High-resolution Atmospheric Model Research Articles

Using atmospheric observations to evaluate the spatiotemporal variability of CO<sub>2</sub> fluxes simulated by terrestrial biospheric models

Abstract. Terrestrial biospheric models (TBMs) are used to extrapolate local observations and process-level understanding of land-atmosphere carbon exchange to larger regions, and serve as predictive tools for examining carbon-climate interactions. Understanding the performance of TBMs is thus crucial to the carbon cycle and climate science communities. In this study, we present and assess an approach to evaluating the spatiotemporal patterns, rather than aggregated magnitudes, of net ecosystem exchange (NEE) simulated by TBMs using atmospheric CO2 measurements. The approach is based on statistical model selection implemented within a high-resolution atmospheric inverse model. Using synthetic data experiments, we find that current atmospheric observations are sensitive to the underlying spatiotemporal flux variability at sub-biome scales for a large portion of North America, and that atmospheric observations can therefore be used to evaluate simulated spatiotemporal flux patterns as well as to differentiate between multiple competing TBMs. Experiments using real atmospheric observations and four prototypical TBMs further confirm the applicability of the method, and demonstrate that the performance of TBMs in simulating the spatiotemporal patterns of NEE varies substantially across seasons, with best performance during the growing season and more limited skill during transition seasons. This result is consistent with previous work showing that the ability of TBMs to model flux magnitudes is also seasonally-dependent. Overall, the proposed approach provides a new avenue for evaluating TBM performance based on sub-biome-scale flux patterns, presenting an opportunity for assessing and informing model development using atmospheric observations.

Testing the Performance of Tropical Cyclone Genesis Indices in Future Climates Using the HiRAM Model

Abstract Tropical cyclone genesis indices (TCGIs) are functions of the large-scale environment that are designed to be proxies for the probability of tropical cyclone (TC) genesis. While the performance of TCGIs in the current climate can be assessed by direct comparison to TC observations, their ability to represent future TC activity based on projections of the large-scale environment cannot. Here the authors examine the performance of TCGIs in high-resolution atmospheric model simulations forced with sea surface temperatures (SST) of future, warmer climate scenarios. They investigate whether the TCGIs derived for the present climate can, when computed from large-scale fields taken from future climate simulations, capture the simulated global mean decreases in TC frequency. The TCGIs differ in their choice of environmental predictors, and several choices of predictors perform well in the present climate. However, some TCGIs that perform well in the present climate do not accurately reproduce the simulated future decrease in TC frequency. This decrease is captured when the humidity predictor is the column saturation deficit rather than relative humidity. Using saturation deficit with relative SST as the other thermodynamic predictor overpredicts the TC frequency decrease, while using potential intensity in place of relative SST as the other thermodynamic predictor gives a good prediction of the decrease’s magnitude. These positive results appear to depend on the spatial and seasonal patterns in the imposed SST changes; none of the indices capture correctly the frequency decrease in simulations with spatially uniform climate forcings, whether a globally uniform increase in SST of 2 K, or a doubling of CO2 with no change in SST.

Community-based flood damage assessment approach for lower West Rapti River basin in Nepal under the impact of climate change

The West Rapti River (WRR), one of the dynamic and economically important basins of Nepal, was focused in this study to understand the impact of climate change in localized scale. The adopted methodology was a community-based field survey followed by a hydrological modeling to estimate present and future flood damages for households and agriculture. Flood disaster occurred in 2007 was simulated and discussed. High-resolution atmospheric general circulation model’s precipitation outputs for emission scenario A1B were utilized with their bias corrections to obtain the precipitation patterns over lower WRR basin for Present (1979–2003) and Future (2075–2099) periods. A conceptual hydrologic model was employed to obtain the time series of daily river runoffs for the above-mentioned time durations followed by frequency analyses for probable flood discharges of 25- and 50-year return periods. Flood inundation simulations of 50-year return period events for Present and Future were carried out with the rainfall–runoff–inundation model followed by a flood damage assessment for household and agriculture based on the inundation simulation results, and field survey data over the target area and potential flood damages were discussed. Integration of hydrological modeling and socioeconomic approach to foresee potential flood damages was achieved, and the adopted damage assessment methodology was systematically explained. The total increments of household and agriculture damages due to climate change were estimated for Present as 1.80 and 1.95, and for Future, it was 2.40 and 2.27, respectively, compared to 2007 flood disaster. Future flood frequencies, intensities, and consequent damages in the area show a significant increment compared to the present situation despite limitations and uncertainties.

