Aires, F, Pierre Gentine, Kirsten L Findell, Benjamin R Lintner, and Christopher Kerr, March 2014: Neural network-based sensitivity analysis of summertime convection over the continental US. Journal of Climate, 27(5), DOI:10.1175/JCLI-D-13-00161.1. Abstract
Although land-atmosphere coupling is thought to play a role in shaping the mean climate and its variability, it remains difficult to quantify precisely. The present study aims to isolate relationships between early morning surface turbulent fluxes partitioning (i.e., evaporative fraction, EF), and subsequent afternoon convective precipitation frequency and intensity. A general approach involving statistical relationships among input and output variables, known as Sensitivity Analysis (SA), is used to develop a reduced complexity meta-model of the linkage between EF and convective precipitation. Two additional quantities characterizing the early morning convective environment, convective triggering potential (CTP) and low-level humidity (HIlow) deficit, are included. The SA approach is applied to the North American Regional Reanalysis (NARR) for June-July-August (JJA) conditions over the entire continental United States, Mexico, and Central America domain. Five land-atmosphere coupling regimes are objectively characterized based on CTP, HIlow and EF. Two western regimes are largely atmospherically controlled, with a positive link to CTP and a negative link to HIlow. The other three regimes occupy Mexico and the eastern half of the domain and show positive links to EF and negative links to HIlow, suggesting that both surface fluxes and atmospheric humidity play a role in the triggering of rainfall in these regions. The regimes associated with high mean EF also tend to have high sensitivity of rainfall frequency to variations in EF. While these results may be sensitive to the choice of dataset, the approach can be applied across observational, reanalysis, and model datasets and thus represents a potentially powerful tool for inter-comparison and validation as well as to characterize land-atmosphere interactions regimes.
A high-resolution regional atmospheric model is used to simulate present-day western North Pacific (WNP) tropical cyclone (TC) activity and investigate the projected changes for the late 21st century. Compared to observations, the model can realistically simulate many basic features of the WNP TC activity climatology, such as the TC genesis location, track, and lifetime. A number of spatial and temporal features of observed TC interannual variability are captured, although observed variations in basin-wide TC number are not. A relatively well-simulated feature is the contrast of years when the Asian summer monsoon trough extends eastward (retreats westward), more (fewer) TCs form within the southeastern quadrant of the WNP, and the corresponding TC activity is above (below) normal over most parts of the WNP east of 125°E. Future projections with the Coupled Model Intercomparison Project 3 (CMIP3) A1B scenario show a weak tendency for decreases in the number of WNP TCs, and of increases in the more intense TCs; these simulated changes are significant at the 80% level. The present-day simulation of intensity is limited to storms of intensity less than about 55 m s-1. There is also a weak (80% significance level) tendency for projected WNP TC activity to shift poleward under global warming. A regional-scale feature is a projected increase of the TC activity north of Taiwan, which would imply an increase in TCs making landfall in North China, the Korean Peninsula and parts of Japan. However, given the weak statistical significance found for the simulated changes, an assessment of the robustness of such regional-scale projections will require further study.
Berg, Alexis, Kirsten L Findell, Benjamin R Lintner, Pierre Gentine, and Christopher Kerr, June 2013: Precipitation sensitivity to surface heat fluxes over North America in reanalysis and model data. Journal of Hydrometeorology, 14(3), DOI:10.1175/JHM-D-12-0111.1. Abstract
A new methodology for assessing the impact of surface heat fluxes on precipitation is applied to data from the North American Regional Reanalysis (NARR) and to output from the Geophysical Fluid Dynamics Laboratory’s model AM2.1. The method assesses the sensitivity of afternoon convective rainfall frequency and intensity to the late-morning partitioning of latent and sensible heating, quantified in terms of evaporative fraction (EF). Over North America, both NARR and AM2.1 indicate sensitivity of convective rainfall triggering to EF but no appreciable influence of EF on convective rainfall amounts. Functional relationships between the triggering feedback strength (TFS) metric and mean EF demonstrate the occurrence of stronger coupling for mean EF in the range of 0.6 to 0.8. To leading order, AM2.1 exhibits spatial distributions and seasonality of the EF impact on triggering resembling those seen in NARR: rainfall probability increases with higher EF over the Eastern US and Mexico and peaks in Northern Hemisphere summer. Over those regions, the impact of EF variability on afternoon rainfall triggering in summer can explain up to 50% of seasonal rainfall variability. However, the AM2.1 metrics also exhibit some features not present in NARR, e.g., strong coupling extending northwest from the central Great Plains into Canada. Sources of disagreement may include model hydroclimatic biases that affect the mean patterns and variability of surface flux partitioning, with EF variability typically much lower in NARR. Finally, we also discuss the consistency of our results with other assessments of land-precipitation coupling obtained from different methodologies.
