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Margaret A. Rozendaal and William B. Rossow

Abstract

The seasonal and intraseasonal variability of boundary layer cloud in the subtropical eastern oceans is studied using combined data from the International Satellite Cloud Climatology Project and the European Centre for Medium-Range Weather Forecasts reanalysis.

Spectral analysis reveals that most of the time variability of cloud properties occurs on seasonal to annual timescales. The variance decreases by one to two orders of magnitude for each decade of timescale decrease, indicating that daily to monthly timescales and their spatial extent have smaller, although nonnegligible, variability. The length of these dominant timescales suggests that the majority of the variability is influenced by the general circulation and its interaction with boundary layer turbulence, rather than being a product of local boundary layer turbulence alone. Although the dominance of seasonal to annual periods in the temporal power spectra of low-cloud fraction—TAU and CTP—justifies the previous focus of effort on seasonal variability, intraseasonal data can be better used to examine the cloud formation/dissipation processes as revealed in relationships between synoptic meteorology and cloud properties.

Previous datasets have lacked the necessary combination of resolution and scope in either time or space coverage to properly characterize variability on synoptic and larger scales; this is remedied by using global satellite-retrieved cloud properties. The intraseasonal subtropical cloud variability in both hemispheres and in different seasons are characterized. In addition to cloud fraction, variability of cloud optical thickness and cloud-top pressure frequency distributions are examined.

The intraseasonal variability is divided into three types. The first type, found in the Californian local summer and Southern Hemisphere regions year round, is characterized by lower-altitude, greater optical thickness, stationary clouds. The second type is found in the Canarian local summer and has more instances of smaller cloud-top pressures and a westward propagation direction. The third type, found in Northern Hemisphere regions during winter, is similar to the second type, but shows an eastward propagation direction. This study focuses on the third type more closely and finds it to be associated with the lower sea level pressure, upward vertical velocity phase of synoptic waves.

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Margaret A. Rozendaal, Conway B. Leovy, and Stephen A. Klein

Abstract

Marine stratiform clouds are important and highly variable contributors to earth's radiation budget over the eastern subtropical oceans and over middle- to high-latitude oceans in summer. Because these clouds influence the radiation budget primarily through their albedo, their diurnal cycle has an important influence on their radiative effectiveness.

The authors have analyzed the diurnal cycle in marine low-cloud fraction inferred from the International Satellite Cloud Climatology Project (ISCCP) dataset, after correcting for overlying clouds by using the assumption of random cloud overlap. The results have been compared with the diurnal cycle of low clouds at fixed ships and ships of opportunity. The diurnal cycles of ISCCP low clouds are in good agreement with surface observations of the diurnal cycle of low stratiform clouds almost everywhere. Analysis of the ISCCP data on a 2.5° × 2.5° grid shows that the largest diurnal range in low-cloud fraction occurs downwind in the mean flow, or westward and equatorward, of the subtropical maxima in low-cloud fraction. This is qualitatively consistent with control of the cloud amount by two competing processes in a partially decoupled cloud-topped planetary boundary layer: heating by solar radiation absorption and advection of moist boundary layer air. A radiative transfer code has been used to show that in eastern subtropical ocean regions, where the diurnal cycle of low clouds is large and the cloud has a small optical thickness, calculations with diurnally averaged cloud fraction overestimate total cloud radiative forcing by up to 3 W m−2 (16%) at the surface and 3 W m−2 (7%) at the top of the atmosphere compared with calculations that account for the diurnal cycle.

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Sungsu Park, Conway B. Leovy, and Margaret A. Rozendaal

Abstract

A new heuristic model of stratocumulus cloudiness in the inversion-capped marine boundary layer is developed and tested. The essential ingredient is a new method for predicting the statistical distribution of temperature and specific humidity at the inversion base under partially decoupled conditions along steady-state marine boundary layer (MBL) trajectories. MBL decoupling is parameterized as an increasing function of the height difference between the inversion base and lifting condensation level (LCL) of the mixed-layer air. Required inputs are sea surface temperature (SST), free air (above inversion) temperature and humidity, subsidence velocity, and mean boundary layer wind speed. Upstream boundary conditions must also be specified but have little influence at sufficient downstream distances (>2000 km).

The model is applied to the cold advection regime of the northeastern subtropical Pacific and to both warm and cold advection regimes of the eastern equatorial Pacific Ocean. The model is conceptually simple and avoids explicit calculation of several important physical processes. Nevertheless, it is at least qualitatively successful in predicting both the climatological mean properties and climate anomaly variations of MBL stratocumulus in both regions. These results suggest that, regardless of other properties, successful MBL stratocumulus models will need to accurately predict inversion base height and the LCL and they will have to account for downstream memory effects.

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