3-D model simulations of dynamical and microphysical interactions in pyroconvective clouds under idealized conditions

Abstract. Dynamical and microphysical processes in pyroconvective clouds in mid-latitude conditions are investigated using idealized three-dimensional simulations with the Active Tracer High resolution Atmospheric Model (ATHAM). A state-of-the-art two-moment microphysical scheme building upon a realistic parameterization of cloud condensation nuclei (CCN) activation has been implemented in order to study the influence of aerosol concentration on cloud development. The results show that aerosol concentration influences the formation of precipitation. For low aerosol concentrations (NCN = 200 cm−3), rain droplets are rapidly formed by autoconversion of cloud droplets. This also triggers the formation of large graupel and hail particles, resulting in an early onset of precipitation. With increasing aerosol concentration (NCN = 1000 cm−3 and NCN = 20 000 cm−3) the formation of rain droplets is delayed due to more but smaller cloud droplets. Therefore, the formation of ice crystals and snowflakes becomes more important for the eventual formation of graupel and hail, which is delayed at higher aerosol concentrations. This results in a delay of the onset of precipitation and a reduction of its intensity with increasing aerosol concentration. This study is the first detailed investigation of the interaction between cloud microphysics and the dynamics of a pyroconvective cloud using the combination of a high-resolution atmospheric model and a detailed microphysical scheme.

Variability of Tropical Cyclone Track Density in the North Atlantic: Observations and High-Resolution Simulations

Interannual–decadal variability of tropical cyclone (TC) track density over the North Atlantic (NA) between 1979 and 2008 is studied using observations and simulations with a 25-km-resolution version of the High Resolution Atmospheric Model (HiRAM) forced by observed sea surface temperatures (SSTs). The variability on decadal and interannual time scales is examined separately. On both time scales, a basinwide mode dominates, with the time series being related to variations in seasonal TC counts. On decadal time scales, this mode relates to SST contrasts between the tropical NA and the tropical northeast Pacific as well as the tropical South Atlantic, whereas on interannual time scales it is controlled by SSTs over the central–eastern equatorial Pacific and those over the tropical NA. The temporal evolution of the spatial distribution of track density is further investigated by normalizing the track density with seasonal TC counts. On decadal time scales, two modes emerge: one is an oscillation between track density over the U.S. East Coast and midlatitude ocean and that over the Gulf of Mexico and the Caribbean Sea and the other oscillates between low and middle latitudes. They might be driven by the preceding winter North Atlantic Oscillation and concurrent Atlantic meridional mode, respectively. On interannual time scales, two similar modes are present in observations but are not well separated in HiRAM simulations. Finally, the internal variability and predictability of TC track density are explored and discussed using HiRAM ensemble simulations. The results suggest that basinwide total TC counts/days are much more predictable than local TC occurrence, posing a serious challenge to the prediction and projection of regional TC threats, especially the U.S. landfall hurricanes.

Global-to-Regional Nested Grid Climate Simulations in the GFDL High Resolution Atmospheric Model

