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This study examines the vertically resolved cloud measurements from the cloud-aerosol lidar with orthogonal polarization instrument on Aqua satellite from June 2006 through May 2007 to estimate the extent to which the mixed cloud-phase composition can vary according to the ambient temperature, an important concern for the uncertainty in calculating cloud radiative effects. At −20°C, the global average fraction of supercooled clouds in the total cloud population is found to be about 50% in the data period. Between −10 and −40°C, the fraction is smaller at lower temperatures. However, there are appreciable regional and temporal deviations from the global mean (> AE 20%) at the isotherm. In the analysis with coincident dust aerosol data from the same instrument, it appears that the variation in the supercooled cloud fraction is negatively correlated with the frequencies of dust aerosols at the −20°C isotherm. This result suggests a possibility that dust particles lifted to the cold cloud layer effectively glaciate supercooled clouds. Observations of radiative flux from the clouds and earth's radiant energy system instrument aboard Terra satellite, as well as radiative transfer model simulations, show that the 20% variation in the supercooled cloud fraction is quantitatively important in cloud radiative effects, especially in shortwave, which are 10 − 20 W m −2 for regions of mixed-phase clouds affected by dust. In particular, our results demonstrate that dust, by glaciating supercooled water, can decrease albedo, thus compensating for the increase in albedo due to the dust aerosols themselves. This has important implications for the determination of climate sensitivity.aerosol-cloud interaction | ice nucleation | mixed cloud phase | super cooled water | cloud radiative effect C old clouds that consist of mixed-phase particles, are ubiquitous in the Earth's upper and middle troposphere. Changes in the liquid/ice-phase composition in such clouds may significantly affect the radiative balance of the earth atmosphere system because the cloud radiative properties for both the shortwave (SW) and longwave (LW) such as cloud optical depth, single-scattering albedo, and emissivity vary according to the phase of cloud particles (1-3). It is therefore of fundamental importance to examine the variation in the cloud-phase composition for accurate calculations of cloud radiative effects.In fact, the cloud-phase composition in mixed-phase clouds is complicatedly affected by several factors other than temperature; e.g., ice nuclei (IN) aerosols. Most of climate models, however, have calculated the cloud-phase composition with limited sophistication, simply as a function of grid-mean temperature (4). For example, it has been typically assumed that clouds are composed entirely of ice and liquid particles below −40°C and above 0°C, respectively, of a mixture of ice and liquid phases between −40 and 0°C. The fraction of supercooled liquid particles within mixed-phase clouds is represented as a linear function of temperatures in genera...

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[1] Weekly cycles of the concentration of anthropogenic aerosols have been observed in many regions around the world. The phase and the magnitude of these cycles, however, vary greatly depending on region and season. In the present study the authors investigated important features of the weekly cycles of aerosol concentration and the covariations in meteorological conditions in major urban regions over east China, one of the most polluted areas in the world, in summertime during the period 2001-2005/2006. The PM10 (aerosol particulate matters of diameter < 10 mm) concentrations at 29 monitoring stations show significant weekly cycles with the largest values around midweek and smallest values in weekend. Accompanying the PM10 cycle, the meteorological variables also show notable and consistent weekly cycles. The wind speed in the lower troposphere is relatively small in the early part of the week and increases after about Wednesday. At the same time, the air temperature anomalies in low levels are positive and then become negative in the later part of the week. The authors hypothesize that the changes in the atmospheric circulation may be triggered by the accumulation of PM10 through diabatic heating of lower troposphere. During the early part of a week the anthropogenic aerosols are gradually accumulated in the lower troposphere. Around midweek, the accumulated aerosols could induce radiative heating, likely destabilizing the middle to lower troposphere and generating anomalously vertical air motion and thus resulting in stronger winds. The resulting circulation could promote ventilation to reduce aerosol concentrations in the boundary layer during the later part of the week. Corresponding to this cycle in anthropogenic aerosols the frequency of precipitation, particularly the light rain events, tends to be suppressed around midweek days through indirect aerosol effects. This is consistent with the observed anthropogenic weather cycles, i.e., more (less) solar radiation near surface, higher (lower) maximum temperature, larger (smaller) diurnal temperature range, and fewer (more) precipitation events in midweek days (weekend).

The variability of observed tropical cyclone (TC) activity (i.e., genesis, track, and landfall) in the western North Pacific (WNP) is examined in relation to the various categories of the Madden-Julian oscillation (MJO) during summer (June-September) for the period 1979-2004. The MJO categories are defined based on the empirical orthogonal function analysis of outgoing longwave radiation data.The number of TCs increases when the MJO-related convection center is located in the WNP. The axis of a preferable genesis region systematically shifts like a seesaw in response to changes in the large-scale environments associated with both the eastward and northward propagation of the MJO and the intraseasonal variability of the WNP subtropical high. Furthermore, the authors show that the density of TC tracks in each MJO category depends on the systematic shift in the main genesis regions at first order. Also, the shift is affected by the prevailing large-scale steering flows in each MJO category. When the MJO-related convection center is found in the equatorial Indian Ocean (the tropical WNP), a dense area of tracks migrates eastward (westward). The effects of extreme ENSO events and the variations occurring during ENSO neutral years are also examined.A statistical analysis of TC landfalls by MJO category is applied in seven selected subareas: the Philippines,