Laxmi Sushama

Precipitation Climatology in an Ensemble of CORDEX-Africa Regional Climate Simulations

AbstractAn ensemble of regional climate simulations is analyzed to evaluate the ability of 10 regional climate models (RCMs) and their ensemble average to simulate precipitation over Africa. All RCMs use a similar domain and spatial resolution of ~50 km and are driven by the ECMWF Interim Re-Analysis (ERA-Interim) (1989–2008). They constitute the first set of simulations in the Coordinated Regional Downscaling Experiment in Africa (CORDEX-Africa) project. Simulated precipitation is evaluated at a range of time scales, including seasonal means, and annual and diurnal cycles, against a number of detailed observational datasets. All RCMs simulate the seasonal mean and annual cycle quite accurately, although individual models can exhibit significant biases in some subregions and seasons. The multimodel average generally outperforms any individual simulation, showing biases of similar magnitude to differences across a number of observational datasets. Moreover, many of the RCMs significantly improve the precip...

Climate simulation over CORDEX Africa domain using the fifth-generation Canadian Regional Climate Model (CRCM5)

The new fifth-generation Regional Climate Model (CRCM5) was driven by ERA reanalyses for the period 1984–2008 over the African continent following the CORDEX experimental protocol. Overall the model succeeds in reproducing the main features of the geographical distribution and seasonal cycle of temperature and precipitation, the diurnal cycle of precipitation, and the West African Monsoon (WAM). Biases in surface temperature and precipitation are discussed in relation with some circulation defects noted in the simulation. In the African regions near the equator, the model successfully reproduces the double peak of rainfall due to the double passage of the tropical rainbelt, although it better simulates the magnitude and timing of the second peak of precipitation. CRCM5 captures the timing of the monsoon onset for the Sahel region but underestimates the magnitude of precipitation. The simulated diurnal cycle is quite well simulated for all of the regions, but is always somewhat in advance for the timing of rainfall peak. In boreal summer the CRCM5 simulation exhibits a weak cold bias over the Sahara and the maximum temperature is located too far south, resulting in a southward bias in the position of the Saharan Heat Low. The region of maximum ascent in the deep meridional circulation of the Hadley cell is well located in the CRCM5 simulation, but it is somewhat too narrow. The core of the African Easterly Jet is of the right strength and almost at the right height, but it is displayed slightly southward, as a consequence of the southward bias in the position of the Saharan Heat Low and the thermal wind relationship. These biases appear to be germane to the WAM rainfall band being narrower and not moving far enough northward, resulting in a dry bias in the Sahel.

Canadian RCM Projected Changes to Extreme Precipitation Characteristics over Canada

Changes to the intensity and frequency of hydroclimatic extremes can have significant impacts on sectors associated with water resources, and therefore it is relevant to assess their vulnerabilities in a changing climate. This study focuses on the assessment of projected changes to selected return levels of 1-, 2-, 3-, 5-, 7- and 10-day annual (April–September) maximum precipitation amounts, over Canada, using an ensemble of five 30-yr integrations each for current reference (1961–90) and future (2040–71) periods performed with the Canadian Regional Climate Model (CRCM); the future simulations correspond to the A2 Special Report on Emissions Scenarios (SRES) scenario. Two methods, the regional frequency analysis (RFA), which operates at the scale of statistically homogenous units of predefined climatic regions, with the possibility of downscaling to gridcell level, and the individual gridbox analysis (GBA), are used in this study, with the time-slice stationarity assumption. Validation of model simulated 20-, 50- and 100-yr return levels of single- and multiday precipitation extremes against those observed for the 1961–90 period using both the RFA and GBA methods suggest an underestimation of extreme events by the CRCM over most of Canada. The CRCM projected changes, realized with the RFA method at regional scale, to selected return levels for the future (2041–70) period, in comparison to the reference (1961–90) period, suggest statistically significant increases in event magnitudes for 7 out of 10 studied climatic regions. Though the results of the RFA and GBA methods at gridcell level suggest positive changes to studied return levels for most parts of Canada, the results corresponding to the 20-yr return period for the two methods agree better, while the agreement abates with increasing return periods, that is, 50 and 100 yr. It is expected that the increase in return levels of short and longer duration precipitation extremes will have severe implications for various water resource–related development and management activities.