A two-way nested grid version of the Geophysical Fluid Dynamics Laboratory High Resolution Atmosphere Model (HiRAM) has been developed that uses simple methods for providing nested grid boundary conditions and mass-conserving nested-to-global communication. Nested grid simulations over the Maritime Continent and over North America were performed, each at two different resolutions: a 110-km mean grid cell width refined by a factor of 3, and a 50-km mean grid cell width refined by a factor of 2. Nested grid simulations were compared against uniform-resolution simulations, and against reanalyses, to determine the effect of grid nesting on both the modeled global climate and the simulation of small-scale features. Orographically forced precipitation was robustly found to be simulated with more detail and greater realism in a nested grid simulation compared with when only the coarse grids were simulated alone. Tropical precipitation biases were reduced in the Maritime Continent region when a nested grid was introduced. Both results were robust to changes in the nested grid parameterization tunings. In North America, cold-season orographic precipitation was improved by nesting, but precipitation biases in the central and eastern United States were little changed. Improving the resolution through nesting also allowed for more intense rainfall events, greater Kelvin wave activity, and stronger tropical cyclones. Nested grid boundary artifacts were more pronounced when a one-way, noninteractive nested grid was used.

Sensitivity of Tropical Cyclone Rainfall to Idealized Global-Scale Forcings*

Abstract Heavy rainfall and flooding associated with tropical cyclones (TCs) are responsible for a large number of fatalities and economic damage worldwide. Despite their large socioeconomic impacts, research into heavy rainfall and flooding associated with TCs has received limited attention to date and still represents a major challenge. The capability to adapt to future changes in heavy rainfall and flooding associated with TCs is inextricably linked to and informed by understanding of the sensitivity of TC rainfall to likely future forcing mechanisms. Here a set of idealized high-resolution atmospheric model experiments produced as part of the U.S. Climate Variability and Predictability (CLIVAR) Hurricane Working Group activity is used to examine TC response to idealized global-scale perturbations: the doubling of CO2, uniform 2-K increases in global sea surface temperature (SST), and their combined impact. As a preliminary but key step, daily rainfall patterns of composite TCs within climate model outputs are first compared and contrasted to the observational records. To assess similarities and differences across different regions in response to the warming scenarios, analyses are performed at the global and hemispheric scales and in six global TC ocean basins. The results indicate a reduction in TC daily precipitation rates in the doubling CO2 scenario (on the order of 5% globally) and an increase in TC rainfall rates associated with a uniform increase of 2 K in SST (both alone and in combination with CO2 doubling; on the order of 10%–20% globally).

Model Simulation and Projection of European Heat Waves in Present-Day and Future Climates

Abstract The synoptic behavior of present-day heat waves (HW) over Europe is studied using the GFDL high-resolution atmospheric model (HiRAM) with 50-km grid spacing. Three regions of enhanced and coherent temperature variability are identified over western Russia, eastern Europe, and western Europe. The simulated HW characteristics are compared with those derived from Climate Forecast System Reanalysis products. Composite charts for outstanding HW episodes resemble well-known recurrent circulation types. The HW region is overlain by a prominent upper-level anticyclone, which blocks the passage of synoptic-scale transients. The altered eddy vorticity transports in turn reinforce the anticyclone. The anticyclone is part of a planetary-scale wave train. The successive downstream development of this wave train is indicative of Rossby wave dispersion. Additional runs of HiRAM are conducted for the “time slices” of 2026–35 and 2086–95 in the climate scenario corresponding to representative concentration pathway 4.5 (RCP4.5). By the end of the twenty-first century, the average duration and frequency of HW in the three European sites are projected to increase by a factor of 1.4–2.0 and 2.2–4.5, respectively, from the present-day values. These changes can be reproduced by adding the mean shift between the present and future climatological temperatures to the daily fluctuations in the present-day simulation. The output from a continuous integration of a coupled general circulation model through the 1901–2100 period indicates a monotonic increase in severity, duration, and HW days during the twenty-first century.

