Where do winds come from? A new theory on how water vapor condensation influences atmospheric pressure and dynamics

Phase transitions of atmospheric water play a ubiquitous role in the Earth's climate system, but their direct impact on atmospheric dynamics has escaped wide attention. Here we examine and advance a theory as to how condensation influences atmospheric pressure through the mass removal of water from the gas phase with a simultaneous account of the latent heat release. Building from the fundamental physical principles we show that condensation is associated with a decline in air pressure in the lower atmosphere. This decline occurs up to a certain height, which ranges from 3 to 4 km for surface temperatures from 10 to 30 deg C. We then estimate the horizontal pressure differences associated with water vapor condensation and find that these are comparable in magnitude with the pressure differences driving observed circulation patterns. The water vapor delivered to the atmosphere via evaporation represents a store of potential energy available to accelerate air and thus drive winds. Our estimates suggest that the global mean power at which this potential energy is released by condensation is around one per cent of the global solar power -- this is similar to the known stationary dissipative power of general atmospheric circulation. We conclude that condensation and evaporation merit attention as major, if previously overlooked, factors in driving atmospheric dynamics

Comment on "The Tropospheric Land-Sea Warming Contrast as the Driver of Tropical Sea Level Pressure Changes" by Bayr and Dommenget

T Bayr and D Dommenget [J. Climate 26 (2013) 1387] proposed a model of temperature-driven air redistribution to quantify the ratio between changes of sea level pressure $p_s$ and mean tropospheric temperature $T_a$ in the tropics. This model assumes that the height of the tropical troposphere is isobaric. Here problems with this model are identified. A revised relationship between $p_s$ and $T_a$ is derived governed by two parameters -- the isobaric and isothermal heights -- rather than just one. Further insight is provided by the model of R S Lindzen and S Nigam [J. Atmos. Sci. 44 (1987) 2418], which was the first to use the concept of isobaric height to relate tropical $p_s$ to air temperature, and did this by assuming that isobaric height is always around 3 km and isothermal height is likewise near constant. Observational data, presented here, show that neither of these heights is spatially universal nor do their mean values match previous assumptions. Analyses show that the ratio of the long-term changes in $p_s$ and $T_a$ associated with land-sea temperature contrasts in a warming climate -- the focus of Bayr and Dommenget [2013] -- is in fact determined by the corresponding ratio of spatial differences in the annual mean $p_s$ and $T_a$. The latter ratio, reflecting lower pressure at higher temperature in the tropics, is dominated by meridional pressure and temperature differences rather than by land-sea contrasts. Considerations of isobaric heights are shown to be unable to predict either spatial or temporal variation in $p_s$. As noted by Bayr and Dommenget [2013], the role of moisture dynamics in generating sea level pressure variation remains in need of further theoretical investigations.Comment: 26 pages, 11 figures. arXiv admin note: text overlap with arXiv:1404.101

Heat engines and heat pumps in a hydrostatic atmosphere: How surface pressure and temperature constrain wind power output and circulation cell size

The kinetic energy budget of the atmosphere's meridional circulation cells is analytically assessed. In the upper atmosphere kinetic energy generation grows with increasing surface temperature difference \$\Delta T_s\$ between the cold and warm ends of a circulation cell; in the lower atmosphere it declines. A requirement that kinetic energy generation is positive in the lower atmosphere limits the poleward cell extension \$L\$ of Hadley cells via a relationship between \$\Delta T_s\$ and surface pressure difference \$\Delta p_s\$: an upper limit exists when \$\Delta p_s\$ does not grow with increasing \$\Delta T_s\$. This pattern is demonstrated here using monthly data from MERRA re-analysis. Kinetic energy generation along air streamlines in the boundary layer does not exceed \$40\$~J~mol\$^{-1}\$; it declines with growing \$L\$ and reaches zero for the largest observed \$L\$ at 2~km height. The limited meridional cell size necessitates the appearance of heat pumps -- circulation cells with negative work output where the low-level air moves towards colder areas. These cells consume the positive work output of the heat engines -- cells where the low-level air moves towards the warmer areas -- and can in theory drive the global efficiency of atmospheric circulation down to zero. Relative contributions of \$\Delta p_s\$ and \$\Delta T_s\$ to kinetic energy generation are evaluated: \$\Delta T_s\$ dominates in the upper atmosphere, while \$\Delta p_s\$ dominates in the lower. Analysis and empirical evidence indicate that the net kinetic power output on Earth is dominated by surface pressure gradients, with minor net kinetic energy generation in the upper atmosphere. The role of condensation in generating surface pressure gradients is discussed.Comment: 26 pages, 9 figures, 2 tables; re-organized presentation, more discussion and a new figure (Fig. 4) added; in Fig. 3 the previously invisible dots (observations) can now be see

