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Maria K. Flatau, Piotr J. Flatau, and Daniel Rudnick

Abstract

Double monsoon onset develops when the strong convection in the Bay of Bengal is accompanied by the monsoonlike circulation and appears in the Indian Ocean in early May, which is about 3 weeks earlier than the climatological date of the onset (1 Jun). The initial “bogus onset” is followed by the flow weakening or reversal and clear-sky and dry conditions over the monsoon region. The best example of such a phenomenon is the development of the summer monsoon in 1995, when monsoonlike perturbations that appeared in mid-May disappeared by the end of the month and were followed by a heat wave in India, delaying onset of the monsoon. The climatology of double onsets is analyzed, and it is shown that they are associated with delay of the monsoon rainfall over India. This analysis indicates that the development of bogus onsets depends on the timing of intraseasonal oscillation in the Indian Ocean and the propagation of convective episodes into the western Pacific. There is evidence that an SST evolution in the Bay of Bengal and the western Pacific plays an important role in this phenomenon. It is shown that in the case of the double monsoon onset it is possible to predict hot and dry conditions in India before the real monsoon onset. In the 32 yr of climatological data, six cases of double monsoon onset were identified.

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Maria Flatau, Piotr J. Flatau, Patricia Phoebus, and Pearn P. Niiler

Abstract

Existing theories of the Madden–Julian oscillation neglect the feedback between the modification of sea surface temperature by the convection and development of a convective cluster itself. The authors show that the convection-generated SST gradient plays an important role in cluster propagation and development. The relative importance of radiative and evaporative fluxes in SST regulation is also discussed. Various Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment and Central Equatorial Pacific Experiment observation platforms are used to estimate the effects of equatorial convection on SST changes during March 1993. The data include drifting buoys and TAO-buoy array measurements, combined with the Navy Operational Global Atmospheric Prediction System analyzed surface wind fields and Geostationary Meteorological Satellite cloud-top temperatures. It is shown that during the equatorial convection episode SST is decreasing under and to the west of the convective heat source due to the large wind velocities and solar flux reduction. To the east of the source, in the convergence region of a Kelvin wave, low wind speeds and high insolation cause the SST to increase.

The data are used to formulate an empirical relationship between wind speed and the 24-h SST change on the equator. Although formulated in terms of wind speed, this relationship implicitly includes radiative effects. This equation is then used in a global circulation model to examine the effect of SST feedback on the behavior of equatorial convection. A series of experiments is performed using an R15 general circulation model of the “aquaplanet” with a zonally symmetric SST distribution. In the case with fixed SSTs, equatorial wind fluctuations have the character of waves propagating around the globe with a phase speed of about 20 m s−1. When the effect of SST modification is included, the fluctuations slow down and become more organized. In addition, a 40–60-day peak appears in the spectral analysis of equatorial precipitation.

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Jacek Piskozub, Piotr J. Flatau, and J. V. Ronald Zaneveld

Abstract

A Monte Carlo model was used to study the scattering error of an absorption meter with a divergent light beam and a limited acceptance angle of the receiver. Reflections at both ends of the tube were taken into account. Calculations of the effect of varying optical properties of water as well as the receiver geometry were performed. A weighting function showing the scattering error quantitatively as a function of angle was introduced. Some cases of practical interest are discussed.

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Piotr J. Flatau, Robert L. Walko, and William R. Cotton

Abstract

The authors describe eighth- and sixth-order polynomial fits to Wexler's and Hyland-Wexler's saturation-vapor-pressure expressions. Fits are provided in both least-squares and relative-error norms. Error analysis is presented. The authors show that their method is faster in comparison with the reference expressions when implemented on a CRAY-YMP.

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Jerome M. Schmidt, Piotr J. Flatau, and Paul R. Harasti

Abstract

The structure of a melting layer associated with a mesoconvective system is examined using a combination of in situ aircraft measurements and a unique Doppler radar operated by the U.S. Navy that has a range resolution as fine as 0.5 m. Interest in this case was motivated by ground-based all-sky camera images that captured the transient development of midlevel billow cloud structures within a precipitating trailing stratiform cloud shield associated with a passing deep convective system. A sequence of high-fidelity time–height radar measurements taken of this storm system reveal that the movement of the billow cloud structure over the radar site corresponded with abrupt transitions in the observed low-level precipitation structure. Of particular note is an observed transition from stratiform to more periodic and vertically slanted rain shaft structures that both radar and aircraft measurements indicate have the same temporal periodicity determined to arise visually between successive billow cloud bands. Doppler, balloon, and aircraft measurements reveal these transient bands are associated with a shallow circulation field that resides just above the melting level in a layer of moist neutral stability and strong negative vertical wind shear. The nature of these circulations and their impact on the evolving precipitation field are described in the context of known nimbostratus cloud types.