Findell, Kirsten L., Pierre Gentine, Benjamin R Lintner, and Christopher Kerr, June 2011: Probability of afternoon precipitation in eastern United States and Mexico enhanced by high evaporation. Nature Geoscience, 4(7), DOI:10.1038/ngeo1174. Abstract
Moisture and heat fluxes from the land surface to the atmosphere form a critical nexus between surface hydrology and atmospheric processes, particularly those relevant to precipitation. Although current theory suggests that soil moisture generally has a positive impact on subsequent precipitation, individual studies have shown support both for and against this positive feedback. Broad assessment of the coupling between soil moisture and evapotranspiration, and evapotranspiration and precipitation, has been limited by a lack of large-scale observations. Quantification of the influence of evapotranspiration on precipitation remains particularly uncertain. Here, we develop and apply physically based, objective metrics for quantifying the impacts of surface evaporative and sensible heat fluxes on the frequency and intensity of convective rainfall during summer, using North American reanalysis data. We show that high evaporation enhances the probability of afternoon rainfall east of the Mississippi and in Mexico. Indeed, variations in surface fluxes lead to changes in afternoon rainfall probability of between 10 and 25% in these regions. The intensity of rainfall, by contrast, is largely insensitive to surface fluxes. We suggest that local surface fluxes represent an important trigger for convective rainfall in the eastern United States and Mexico during the summer, leading to a positive evaporation–precipitation feedback.
Orlanski, Isidoro, and Christopher Kerr, 2007: Project TERRA: A glimpse into the future of weather and climate modeling In High Resolution Numerical Modelling of the Atmosphere and Ocean, New York, NY, Springer-Verlag, 45-50. Abstract
One major challenge in obtaining useful numerical simulations of weather and climate is addressing the sensitivity of these simulations to the characteristics and distribution of clouds in the model(s). Latent heat release produced in clouds as a consequence of moist convection can dramatically affect the dynamics that govern the development of larger scale weather systems and storm tracks. Also, given the profound effects of cloud distribution on the radiative characteristics of the atmosphere, these interactions critically affect the models' climate and thus our conclusions regarding climate change.
A very high resolution global model has recently been run at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) to investigate the potential value of cloud-resolving numerical models to weather forecasts and climate simulations. Dubbed "Project TERRA", this experiment was conceived as an experimental 1-day simulation with GFDL's ZETAC model.
Jones, P, Christopher Kerr, and Richard S Hemler, 1995: Practical considerations in development of a parallel SKYHI general circulation model. Parallel Computing, 21(10), 1677-1694. Abstract PDF
We have developed a parallel version of the SKYHI atmospheric general circulation model. The new parallel model has been designed for shared and distributed memory machines that support data parallel, message passing or worksharing programming paradigms. The newly developed model has a framework that makes the code easier to understand, maintain, and modify, increasing the model's flexibility and scientific productivity. Numerous model changes are described (code design, programming models, language choice, data decomposition, communications, table lookups, memory management, and i/o) that were necessary to develop the model. The performance and verification of the model is described on several systems including a shared-memory machine with high-level worksharing and a distributed-memory system with a data parallel programming paradigm.
Kurihara, Yoshio, Christopher Kerr, and Morris A Bender, 1989: An improved numerical scheme to treat the open lateral boundary of a regional model. Monthly Weather Review, 117(12), 2714-2722. Abstract PDF
A numerical scheme proposed by Kurihara and Bender is modified so as to improve the behavior of open lateral boundaries of a regional model. In the new scheme, both the local values and the gradients of fields from a larger model are used to define the time-dependent reference values toward which the boundary gridpoint values of the regional model prediction are relaxed at each step of the model integration. Use of the gradients in the boundary forcing imposes constraints on the vorticity, divergence and baroclinicity fields for the regional model. The relaxation time of forcing is set to be short for the normal component of wind. For other variables, the relaxation time at a given boundary gridpoint depends on the wind direction at that gridpoint, with a minimum at a point of normal inflow and a maximum at a point of normal outflow. The forcing strength is reduced in the planetary boundary layer so that the boundary layer structure is determined mainly by the surface condition of the regional model. Also, a simple method to control the total mass in the regional model is described. Numerical results from 96-hour integrations with the improved scheme are compared with those from the previous scheme for the cases of the propagations of a wave and a vortex. The behavior of the model at the lateral boundary was noticeably improved with the use of the new scheme, while the solution in the interior domain was little affected by the scheme modification.