Livestock methane emissions in the United States

The recent study by Miller et al. (1) provides a comprehensive, quantitative analysis of anthropogenic methane sources in the United States using atmospheric methane observations, spatial datasets, and a high-resolution atmospheric transport model. The authors conclude that “…emissions due to ruminants and manure are up to twice the magnitude of existing [i.e., US Environmental Protection Agency (US EPA); www.epa.gov/climatechange/ghgemissions/usinventoryreport.html] inventories” (1). The validity of this “top-down” approach can be verified by a relatively simple “bottom-up” method using current livestock inventories and enteric or manure methane emission factors. Animal scientists have generated large datasets of enteric methane production estimates per unit of feed or energy intake. Methanogenesis in the rumen is substrate-dependent and methane production data derived from studies using respiratory chambers (or other techniques) expressed on feed intake basis are representative of field emissions, if feed intake is known. We used the US Department of Agriculture-National Agricultural Statistics Service (USDA-NASS) livestock inventory estimates for 2013 (www.nass.usda.gov) and methane emission rates per unit of feed dry matter intake from two large datasets [Hristov et al. (2) and Hales et al. (3)] to estimate total methane emission from enteric fermentation for the United States. Total cattle inventories for 2013 were 89,299,600 head (including 29,295,200 beef cows, 9,219,900 dairy cows, and 13,351,700 cattle on feed, among other categories). Feed dry matter intake was estimated based on beef and dairy cattle requirements and ranged from 3.8 (calves 500 lbs), 11 (beef cows), and 22 kg/d (dairy cows). Methane production rates were estimated at 8–13 (cattle on feed) or 20 g/kg (all other categories; SD = 4) feed dry matter intake. Contributions to methane emissions by other ruminants or nonruminant herbivores (sheep, goats, wild ruminants, horses, and so forth) are small in the United States and were not included in this analysis. With the above assumptions, total methane emissions from enteric fermentation were estimated at 6.241 Tg/yr (minimum = 4.972 and maximum = 7.511), which is comparable to the current, 2011 US EPA estimates of 6.542 Tg/yr and was also independently verified using equations proposed by Moraes et al. (4). USDA-NASS inventories for cattle, swine (59,387,000 market swine and 5,834,000 breeding swine), and poultry (a total of 8.562 billion birds) and Intergovernmental Panel on Climate Change (5) manure methane emission factors [from 0.02 (most poultry categories), to 1 (beef cattle) and 53 (dairy cows) kilograms per head per year] were used to estimate emissions from manure management. Using this approach, manure emissions in the United States were estimated at 1.604 Tg/yr, which is lower than the 2011 US EPA estimate of 2.478 Tg/yr (with the latter figure perhaps being more representative of manure systems in the United States). Thus, the conclusions by Miller et al. (1) that US EPA estimates for livestock methane emissions are grossly underestimated appears to be unsubstantiated by the above “bottom-up” approach. There is a need for a detailed inventory of manure systems for all farm animal species and categories, which will help to more accurately estimate greenhouse gas (and ammonia) emissions from animal manure in the United States.

Development of local-scale high-resolution atmospheric dispersion model using large-eddy simulation. Part 4: turbulent flows and plume dispersion in an actual urban area

We have developed a local-scale high-resolution atmospheric dispersion model using large-eddy simulation (LOHDIM-LES) to assess the safety at nuclear facilities and to respond to emergency situations resulting from accidental or deliberate releases of radioactive materials (e.g., a terrorist attack in an urban area). In Part 1, the unsteady behavior of a plume dispersing over a flat terrain was successfully simulated. In Parts 2 and 3, LESs of turbulent flows and plume dispersion around an isolated building and in building arrays with different obstacle densities were performed, which showed the basic performance comparable to wind tunnel experimental technique. In this study, we apply the LES model to turbulent flows and plume dispersion in an actual urban area. Although some of the turbulence and dispersion characteristics are quantitatively different from the wind tunnel experimental data, the distribution patterns are generally similar to those of the experiments. It is concluded that our LES model simulates reasonably the unsteady behavior of turbulent flows and plume dispersion even for complex heterogeneous urban areas.

