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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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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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Anne K. Smith and Susan K. Avery

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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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Darren McKague, K. Franklin Evans, and Susan Avery

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Vertical profiles of drop size distribution (DSD) parameters are produced from data collected with the National Oceanographic and Atmospheric Administration 915- and 50-MHz Doppler radars at Darwin, Australia, for the 1993–94 monsoon season. An existing algorithm is used to retrieve gamma size distribution parameters from the VHF and UHF Doppler radar spectra. The clear-air mean velocities and spectral widths obtained from the VHF radar are used to fit DSDs accurately to UHF spectra. Uncertainties in retrieved precipitation parameters are estimated from errors in both VHF and UHF spectra. The statistics of the retrieved profiles of DSD parameters are summarized and compared with surface disdrometer data from a site near Darwin. Retrieved vertical profiles of gamma DSDs are input to a microwave radiative transfer model to determine realistic variations in upwelling 10- and 19-GHz brightness temperatures due to uncertainties in drop size distribution. These brightness temperature variations are then used to estimate the error in simple emission-based passive microwave remote sensing algorithms for tropical rainfall due to the Marshall–Palmer assumption. For a viewing angle of 53.1° and for vertical polarization, the two-sigma scatter in brightness temperature is estimated to be ±7.0 K at 10 GHz and ±6.8 K at 19 GHz. The rms difference in brightness temperatures from the Marshall–Palmer brightness temperature, rain rate curve is estimated to be 6.7 K at 10 GHz and 4.9 K at 19 GHz. It is concluded that the scatter in the modeled brightness temperatures is primarily due to variations in the retrieved DSDs, though some scatter can be attributed to vertical inhomogeneity within the DSD profiles. The rms difference in the brightness temperature–rain rate relationship from Marshall–Palmer is consistent with a systematic shift in the retrieved DSDs toward smaller raindrops for a given rain rate than is predicted with the Marshall–Palmer DSD.

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