REDD at the crossroads? The opportunities and challenges of REDD for conservation and human welfare in South West Uganda

Reducing Emissions from Deforestation and forest Degradation (REDD) in the tropics could slow climate change while contributing to biodiversity conservation and to improvement of people’s livelihoods. In this study we assessed the opportunities and challenges of implementing REDD in South West (SW) Uganda. We consulted key stakeholders and reviewed regional literature particularly focusing on the opportunities for conservation and human welfare benefits. We structured our study using the Simpson and Vira (2010) framework for assessing policy interventions. The leading drivers of forest loss and degradation include escalating timber trade, fuel-wood extraction and agricultural expansion. Generally, local stakeholders had limited awareness of REDD, and local expectations appeared un-realistically high. Mechanisms for allocating and administering REDD payments remained unknown. However, Civil Society Organisations appeared the most popular option to manage REDD funding as government agencies had limited credibility. For REDD to succeed, the challenges we have highlighted will need to be addressed: key to success will be improvements in foundational knowledge, enabling institutions and social conditions. Our results have implications for potential REDD activities around the world which face similar challenges.This work was supported by a grant from the John D. and Catherine T. MacArthur Foundation to Mbarara University of Science and Technology.This is the author accepted manuscript. The final version is available from Inderscience via http://dx.doi.org/10.1504/IJESD.2015.07013

The water budget of a hurricane as dependent on its movement

Despite the dangers associated with tropical cyclones and their rainfall, the origins of storm moisture remains unclear. Existing studies have focused on the region 40-400 km from the cyclone center. It is known that the rainfall within this area cannot be explained by local processes alone but requires imported moisture. Nonetheless, the dynamics of this imported moisture appears unknown. Here, considering a region up to three thousand kilometers from storm center, we analyze precipitation, atmospheric moisture and movement velocities for North Atlantic hurricanes. Our findings indicate that even over such large areas a hurricane's rainfall cannot be accounted for by concurrent evaporation. We propose instead that a hurricane consumes pre-existing atmospheric water vapor as it moves. The propagation velocity of the cyclone, i.e. the difference between its movement velocity and the mean velocity of the surrounding air (steering flow), determines the water vapor budget. Water vapor available to the hurricane through its movement makes the hurricane self-sufficient at about 700 km from the hurricane center obviating the need to concentrate moisture from greater distances. Such hurricanes leave a dry wake, whereby rainfall is suppressed by up to 40 per cent compared to its long-term mean. The inner radius of this dry footprint approximately coincides with the radius of hurricane self-sufficiency with respect to water vapor. We discuss how Carnot efficiency considerations do not constrain the power of such open systems that deplete the pre-existing moisture. Our findings emphasize the incompletely understood role and importance of atmospheric moisture supplies, condensation and precipitation in hurricane dynamics.Comment: 38 pages, 17 figures, 1 Table; extended analyses: available E/P ratios reviewed and explained (Table 1); rainfall and moisture distributions 3 days before and after hurricanes, propagation velocity and its relationship to radial velocity; efficiency for non-steady hurricanes; hurricane motion and rainfall asymmetries discusse

Hope for Bohemian ecologists – comments on “A possible role of social activity to explain differences in publication output among ecologists?” by Tomáš Grim, Oikos 2008

No abstract available

The equations of motion for moist atmospheric air

How phase transitions affect the motion of moist atmospheric air remains controversial. In the early 2000s two distinct differential equations of motion were proposed. Besides their contrasting formulations for the acceleration of condensate, the equations differ concerning the presence/absence of a term equal to the rate of phase transitions multiplied by the difference in velocity between condensate and air. This term was interpreted in the literature as the "reactive motion" associated with condensation. The reasoning behind this "reactive motion" was that when water vapor condenses and droplets begin to fall the remaining gas must move upwards to conserve momentum. Here we show that the two contrasting formulations imply distinct assumptions about how gaseous air and condensate particles interact. We show that these assumptions cannot be simultaneously applicable to condensation and evaporation. "Reactive motion" leading to an upward acceleration of air during condensation does not exist. The "reactive motion" term can be justified for evaporation only; it describes the downward acceleration of air. We emphasize the difference between the equations of motion (i.e., equations constraining velocity) and those constraining momentum (i.e., equations of motion and continuity combined). We show that, owing to the imprecise nature of the continuity equations, consideration of total momentum can be misleading and that this led to the "reactive motion" controversy. Finally, we provide a revised and generally applicable equation for the motion of moist air.Comment: 11 pages, two figure