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During the Marine Stratus/Stratocumulus Experiment, cloud and aerosol microphysics were measured in the eastern Pacific off the coast of northern California on board Department of Energy Gulfstream-1 in July 2005. Three cases with uniform aerosol concentration and minimal drizzle concentration were examined to study cloud microphysical behavior. For these three cases, the average droplet number concentration increased with increasing altitude, while the average interstitial aerosol concentration decreased with altitude. The data show enhanced growth of large droplets and spectral broadening in cloud parcels with low liquid water mixing ratio. Three mixing models, including inhomogeneous mixing, entity type entrainment mixing, and circulation mixing proposed in this study, are examined with regard to their influences on cloud microphysics. The observed cloud microphysical behavior is most consistent with the circulation mixing, which describes the mixing between cloud parcels with different lifting condensation levels during their circulations driven by evaporative and radiative cooling. The enhanced growth and spectrum broadening resulting from the circulation mixing reduce cloud albedo at the same liquid water path and facilitate the formation of precipitation embryos.

[1] In 2006, the ARM Mobile Facility (AMF) completed a 1-year deployment at Niamey, Niger, Africa, in support of the Radiative Atmospheric Divergence using ARM Mobile Facility, GERB data and AMMA Stations (RADAGAST) field campaign, which is the subject of this special issue. Observations from the AMF instrumentation are used to analyze the relationship between clouds, precipitation, and the thermodynamic environment in this rarely observed region and to evaluate the cloud fields in the National Center for Environmental Prediction Global Forecast System (GFS) initialization product. The 1-year deployment period enabled measurements in the dry and wet (monsoon) seasons and through the transitions in May and September, respectively. Cirrus clouds in the 10- to 15-km layer with modest monthly cloud fraction and mean depth of � 1k m are ubiquitous through the observing period as observed in other regions of the tropics. The monsoon season from May to September is characterized by convective clouds of varying depth that produce precipitation of varying intensity, as indicated by cloud radar. Peak surface rainfall is observed during August, and the largest daily rainfall rates are observed during the period from July to September. The lifting condensation level (LCL) is observed to decrease as the monsoon season progresses, and a strong correlation between the height of the LCL and precipitation is demonstrated. Cooling of the lower troposphere is implicated as the probable cause of the lowering of the LCL. Conversely, the amount of convective available potential energy is found to be poorly correlated with precipitation. As in other tropical regions, the physical height at which the zero-degree isotherm is observed corresponds to gradients in the thermodynamic profiles and a gradient in the profile of cloud occurrence. Comparisons with the GFS initialization data, which are derived from a number of sources including satellites, show some systematic biases when compared to AMF measurements. There is general correspondence between the locations of clouds and the profile of vertical velocity diagnosed by the GFS initialization early in the monsoon season, but vague correspondence thereafter. The relative humidity in the GFS initialization is too large above 10 km and too small in the monsoon layer near the surface, and it seriously underestimates the amount of cloud below 10 km during August, which is the height of the West African monsoon in Niamey.

becomes more discernable after 1 month. The model results suggest that the landatmosphere feedback is an important mechanism for summer precipitation over the Central United States. Vegetation growth and groundwater dynamics play a significant role in enhancing the persistence of intraseasonal precipitation in regional climate models. Their combined effects act to favor a stronger land-atmosphere feedback during the summer season. The simulated diurnal cycle of precipitation is improved by the WRF model with the augmented Noah LSM. Moreover, we found that the coupling between the soil moisture and the lifting condensation level (LCL) is enhanced by adding the two components to the WRF model. The impact of groundwater is significant when the soil moisture is relatively dry. This study suggests that incorporating vegetation and groundwater dynamics into a regional climate model would be especially beneficial for seasonal precipitation forecast in the transition zones.

