Intensification of precipitation extremes with warming in a cloud resolving model

A cloud-resolving model is used to investigate the effect of warming on high percentiles of precipitation (precipitation extremes) in the idealized setting of radiative-convective equilibrium. While this idealized setting does not allow for several factors that influence precipitation in the tropics, it does allow for an evaluation of the response of precipitation extremes to warming in simulations with resolved rather than parameterized convection. The methodology developed should also be applicable to less idealized simulations. Modeled precipitation extremes are found to increase in magnitude in response to an increase in sea surface temperature. A dry static energy budget is used to relate the changes in precipitation extremes to changes in atmospheric temperature, vertical velocity, and precipitation efficiency. To first order, the changes in precipitation extremes are captured by changes in the mean temperature structure of the atmosphere. Changes in vertical velocities play a secondary role and tend to weaken the strength of precipitation extremes, despite an intensification of updraft velocities in the upper troposphere. The influence of changes in condensate transports on precipitation extremes is quantified in terms of a precipitation efficiency; it does not change greatly with warming. Tropical precipitation extremes have previously been found to increase at a greater fractional rate than the amount of atmospheric water vapor in observations of present-day variability and in some climate model simulations with parameterized convection. But the fractional increases in precipitation extremes in the cloud-resolving simulations are comparable in magnitude to those in surface water vapor concentrations (owing to a partial cancellation between dynamical and thermodynamical changes), and are substantially less than the fractional increases in column water vapor.Texas Advanced Computing CenterNational Science Foundation (U.S.) (TeraGrid resources

Methodological entanglements in the field: Methods, transitions and transmissions

While much discussion of art practice within research and university contexts tends to draw from 'practice-led' or 'practice-based' research, those practices outside the visual arts that deploy art-related methods and techniques often sit uncomfortably within other disciplines and struggle to be accounted for within official university accountabilities. This situation creates a divide between visual art accountable practices and those that do not fit. It is the latter category we wish to explore. As ethnographic researchers within cultural studies and sociology, the process of making and thinking through art-based methods is an integral part of doing research. Through the interdisciplinary process we seek to explore overlaps between traditional and non-traditional modes of making, presenting and transmitting knowledge to audiences

A Simple Model of Climatological Rainfall and Vertical Motion Patterns over the Tropical Oceans

A simple model is developed that predicts climatological rainfall, vertical motion, and diabatic heating profiles over the tropical oceans given the sea surface temperature (SST), using statistical relationships deduced from the 40-yr ECMWF Re-Analysis (ERA-40). The model allows for two modes of variability in the vertical motion profiles: a shallow mode responsible for all “boundary layer” convergence between 850 hPa and the surface, and a deep mode with no boundary layer convergence. The model is based on the argument expressed in the authors’ companion paper that boundary layer convergence can be usefully viewed as a forcing on deep convection, not just a result thereof. The shallow mode is either specified from satellite observations or modeled using a simple mixed-layer model that has SST as well as 850-hPa geopotential height, winds, and temperature as boundary conditions. The deep-mode amplitude is empirically shown to be proportional to a simple measure of conditional instability in convecting regions, and is determined by the constraint that radiative cooling must balance adiabatic warming in subsidence regions. This two-mode model is tested against a reanalysis-derived dry static energy budget and in a reanalysis-independent framework based on satellite-derived surface convergence and using SST as a proxy for conditional instability. It can predict the observed annual mean and seasonal cycle of rainfall, vertical motion, and diabatic heating profiles across the tropical oceans with significantly more skill than optimized predictions using a thresholded linear relationship with SST. In most warm-ocean regions, significant rainfall only occurs in regions of monthly-mean boundary layer convergence. In such regions, deep-mode amplitude and rainfall increase linearly with SST, with an additional rainfall contribution from the shallow mode directly tied to boundary layer convergence. This second contribution is significant mainly in the east and central Pacific ITCZ, where it is responsible for that region’s “bottom-heavy” vertical-velocity, diabatic heating, and cloud profiles.United States Department of Energy. Atmospheric Radiation Measurement (ARM) Program (Grant DE-FG02-05ER63959)National Oceanic and Atmospheric Administration. Climate Prediction Program for the Americas (Grant NA06OAR4310055

On the Relationship between SST Gradients, Boundary Layer Winds, and Convergence over the Tropical Oceans

A linear mixed layer model that skillfully reproduces observed surface winds and convergence over the tropical oceans is used to examine the relative influence of boundary layer and free-tropospheric processes on the distribution of climatological surface winds and convergence. The semiempirical model assumes a subcloud-layer momentum force balance between pressure gradients, Coriolis acceleration, linearized friction, and downward momentum mixing, and it utilizes boundary conditions from the 40-yr ECMWF Re-Analysis (ERA-40). Observed pressure gradients are linearly decomposed into boundary layer (defined as the region below 850 hPa) and free-tropospheric components, and the surface winds and convergence associated with these components are computed. Results show that surface zonal winds are predominantly associated with a combination of free-tropospheric pressure gradients and downward momentum mixing, whereas the distribution of convergence is primarily due to boundary layer temperature gradients, which are closely related to SST gradients. The authors conclude that the climatological distribution of boundary layer convergence is primarily a function of the pattern of SST gradients and is better regarded as a cause rather than a consequence of deep convection.United States. Dept. of Energy (ARM DE-FG02-05ER63959)United States. National Oceanic and Atmospheric Administration (CPPA program grant Grant NA06OAR4310055

Importance of Laplacian of low-level warming for the response of precipitation to climate change over tropical oceans

AbstractSeveral physical mechanisms have been proposed for projected changes in mean precipitation in the tropics under climate warming. In particular, the “wet-get-wetter” mechanism describes an amplification of the pattern of precipitation in a moister atmosphere, and the “warmer-get-wetter” mechanism describes enhanced upward motion and precipitation in regions where the increase in SST exceeds the tropical-mean increase. Studies of the current climate have shown that surface convergence over the tropical oceans is largely driven by horizontal gradients of low-level temperature, but the influence of these gradients on the precipitation response under climate warming has received little attention. Here, a simple model is applied to give a decomposition of changes in precipitation over tropical oceans in twenty-first-century climate model projections. The wet-get-wetter mechanism and changes in surface convergence are found to be of widespread importance, whereas the warmer-get-wetter mechanism is primarily limited to negative anomalies in the tropical southern Pacific. Furthermore, surface convergence is linked to gradients of boundary layer temperature using an atmospheric mixed layer model. Changes in surface convergence are found to be strongly related to changes in the Laplacian of boundary layer virtual temperature, and, to a lesser extent, the Laplacian of SST. Taken together, these results suggest that a “Laplacian-of-warming” mechanism is of comparable importance to wet get wetter and warmer get wetter for the response of precipitation to climate change over tropical oceans.</jats:p

A model for the relationship between tropical precipitation and column water vapor

Several observational studies have shown a tight relationship between tropical precipitation and column-integrated water vapor. We show that the observed relationship in the tropics between column-integrated water vapor, precipitation, and its variance can be qualitatively reproduced by a simple and physically motivated two-layer model. It has previously been argued that features of this relationship could be explained by analogy with the theory of continuous phase transitions. Instead, our model explicitly assumes that the onset of precipitation is governed by a stability threshold involving boundary-layer water vapor. This allows us to explain the precipitation-humidity relationship over a broader range of water vapor values, and may explain the observed temperature dependence of the relationship