ABSTRACT
Convective cloud formation and evolution strongly depend on environmental temperature and humidity profiles. The forming clouds change the profiles that created them by redistributing heat and moisture. Here we show that the evolution of the field's thermodynamic properties depends heavily on the concentration of aerosol, liquid or solid particles suspended in the atmosphere. Under polluted conditions, rain formation is suppressed and the non-precipitating clouds act to warm the lower part of the cloudy layer (where there is net condensation) and cool and moisten the upper part of the cloudy layer (where there is net evaporation), thereby destabilizing the layer. Under clean conditions, precipitation causes net warming of the cloudy layer and net cooling of the sub-cloud layer (driven by rain evaporation), which together act to stabilize the atmosphere with time. Previous studies have examined different aspects of the effects of clouds on their environment. Here, we offer a complete analysis of the cloudy atmosphere, spanning the aerosol effect from instability-consumption to enhancement, below, inside and above warm clouds, showing the temporal evolution of the effects. We propose a direct measure for the magnitude and sign of the aerosol effect on thermodynamic instability.
ABSTRACT
The cycling of atmospheric aerosols through clouds can change their chemical and physical properties and thus modify how aerosols affect cloud microphysics and, subsequently, precipitation and climate. Current knowledge about aerosol processing by clouds is rather limited to chemical reactions within water droplets in warm low-altitude clouds. However, in cold high-altitude cirrus clouds and anvils of high convective clouds in the tropics and midlatitudes, humidified aerosols freeze to form ice, which upon exposure to subsaturation conditions with respect to ice can sublimate, leaving behind residual modified aerosols. This freeze-drying process can occur in various types of clouds. Here we simulate an atmospheric freeze-drying cycle of aerosols in laboratory experiments using proxies for atmospheric aerosols. We find that aerosols that contain organic material that undergo such a process can form highly porous aerosol particles with a larger diameter and a lower density than the initial homogeneous aerosol. We attribute this morphology change to phase separation upon freezing followed by a glass transition of the organic material that can preserve a porous structure after ice sublimation. A porous structure may explain the previously observed enhancement in ice nucleation efficiency of glassy organic particles. We find that highly porous aerosol particles scatter solar light less efficiently than nonporous aerosol particles. Using a combination of satellite and radiosonde data, we show that highly porous aerosol formation can readily occur in highly convective clouds, which are widespread in the tropics and midlatitudes. These observations may have implications for subsequent cloud formation cycles and aerosol albedo near cloud edges.
ABSTRACT
BACKGROUND: Obstructive sleep apnea syndrome (OSAS) is a common disorder that affects 2% to 9% of the population. Health care policy makers have noted increased referrals for sleep studies. OBJECTIVE: In this article, the authors conduct a cost-effectiveness analysis to determine the optimal technology for the diagnosis of OSAS using polysomnography (PSG) or partial sleep monitoring (PSM). DESIGN: The target population was a hypothetical cohort of patients suspected of having OSAS. A 2-level decision tree wasformulated that reflects all possible steps of OSAS diagnosis and therapy. The method represents a comprehensive strategy to determine which of the 2 systems-PSG or PSM-has cost advantages. The financial and operational aspects of OSAS diagnosis and therapy were analyzed. A sensitivity analysis was performed over all uncertain parameters (i.e., diagnostic agreement, data loss, and referral to therapy). RESULTS: Unattended at-home sleep monitoring was the most expensive method. The combination of 1:2 PSG and attended PSM strategy was the optimal strategy with respect tofinancing and operations. Compared to the PSG-only strategy, this combination may lead to a 10% reduction of the annual expenditure. CONCLUSION: This study provides proof of concept (under a wide range of sensitivity assumptions) that the cost of sleep study techniques can be modeled. It rejects the assumption that athome portable sleep monitoring is cost advantageous. The combination of PSG and attended PSM OSAS is the most cost-effective approach to sleep evaluation.