Hydrological Flood Simulation Using a Design Hyetograph Created from Extreme Weather Data of a High-Resolution Atmospheric General Circulation Model

To understand the characteristics of severe floods under global climate change, we created a design hyetograph for a 100-year return period. This incorporates a modified ranking method using the top 10 extreme rainfall events for present, near-future, and far-future periods. The rainfall data sets were projected with a general circulation model with high spatial and temporal resolution and used with a flood model to simulate the higher discharge peaks for the top 10 events of each term in a local watershed. The conventional-like ranking method, in which only a dimensionless shape is considered for the creation of a design hyetograph for a temporal distribution of rainfall, likely results in overestimates of discharge peaks because, even with a lower peak of rainfall intensity and a smaller amount of cumulative rainfall, the distribution shape is the only the factor for the design hyetograph. However, the modified ranking method, which considers amounts of cumulative rainfalls, provides a discharge peak from the design hyetograph less affected by a smaller cumulative rainfall depth for extreme rainfall. Furthermore, the effects of global climate change indicate that future discharge peaks will increase by up to three times of those of Present-term peaks, which may result in difficult flood control for the downstream river reaches.

Trends in Storm-Triggered Landslides over Southern California

AbstractChanges in storm-triggered landslide activity for Southern California in a future warming climate are estimated using an advanced, fully three-dimensional, process-based landslide model, the Scalable and Extensible Geofluid Modeling System for landslides (SEGMENT-Landslide). SEGMENT-Landslide is driven by extreme rainfall projections from the Geophysical Fluid Dynamics Laboratory High Resolution Atmospheric Model (GFDL-HIRAM). Landslide changes are derived from GFDL-HIRAM forcing for two periods: 1) the twentieth century (CNTRL) and 2) the twenty-first century under the moderate Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios A1B enhanced greenhouse gas emissions scenario (EGHG). Here, differences are calculated in landslide frequency and magnitude between the CNTRL and EGHG projections; kernel density estimation (KDE) is used to determine differences in projected landslide locations. This study also reveals that extreme precipitation events in Southern California are strongly correlated with several climate drivers and that GFDL-HIRAM simulates well the southern (relative to Aleutian synoptic systems) storm tracks in El Niño years and the rare (~27-yr recurrence period) hurricane-landfalling events. GFDL-HIRAM therefore can provide satisfactory projections of the geographical distribution, seasonal cycle, and interannual variability of future extreme precipitation events (>50 mm) that have possible landslide consequences for Southern California. Although relatively infrequent, extreme precipitation events contribute most of the annual total precipitation in Southern California. Two findings of this study have major implications for Southern California. First is a possible increase in landslide frequency and areal distribution during the twenty-first century. Second, the KDE reveals three clusters in both the CNTRL and EGHG model mean scarp positions, with a future eastward (inland) shift of ~0.5° and a northward shift of ~1°. These findings suggest that previously stable areas might become susceptible to storm-triggered landslides in the twenty-first century.

The terraced thermal contrast among the Tibetan Plateau, the East Asian plain, and the western North Pacific and its impacts on the seasonal transition of East Asian climate

The heating sources over the Tibetan Plateau (TP), the East Asian plain, and the western North Pacific (WNP) form a terraced thermal contrast in the west-east direction. Over East Asia and the WNP, this zonal thermal contrast contributes as high as 45 % to the seasonal variance based on the EOF analysis and exerts a significant impact on the seasonal transition of the East Asian climate through the enhancement of the year-round southerly to the southeast of the TP in late March and early April. This effect is investigated in this study using a high-resolution regional atmospheric model by doubling the surface sensible heat flux, respectively, over the TP, the East Asian plain, and the WNP in three sensitivity experiments. Comparisons among the experiments reveal that doubling the surface sensible heat flux over the WNP has little upstream response over East Asia. The increased zonal thermal contrast between the TP and the East Asian plain due to doubled heat flux over the TP would induce anomalous northerly over the region with year-round southerly to the southeast of the TP and weaken its seasonal enhancement. Doubling the surface sensible heat flux over the East Asian plain decreases the zonal thermal contrast and leads to southerly anomaly over the region with year-round southerly to the southeast of the TP and South China, which is favorable for the enhancement of the year-round southerly and its eastward extension.