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Jerome M. Schmidt, Piotr J. Flatau, and Robert D. Yates

Abstract

Very-high-resolution Doppler radar observations are used together with aircraft measurements to document the dynamic and thermodynamic structure of a dissipating altocumulus cloud system associated with a deep virga layer. The cloud layer circulation is shown to consist of shallow vertical velocity couplets near cloud top and a series of subkilometer-scale Rayleigh–Bénard-like cells that extend vertically through the depth of the cloud layer. The subcloud layer was observed to contain a number of narrow virga fall streaks that developed below the more dominant Rayleigh–Bénard updraft circulations in the cloud layer. These features were discovered to be associated with kilometer-scale horizontally orientated rotor circulations that formed along the lateral flanks of the streaks collocated downdraft circulation. The Doppler analysis further reveals that a layer mean descent was present throughout both the cloud and subcloud layers. This characteristic of the circulation is analyzed with regard to the diabatic and radiative forcing on horizontal length scales ranging from the Rayleigh–Bénard circulations to the overall cloud layer width. In particular, linear analytical results indicate that a deep and broad mesoscale region of subsidence is quickly established in middle-level cloud layers of finite width when a layer-wide horizontal gradient in the cloud-top radiative cooling rate is present. A conceptual model summarizing the primary observed and inferred circulation features of the altocumulus layer is presented.

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Johannes Verlinde, Piotr J. Flatau, and William R. Cotton

Abstract

A closed form solution for the collection growth equation as used in bulk microphysical parameterizations is derived. Although the general form is mathematically complex, it can serve as a benchmark for testing a variety of approximations. Two special cases that can immediately be implemented in existing cloud models are also presented. This solution is used to evaluate two commonly used approximations. The effect of the selection of different basis functions is also investigated.

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Adrian J. Matthews, Dariusz B. Baranowski, Karen J. Heywood, Piotr J. Flatau, and Sunke Schmidtko

Abstract

A surface diurnal warm layer is diagnosed from Seaglider observations and develops on half of the days in the Cooperative Indian Ocean Experiment on Intraseasonal Variability/Dynamics of the Madden–Julian Oscillation (CINDY/DYNAMO) Indian Ocean experiment. The diurnal warm layer occurs on days of high solar radiation flux (>80 W m−2) and low wind speed (<6 m s−1) and preferentially in the inactive stage of the Madden–Julian oscillation. Its diurnal harmonic has an exponential vertical structure with a depth scale of 4–5 m (dependent on chlorophyll concentration), consistent with forcing by absorption of solar radiation. The effective sea surface temperature (SST) anomaly due to the diurnal warm layer often reaches 0.8°C in the afternoon, with a daily mean of 0.2°C, rectifying the diurnal cycle onto longer time scales. This SST anomaly drives an anomalous flux of 4 W m−2 that cools the ocean. Alternatively, in a climate model where this process is unresolved, this represents an erroneous flux that warms the ocean. A simple model predicts a diurnal warm layer to occur on 30%–50% of days across the tropical warm pool. On the remaining days, with low solar radiation and high wind speeds, a residual diurnal cycle is observed by the Seaglider, with a diurnal harmonic of temperature that decreases linearly with depth. As wind speed increases, this already weak temperature gradient decreases further, tending toward isothermal conditions.

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Graeme L. Stephens, Si-Chee Tsay, Paul W. Stackhouse Jr., and Piotr J. Flatau

Abstract

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.

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Beata Latos, Thierry Lefort, Maria K. Flatau, Piotr J. Flatau, Donaldi S. Permana, Dariusz B. Baranowski, Jaka A.I. Paski, Erwin Makmur, Eko Sulystyo, Philippe Peyrillé, Zhe Feng, Adrian J. Matthews, and Jerome M. Schmidt

Abstract

On the basis of detailed analysis of a case study and long-term climatology, it is shown that equatorial waves and their interactions serve as precursors for extreme rain and flood events in the central Maritime Continent region of southwest Sulawesi, Indonesia. Meteorological conditions on January 22, 2019 leading to heavy rainfall and devastating flooding in this area are studied. It is shown that a convectively coupled Kelvin wave (CCKW) and a convectively coupled equatorial Rossby wave (CCERW) embedded within the larger-scale envelope of the Madden-Julian Oscillation (MJO) enhanced convective phase, contributed to the onset of a mesoscale convective system which developed over the Java Sea. Low-level convergence from the CCKW forced mesoscale convective organization and orographic ascent of moist air over the slopes of southwest Sulawesi. Climatological analysis shows that 92% of December-January-February floods and 76% of extreme rain events in this region were immediately preceded by positive low-level westerly wind anomalies. It is estimated that both CCKWs and CCERWs propagating over Sulawesi double the chance of floods and extreme rain event development, while the probability of such hazardous events occurring during their combined activity is eight times greater than on a random day. While the MJO is a key component shaping tropical atmospheric variability, it is shown that its usefulness as a single factor for extreme weather-driven hazard prediction is limited.

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