[1] Geostationary and Sun-synchronous satellite data and in situ observations from ship cruises are used to investigate the formation of open cellular structure in marine stratocumulus clouds over the southeast Pacific (SEP). Open cellular convection either forms spontaneously as pockets of open cells (POCs) within overcast stratocumulus, or is advected into the region from midlatitude regions. POC formation occurs most frequently during the latter part of the night, demonstrating that this transition is not caused by solar absorption-driven decoupling. The transition preferentially occurs in clouds with low 11-3.9 mm nighttime brightness temperature difference (BTD) which is found to be well correlated with both in situ measured accumulation mode aerosol concentration and cloud droplet concentration estimates derived from MODIS. Besides indicating that nighttime BTD is an excellent proxy for stratocumulus cloud droplet concentration N d , this also suggests that low aerosol concentrations favor POC formation. Indeed, extremely low accumulation mode aerosol concentrations are found during the passage of open cell events over the ship. Free-tropospheric moisture is not found to be an important factor in POC formation. Significant subseasonal variability occurs in the fractional coverage of open cellular convection over the broader SEP. This coverage is well correlated with a MODIS-derived drizzle proxy (MDP) proportional to the ratio of liquid water path (LWP) to N d for predominantly overcast regions. Both LWP and N d variability influences the MDP. Periods of low MDP have significant positive large-scale N d anomalies and are preceded by offshore winds at 850 hPa, which suggests a potential continental influence upon open cell formation over the SEP. Together, the results suggest important two-way interactions between aerosols and drizzle in marine stratocumulus and a role for drizzle in modulating the large-scale albedo of these cloud systems.

Statistical tools based on the maximal overlap discrete wavelet transform (MODWT) are reviewed, and then applied to a dataset of aircraft observations of the atmospheric boundary layer from the tropical eastern Pacific, which includes quasi-stationary and non-stationary segments. The wavelet methods provide decompositions of variances and covariances, e.g. fluxes, between time scales that effectively describe a broadband process like atmospheric turbulence. Easily understood statistical confidence bounds are discussed and applied to these scale decompositions, and results are compared to Fourier methods for quasistationary turbulence. The least asymmetric LA(8) wavelet filter yields coefficients that exhibit better uncorrelatedness across scales than the Haar filter and is better suited for decomposition of broadband turbulent signals. An application to a non-stationary segment of our dataset, namely vertical profiles of the turbulent dissipation rate, highlights the flexibility of wavelet methods.

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We report the first clear-air observations of vertical velocities in the tropical upper troposphere and lower stratosphere (8-22 km) using the Arecibo 430-MHz radar. Oscillations in the vertical velocity near the Brunt-V•iis•il•i period are observed in the lower stratosphere during the 12-hour observation period. Frequency power spectra from the vertical velocity time series show a slope between -0.5 and -1.0. Vertical wave number spectra computed from the height profiles of vertical velocities have slopes between -1.0 and -1.5. These observed slopes do not agree well with the slopes of +1/3 and -2.5 for frequency and vertical wave number spectra, respectively, predicted by a universal gravity-wave spectrum model. The spectral power of wave number spectra of a radial beam directed 15 ø off-zenith is enhanced by an order of magnitude over the spectral power levels of the vertical beam. This enhancement suggests that other geophysical processes besides gravity waves are present in the horizontal flow. The steepening of the wave number spectrum of the off-vertical beam in the lower stratosphere to near -2.0 is attributed to a quasi-inertial period wave, which was present in the horizontal flow during the observation period. 1. •ODUCTION The development of the stratospheric tropospheric (ST) radar technique in the past decade has afforded the capability of direct measurement of vertical velocities in the lower middle atmosphere. Such measurements are useful in studies of gravity waves and mesoscale flows, both of which can have a significant true or apparent vertical component. The first radar observations of vertical velocities in the stratosphere were made using the Jicamarca 50-MHz radar in Peru by Woodman and Guillen [1974], who pointed out the potential for studies of gravity waves near the Brunt-V•iis•il•i frequency by mesospheric-stratospheric-tropospheric (MST) radar. Further observations at Jicamarca by Fukao et al. [1981] showed that oscillations near the Brunt-V•iis•il•i period were a dominant and frequent feature in the tropical lower stratosphere. Observations of vertical velocities by VHF radars elsewhere have shown pronounced variability in the activity of vertical winds. Observations over a 30-day period at Poker Flat by Ecklund et al. [1981] revealed that the magnitude of the vertical velocity fluctuations in the troposphere and lower stratosphere is controlled by propagating planetary waves, which modulate the large-scale wind field. In addition, Ecklund et al. [1981] observed active periods, during which the power in vertical fluctuations increased to about 25 times over that observed during quiet conditions, when horizontal winds were weak. They attributed the increase during active periods to enhanced gravity wave activity in the vertical winds. Ecklund et al. [ 1982] compared vertical velocities measured simultaneously at the Sunset and Platteville radars, located east of the continental divide in Colorado. Their observations indicated that lee wave effects were present. In addition, they noted ...

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