Publications

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2012
Schmidt, JM, Flatau PJ, Harasti PR, Yates RD, Littleton R, Pritchard MS, Fischer JM, Fischer EJ, Kohri WJ, Vetter JR, Richman S, Baranowski DB, Anderson MJ, Fletcher E, Lando DW.  2012.  Radar observations of individual rain drops in the free atmosphere. Proceedings of the National Academy of Sciences of the United States of America. 109:9293-9298.   10.1073/pnas.1117776109   AbstractWebsite

Atmospheric remote sensing has played a pivotal role in the increasingly sophisticated representation of clouds in the numerical models used to assess global and regional climate change. This has been accomplished because the underlying bulk cloud properties can be derived from a statistical analysis of the returned microwave signals scattered by a diverse ensemble comprised of numerous cloud hydrometeors. A new Doppler radar, previously used to track small debris particles shed from the NASA space shuttle during launch, is shown to also have the capacity to detect individual cloud hydrometeors in the free atmosphere. Similar to the traces left behind on film by subatomic particles, larger cloud particles were observed to leave a well-defined radar signature (or streak), which could be analyzed to infer the underlying particle properties. We examine the unique radar and environmental conditions leading to the formation of the radar streaks and develop a theoretical framework which reveals the regulating role of the background radar reflectivity on their observed characteristics. This main expectation from theory is examined through an analysis of the drop properties inferred from radar and in situ aircraft measurements obtained in two contrasting regions of an observed multicellular storm system. The observations are placed in context of the parent storm circulation through the use of the radar's unique high-resolution waveforms, which allow the bulk and individual hydrometeor properties to be inferred at the same time.

2000
Flatau, PJ, Flatau M, Zaneveld JRV, Mobley CD.  2000.  Remote sensing of bubble clouds in sea water. Quarterly Journal of the Royal Meteorological Society. 126:2511-2523.   10.1256/smsqj.56807   AbstractWebsite

We report on the influence of submerged bubble clouds on the remote-sensing properties of water. We show that the optical effect of bubbles on radiative transfer and on the estimate of the ocean colour is significant. We present a global map of the volume fraction of air in water derived from daily wind speed data. This map. together with the parametrization of the microphysical properties, shows the possible significance of bubble clouds on the albedo to incoming solar energy.

1996
Collins, WD, Valero FPJ, Flatau PJ, Lubin D, Grassl H, Pilewskie P.  1996.  Radiative effects of convection in the tropical Pacific. Journal of Geophysical Research-Atmospheres. 101:14999-15012.   10.1029/95jd02534   AbstractWebsite

The radiative effects of tropical clouds at the tropopause and the ocean surface have been estimated by using in situ measurements from the Central Equatorial Pacific Experiment (CEPEX). The effect of clouds is distinguished from the radiative effects of the surrounding atmosphere by calculating the shortwave and longwave cloud forcing. These terms give the reduction in insolation and the increase in absorption of terrestrial thermal emission associated with clouds. At the tropopause the shortwave and longwave cloud forcing are nearly equal and opposite, even on daily timescales. Therefore the net effect of an ensemble of convective clouds is small compared to other radiative terms in the surface-tropospheric heat budget. This confirms the statistical cancellation of cloud forcing observed in Earth radiation budget measurements from satellites. At the surface the net effect of clouds is to reduce the radiant energy absorbed by the ocean. Under deep convective clouds the diurnally averaged reduction exceeds 150 W m(-2). The divergence of flux in the cloudy atmosphere can be estimated from the difference in cloud forcing at the surface and tropopause. The CEPEX observations show that the atmospheric cloud forcing is nearly equal and opposite to the surface forcing. Based upon the frequency of convection, the atmospheric forcing approaches 100 W m(-2) when the surface temperature is 303 K. The cloud forcing is closely related to the frequency of convective cloud systems. This relation is used in conjunction with cloud population statistics derived from satellite to calculate the change in surface cloud forcing with sea surface temperature. The net radiative cooling of the surface by clouds increases at a rate of 20 W m(-2)K(-1)during the CEPEX observing period.

1994
Flatau, M, Schubert WH, Stevens DE.  1994.  The Role of Baroclinic Processes in Tropical Cyclone Motion - the Influence of Vertical Tilt. Journal of the Atmospheric Sciences. 51:2589-2601.   10.1175/1520-0469(1994)051<2589:trobpi>2.0.co;2   AbstractWebsite

The numerical study presented here focuses on baroclinic processes that contribute to tropical cyclone (TC) propagation. A three-dimensional, semispectral, primitive equation model of baroclinic vortex was developed to study TC motion. In a tiled vortex, interaction between upper- and lower-level vorticity anomalies leads to vortex propagation relative to the steering flow. On a beta plane, with no environmental flow, the vortex is tilted toward the south and the interaction between the layers reduces the westward movement of the vortex. The vortex tilting can also occur due to the vertical shear in the environmental wind. On an f plane, the interaction between the layers causes the northward movement of the vortex in westerly linear shear, and southward movement in easterly linear shear, with a meridional velocity of about 1 ms-1. This velocity increases with increasing vortex intensity and vertical motion.

1993
Flatau, M, Stevens DE.  1993.  The Role of Outflow-Layer Instabilities in Tropical Cyclone Motion. Journal of the Atmospheric Sciences. 50:1721-1733.   10.1175/1520-0469(1993)050<1721:trooli>2.0.co;2   AbstractWebsite

The paper examines the role of the development of outflow-layer instabilities on the motion of tropical cyclones. The influence of barotropic instability is examined by comparing the time changes in the storm tracks with the frequencies of free, unstable barotropic modes. For intense vortices barotropic instability is shown to contribute to the slow (periods of a few days) trochoidal motion of a cyclone. The development of instability depends on the horizontal distribution and frequency of environmental forcing. The strongest response occurs when the frequency of the forcing matches the frequency of an unstable mode.

1990
Stephens, GL, Tsay SC, Stackhouse PW, Flatau PJ.  1990.  The Relevance of the Microphysical and Radiative Properties of Cirrus Clouds to Climate and Climatic Feedback. Journal of the Atmospheric Sciences. 47:1742-1753.   10.1175/1520-0469(1990)047<1742:trotma>2.0.co;2   AbstractWebsite

This paper examines the effects of the relationship between cirrus cloud ice water content and cloud temperature on climate change. A simple mechanistic climate model is used to study the feedback between ice water content and temperature. The central question studied in this paper concerns the extent to which both the radiative and microphysical properties of cirrus cloud influence such a feedback. To address this question, a parameterization of the albedo and emissivity of clouds is introduced. Observations that relate the ice water content to cloud temperature are incorporated in the parameterization to introduce a temperature dependence to both albedo and emittance. The cloud properties relevant to the cloud feedback are expressed as functions of particles size re, asymmetry parameter g and cloud temperature and analyses of aircraft measurements, lidar and ground based radiometer data are used to select re and g. It was shown that scattering calculations assuming spherical particles with a distribution described by re = 16 μm reasonably matched the lidar and radiometer data. However, comparison of cloud radiation properties measured from aircraft to those parameterized in this study required values of g significantly smaller than those derived for spheres but consistent with our understanding of nonspherical particle scattering.The climate simulations revealed that the influence of cirrus cloud on climate was strongly affected by the choice of re and g: parameters that are both poorly known for cirrus. It was further shown that the effect of ice water feedback on a CO2 warming simulation could be either positive or negative depending on the value of re assumed. Based on these results, it was concluded that prediction of cirrus cloud feedback on climate is both premature and limited by our lack of understanding of the relationship between size and shape of ice crystals and the gross radiative properties of cirrus.