[1] Large eddy simulations (LESs) of continental boundary layer clouds (BLCs) observed at the southern Great Plains (SGP) are used to study issues associated with the parameterization of sub-grid BLCs in large scale models. It is found that liquid water potential temperature θl and total specific humidity qt, which are often used as parameterization predictors in statistical cloud schemes, do not share the same probability distribution in the cloud layer with θl skewed to the left (negatively skewed) and qt skewed to the right (positively skewed). The skewness and kurtosis change substantially in time and space when the development of continental BLCs undergoes a distinct diurnal variation. The wide range of skewness and kurtosis of θl and qt can hardly be described by a single probability distribution function. To extend the application of the statistical cloud parameterization approach, this paper proposes an innovative cloud parameterization scheme that uses the boundary layer height and the lifting condensation level as the primary parameterization predictors. The LES results indicate that the probability distribution of these two quantities is relatively stable compared with that of θl and qt during the diurnal variation and nearly follows a Gaussian function. Verifications using LES output and the observations collected at the Atmospheric Radiation Measurement (ARM) Climate Research Facility (ARCF) SGP site indicate that the proposed scheme works well to represent continental BLCs.

[1] An integrated data set with simultaneous observations at the surface, from tethered balloons within the boundary layer and from rawinsonde ascents, was collected during the wet season experiment of the Large-Scale Biosphere-Atmosphere (LBA) Experiment in Amazonia during January and February of 1999 in support of the ground validation for the Tropical Rainfall Measuring Mission (TRMM). We analyze the surface diurnal cycles of temperature, humidity, lifting condensation level, equivalent potential temperature, and the surface fluxes of sensible and latent heat, ground heat flux and net radiation, for easterly and westerly wind regimes in the lower troposphere. During the easterly wind regimes, the diurnal evolution of mixing ratio shows that the flux of water vapor through cloud base exceeds the large surface evaporation. There is a trend toward a wetter and cooler subcloud layer as the rainy season progresses. Daytime surface Bowen ratio for this pasture site is about 0.4, and falls slightly as the rainy season progresses. Typically in the afternoon, evaporatively driven downdrafts from convective rainbands transform the boundary layer. The fall of equivalent potential temperature in the boundary layer is similar for both regimes, but the boundary layer cooling by convective events during the westerly regimes is reduced, because the subcloud layer is shallower on average. Tethersonde ascents through the edges of gust fronts show that subcloud air is first cooled and moistened by rainfall evaporation before the arrival of downdraft air at the surface. These measurements provide a detailed observational basis for the validation and improvement of parameterizations for shallow and deep convection in numerical forecast models.

Snow accumulation rates (centimeters of water equivalent per year) along 96°W and 131°W are discussed in terms of meteorological factors and topographic influences. The circulation in the lower and middle troposphere favors transport of moisture from the oceanic sources in the north and west, although there is some transport from the east. A consideration of the topographic features and the lifting condensation level of the air indicates that the combined topographic and atmospheric factors reasonably explain the greater accumulation rate along 96°W than along 131°W, as well as the decrease from the coast toward the interior.

Airborne measurements of NOx, NOy, CO, and O3 were conducted in the vicinity of a line of towering cumulus and cumulonimbus clouds (approximate altitude, 9 km) in south-central Oklahoma, on the afternoon of June 17, 1985. NOx, NOy, and CO concentrations in the upper tropospheric outflow region of these clouds were near environmental levels. Meteorological analyses show that these clouds were located above a cold front, which prevented the entry of air from the boundary layer directly below and near the cloud. Examination of the [NOy]/[CO] and [NOx]/[CO] ratios, as well as determination of the lifting condensation level and other thermodynamic analyses, indicated that the most likely origin of the air in the cloud outflow region was the layer immediately above the top of the boundary layer, which was quite clean. It appears that very little NOy was available to be scavenged by the clouds. The results for this case suggest that the large amount of vertical trace gas transport that has been noted in previous observations and model results cannot simply be extrapolated to all convective cells. The results also show the usefulness of CO as a conserved tracer.