Present climate and climate change over North America as simulated by the fifth-generation Canadian regional climate model

The fifth-generation Canadian Regional Climate Model (CRCM5) was used to dynamically downscale two Coupled Global Climate Model (CGCM) simulations of the transient climate change for the period 1950–2100, over North America, following the CORDEX protocol. The CRCM5 was driven by data from the CanESM2 and MPI-ESM-LR CGCM simulations, based on the historical (1850–2005) and future (2006–2100) RCP4.5 radiative forcing scenario. The results show that the CRCM5 simulations reproduce relatively well the current-climate North American regional climatic features, such as the temperature and precipitation multiannual means, annual cycles and temporal variability at daily scale. A cold bias was noted during the winter season over western and southern portions of the continent. CRCM5-simulated precipitation accumulations at daily temporal scale are much more realistic when compared with its driving CGCM simulations, especially in summer when small-scale driven convective precipitation has a large contribution over land. The CRCM5 climate projections imply a general warming over the continent in the 21st century, especially over the northern regions in winter. The winter warming is mostly contributed by the lower percentiles of daily temperatures, implying a reduction in the frequency and intensity of cold waves. A precipitation decrease is projected over Central America and an increase over the rest of the continent. For the average precipitation change in summer however there is little consensus between the simulations. Some of these differences can be attributed to the uncertainties in CGCM-projected changes in the position and strength of the Pacific Ocean subtropical high pressure.

Interactive lakes in the Canadian Regional Climate Model, version 5: the role of lakes in the regional climate of North America

ABSTRACT Two one-dimensional (1-D) column lake models have been coupled interactively with a developmental version of the Canadian Regional Climate Model. Multidecadal reanalyses-driven simulations with and without lakes revealed the systematic biases of the model and the impact of lakes on the simulated North American climate. The presence of lakes strongly influences the climate of the lake-rich region of the Canadian Shield. Due to their large thermal inertia, lakes act to dampen the diurnal and seasonal cycle of low-level air temperature. In late autumn and winter, ice-free lakes induce large sensible and latent heat fluxes, resulting in a strong enhancement of precipitation downstream of the Laurentian Great Lakes, which is referred to as the snow belt. The FLake (FL) and Hostetler (HL) lake models perform adequately for small subgrid-scale lakes and for large resolved lakes with shallow depth, located in temperate or warm climatic regions. Both lake models exhibit specific strengths and weaknesses. For example, HL simulates too rapid spring warming and too warm surface temperature, especially in large and deep lakes; FL tends to damp the diurnal cycle of surface temperature. An adaptation of 1-D lake models might be required for an adequate simulation of large and deep lakes.

An RCM projection of soil thermal and moisture regimes for North American permafrost zones

The fourth-generation Canadian Regional Climate Model (CRCM4) projected changes to the soil thermal and moisture regimes for the continuous, discontinuous, sporadic and isolated permafrost regions in North America, for the 2041–2070 period with respect to the 1961–1990 base period, for the SRES (Special Report on Emissions Scenarios) A2 scenario are presented. The projections indicate significant increase in the near-surface soil temperatures for all permafrost zones, with maximum warming for the continuous permafrost zone. No significant changes in the timing of minimum and maximum near-surface soil temperatures are projected by the CRCM4. However, the distributions of both minimum and maximum temperatures, at the surface and for the various near-surface soil layers, for the future climate, are significantly different from those for current climate. Intensification of the hydrologic cycle in future climate for the various permafrost zones is projected with important changes to the soil moisture regime, which are reflected in the reduction of the frozen soil moisture content, which in turn increases the deep drainage for all permafrost zones.

Baroclinic and barotropic aspects of the wind-driven ocean circulation ☆

Abstract The double-gyre circulation induced by a symmetric wind-stress pattern in a quasi-geostrophic model of the mid-latitude ocean is studied analytically and numerically. The model is discretized vertically by projection onto normal modes of the mean stratification. Within its horizontally rectangular domain, the numerical model captures the wind-driven circulation’s three dynamic regimes: (1) a basin-scale double-gyre circulation, cyclonic in the basin’s northern part and anticyclonic in the south, which is dominated by Sverdrup balance; (2) a swift western boundary current in either gyre, with dissipation most important near the coast and inertial balance further out; and (3) a strong recirculating dipole near the intersection of the western boundary with the symmetry line of zero wind-stress curl. The flow inside this stationary dipole is highly nonlinear, and equivalent-barotropic. An analytical solution to the potential vorticity equation with variable stratification describes the dipole, and fits well the full numerical model’s steady-state solutions. Changes in the numerical model’s solutions are investigated systematically as a function of changes in the strength of the wind stress τ and the Rossby radius of deformation LR. The main changes occur in the recirculation region, while the basin-scale gyres and the western boundary currents are affected but little. A unique symmetric dipole is observed for small τ, and agrees in its properties with the analytical solution. As τ increases, multiple asymmetric equilibria arise due to pitchfork bifurcation and are stable for large enough LR. The numerically obtained asymmetric equilibria also agree in their main properties with the analytical ones, as well as with the corresponding solutions of a shallow-water model. Increasing τ further results in two successive Hopf bifurcations, that lead to limit cycles with periods near 10 and 1 years, respectively. Both oscillatory instabilities have a strong baroclinic component. Above a certain threshold in τ the solutions become chaotic. Flow pattern evolution in this chaotic regime resembles qualitatively the circulation found in the Gulf Stream and Kuroshio current systems after their separation from the continent.

