rain gauge climatology is less than 20%.

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