Model Projections of the Changes in Atmospheric Circulation and Surface Climate over North America, the North Atlantic, and Europe in the Twenty-First Century

The impacts of climate change on the North America–North Atlantic–Europe sector are studied using a coupled general circulation model: the Climate Model, version 3 (CM3) and a high-resolution atmosphere-only model, the High Resolution Atmospheric Model (HiRAM)—both developed at the Geophysical Fluid Dynamics Laboratory. The CM3 experiment is conducted under two climate change scenarios for the 1860–2100 period. The sea surface temperature (SST) forcing prescribed in the “time slice” integrations with HiRAM is derived from observations for the 1979–2008 period and projection by CM3 for the 2086–95 period. The wintertime response in the late twenty-first century is characterized by an enhancement of the positive phase of the North Atlantic Oscillation in sea level pressure (SLP) and poleward and eastward displacements of the Atlantic jet stream and storm track. The forcing pattern due to eddy vorticity fluxes in the perturbed storm track matches well with the response pattern of the SLP field in the late twenty-first century. The model results suggest that the above circulation changes are linked to the gradient of the altered SST forcing in the North Atlantic. In summer, the projected enhancement of convection over the eastern tropical Pacific is accompanied by a wave train spanning the North America–North Atlantic–Europe sector. This quasi-stationary circulation pattern is associated with diminished storm track activity at 40°–50°N and an eddy forcing pattern similar to the summertime SLP response in the late twenty-first century.

Observed versus GCM-Generated Local Tropical Cyclone Frequency: Comparisons Using a Spatial Lattice

Abstract Of broad scientific and public interest is the reliability of global climate models (GCMs) to simulate future regional and local tropical cyclone (TC) occurrences. Atmospheric GCMs are now able to generate vortices resembling actual TCs, but questions remain about their fidelity to observed TCs. Here the authors demonstrate a spatial lattice approach for comparing actual with simulated TC occurrences regionally using observed TCs from the International Best Track Archive for Climate Stewardship (IBTrACS) dataset and GCM-generated TCs from the Geophysical Fluid Dynamics Laboratory (GFDL) High Resolution Atmospheric Model (HiRAM) and Florida State University (FSU) Center for Ocean–Atmospheric Prediction Studies (COAPS) model over the common period 1982–2008. Results show that the spatial distribution of TCs generated by the GFDL model compares well with observations globally, although there are areas of over- and underprediction, particularly in parts of the Pacific Ocean. Difference maps using the spatial lattice highlight these discrepancies. Additionally, comparisons focusing on the North Atlantic Ocean basin are made. Results confirm a large area of overprediction by the FSU COAPS model in the south-central portion of the basin. Relevant to projections of future U.S. hurricane activity is the fact that both models underpredict TC activity in the Gulf of Mexico.

Microwave Radiometric Remote Sensing of Volcanic Ash Clouds From Space: Model and Data Analysis

The potential of satellite passive microwave sensors to provide quantitative information about near-source volcanic ash cloud parameters is assessed. To this aim, ground-based microwave weather radar and spaceborne microwave radiometer observations are used together with forward-model simulations. The latter are based on 2-D simulations with the numerical plume model Active Tracer High-Resolution Atmospheric Model (ATHAM), in conjunction with the radiative transfer model Satellite Data Simulator Unit (SDSU) that is based on the deltaEddington approximation and includes Mie scattering. The study area is the Icelandic subglacial volcanic region. The analyzed case study is that of the Grimsvotn eruption in May 2011. ATHAM input parameters are adjusted using available ground data, and sensitivity tests are conducted to investigate the observed brightness temperatures and their variance. The tests are based on the variation of environmental conditions like the terrain emissivity, water vapor, and ice in the volcanic plume. Quantitative correlation analysis between ATHAM/SDSU forward-model columnar content simulations and available microwave radiometric brightness temperature measurements, derived from the Special Sensor Microwave Imager/Sounder (SSMIS), are encouraging in terms of both dynamic range and correlation coefficient. The correlation coefficients are found to vary from -0.37 to -0.63 for SSMIS channels from 91 to 183 ± 1 GHz, respectively. The larger sensitivity of the brightness temperature at 183 ± 1 GHz to the columnar content, with respect to other channels, allowed us to consider this channel as the basis for a model-based polynomial relationship of volcanic plume height as a function of the measured SSMIS brightness temperature.