Analysis of streamflow characteristics over Northeastern Canada in a changing climate

An analysis of streamflow characteristics (i.e. mean annual and seasonal flows and extreme high and low flows) in current and future climates for 21 watersheds of north-east Canada covering mainly the province of Quebec is presented in this article. For the analysis, streamflows are derived from a 10-member ensemble of Canadian Regional Climate Model (CRCM) simulations, driven by the Canadian Global Climate Model simulations, of which five correspond to current 1970–1999 period, while the other five correspond to future 2041–2070 period. For developing projected changes of streamflow characteristics from current to future periods, two different approaches are used: one based on the concept of ensemble averaging while the other approach is based on merged samples of current and similarly future simulations following multiple comparison tests. Verification of the CRCM simulated streamflow characteristics for the 1970–1999 period suggests that the model simulated mean hydrographs and high flow characteristics compare well with those observed, while the model tends to underestimate low flow extremes. Results of projected changes to mean annual streamflow suggest statistically significant increases nearly all over the study domain, while those for seasonal streamflow show increases/decreases depending on the season. Two- and 5-year return levels of 15-day low flows are projected to increase significantly over most part of the study domain, though the changes are small in absolute terms. Based on the ensemble averaging approach, changes to 10- and 30-year return levels of high flows are not generally found significant. However, when a similar analysis is performed using longer samples, significant increases to high flow return levels are found mainly for northernmost watersheds. This study highlights the need for longer samples, particularly for extreme events in the development of robust projections.

Land‐atmosphere coupling over North America in CRCM5

Land-atmosphere coupling and its impact on extreme precipitation and temperature events over North America are studied using the fifth generation of the Canadian Regional Climate Model (CRCM5). To this effect, two 30 year long simulations, spanning the 1981–2010 period, with and without land-atmosphere coupling, have been performed with CRCM5, driven by the European Centre for Medium-Range Weather Forecasts reanalysis at the boundaries. In the coupled simulation, the soil moisture interacts freely with the atmosphere at each time step, while in the uncoupled simulation, soil moisture is replaced with its climatological value computed from the coupled simulation, thus suppressing the soil moisture-atmosphere interactions. Analyses of the coupled and uncoupled simulations, for the summer period, show strong soil moisture-temperature coupling over the Great Plains, consistent with previous studies. The maxima of soil moisture-precipitation coupling is more spread out and covers the semiarid regions of the western U.S. and parts of the Great Plains. However, the strength of soil moisture-precipitation coupling is found to be generally weaker than that of soil moisture-temperature coupling. The study clearly indicates that land-atmosphere coupling increases the interannual variability of the seasonal mean daily maximum temperature in the Great Plains. Land-atmosphere coupling is found to significantly modulate selected temperature extremes such as the number of hot days, frequency, and maximum duration of hot spells over the Great Plains. Results also suggest additional hot spots, where soil moisture modulates extreme events. These hot spots are located in the southeast U.S. for the hot days/hot spells and in the semiarid regions of the western U.S. for extreme wet spells. This study thus demonstrates that climatologically wet/dry regions can become hot spots of land-atmosphere coupling when the soil moisture decreases/increases to an intermediate transitional level where evapotranspiration becomes moisture sensitive and large enough to affect the climate.

On the Arctic near-surface permafrost and climate sensitivities to soil and snow model formulations in climate models

Abstract This study investigates the sensitivity of the Canadian Regional Climate Model (CRCM5) simulated near surface permafrost and its climate interactions to soil and snow formulations. In particular, sensitivities to the depth of the soil column, inclusion of organic soils and modified snow conductivity formulation are investigated. The impact of these modifications are first assessed in offline simulations performed with the Canadian Land Surface Scheme (CLASS), which is the land surface scheme used in CRCM5, when driven by ERA-40/ERA-Interim for the 1957–2008 period. Analysis of CLASS simulations shows major improvements in the simulated permafrost extent, particularly with a deeper soil column. Inclusion of organic soil decreased the summer ground heat flux and therefore the summer soil temperatures, leading to improvements in the simulated active layer thickness (ALT). The impact of the new snow thermal conductivity formulation is moderate compared to the effect of organic soils, but reduces the cold biases in winter soil temperatures. CRCM5 experiments revealed similar sensitivities to soil depth, organic soil and snow conductivity changes as with the offline simulations. Significant changes are noted in the land–atmosphere interactions, through modified energy and moisture partitioning at the surface resulting from the inclusion of the organic soils. The inter-annual variability of the ALT shows larger sensitivities to summer temperatures for mineral soil while experiments including organic soils show increased sensitivities to annual temperatures. The ALT trends in the CRCM5 are similar to the observed values, despite the overestimation of ALT associated with a warm bias in the CRCM5 climate.