A modelling investigation into lake-breeze development and convection triggering in the Nam Co Lake basin, Tibetan Plateau

This paper uses the cloud resolving Active Tracer High-resolution Atmospheric Model coupled to the interactive surface model Hybrid in order to investigate the diurnal development of a lake-breeze system at the Nam Co Lake on the Tibetan Plateau. Simulations with several background wind speeds are conducted, and the interaction of the lake breeze with topography and background wind in triggering moist and deep convection is studied. The model is able to adequately simulate the systems most important dynamical features such as turbulent surface fluxes and the development of a lake breeze for the different wind conditions. We identify two different mechanisms for convection triggering that are dependent on the direction of the background wind: triggering over topography, when the background wind and the lake breeze have the same flow direction, and triggering due to convergence between the lake-breeze front and the background wind. Our research also suggests that precipitation measurements at the centre of the basins on the Tibetan Plateau are not representative for the basin as a whole as precipitation is expected to occur mainly in the vicinity of the topography.

A Hierarchical Evaluation of Regional Climate Simulations

Global climate models (GCMs) are the primary tools for predicting the evolution of the climate system. Through decades of development, GCMs have demonstrated useful skill in simulating climate at continental to global scales. However, large uncertainties remain in projecting climate change at regional scales, which limits the ability of scientists to help decision makers in cities and communities strategize how best to adapt to and mitigate climate change.

Sensitivity of Limiting Hurricane Intensity to SST in the Atlantic from Observations and GCMs

Abstract A statistical model for the intensity of the strongest hurricanes has been developed and a new methodology introduced for estimating the sensitivity of the strongest hurricanes to changes in sea surface temperature. Here, the authors use this methodology on observed hurricanes and hurricanes generated from two global climate models (GCMs). Hurricanes over the North Atlantic Ocean during the period 1981–2010 show a sensitivity of 7.9 ± 1.19 m s−1 K−1 (standard error; SE) when over seas warmer than 25°C. In contrast, hurricanes over the same region and period generated from the GFDL High Resolution Atmospheric Model (HiRAM) show a significantly lower sensitivity with the highest at 1.8 ± 0.42 m s−1 K−1 (SE). Similar weaker sensitivity is found using hurricanes generated from the Florida State University Center for Ocean–Atmospheric Prediction Studies (FSU-COAPS) model with the highest at 2.9 ± 2.64 m s−1 K−1 (SE). A statistical refinement of HiRAM-generated hurricane intensities heightens the sensitivity to a maximum of 6.9 ± 3.33 m s−1 K−1 (SE), but the increase is offset by additional uncertainty associated with the refinement. Results suggest that the caution that should be exercised when interpreting GCM scenarios of future hurricane intensity stems from the low sensitivity of limiting GCM-generated hurricane intensity to ocean temperature.

Analysis of Vertical Turbulent Heat Flux Limit in Stable Conditions with a Local Equilibrium, Turbulence Closure Model

Assuming that the vertical turbulent heat flux vanishes at extremely stable conditions, one should expect its maximal absolute value to occur somewhere at moderate stability, between a neutral and extremely stable equilibrium. Consequently, in some situations duality of solutions may be encountered (e.g. two different values of temperature difference associated with the same values of heat flux and wind speed). A quantitative analysis of this feature with a local equilibrium Reynolds-stress model is presented. The fixed-wind / fixed-shear maximum has been identified both in the bulk and in single-point flux–gradient relationships (that is, in the vertical temperature gradient and wind-shear parameter domain). The value of the Richardson number corresponding to this maximum is derived from the model equations. To study the possible feedback in strongly stable conditions, weak and intense cooling scenarios have been simulated with a one-dimensional numerical, high-resolution atmospheric boundary-layer model. Despite the rapid cooling, flow decoupling at the surface has not been observed; instead, a stability-limited heat flux is maintained, with a gradual increase of the Richardson number towards the top of the turbulent layer, with some signs of oscillatory behaviour at intermediate heights. Vertical changes of wind shear and the Brunt–Väisälä frequency display a remarkably non-monotonic character, with some signs of a gradually developing instability.