Verification of the regional atmospheric model CCLM v5.0 with conventional data and lidar measurements in Antarctica

The nonhydrostatic regional climate model CCLM was used for a long-term hindcast run (2002–2016) for the Weddell Sea region with resolutions of 15 and 5 km and two different turbulence parametrizations. CCLM was nested in ERA-Interim data and used in forecast mode (suite of consecutive 30 h long simulations with 6 h spin-up). We prescribed the sea ice concentration from satellite data and used a thermodynamic sea ice model. The performance of the model was evaluated in terms of temperature and wind using data from Antarctic stations, automatic weather stations (AWSs), an operational forecast model and reanalyses data, and lidar wind profiles. For the reference run we found a warm bias for the near-surface temperature over the Antarctic Plateau. This bias was removed in the second run by adjusting the turbulence parametrization, which results in a more realistic representation of the surface inversion over the plateau but resulted in a negative bias for some coastal regions. A comparison with measurements over the sea ice of the Weddell Sea by three AWS buoys for 1 year showed small biases for temperature around ±1 K and for wind speed of 1 m s−1. Comparisons of radio soundings showed a model bias around 0 and a RMSE of 1–2 K for temperature and 3–4 m s−1 for wind speed. The comparison of CCLM simulations at resolutions down to 1 km with wind data from Doppler lidar measurements during December 2015 and January 2016 yielded almost no bias in wind speed and a RMSE of ca. 2 m s−1. Overall CCLM shows a good representation of temperature and wind for the Weddell Sea region. Based on these encouraging results, CCLM at high resolution will be used for the investigation of the regional climate in the Antarctic and atmosphere–ice–ocean interactions processes in a forthcoming study.

A DNS-based Turbulence Parametrization for Global Climate Models: Doubly Diffusive Convection

<p>Global coupled climate modeling requires the representation of multiple widely separated scales in each model component. In the ocean component, the separation of scales is especially dramatic as small scale turbulence exerts significant control on the global scale overturning circulation.  The importance of this control is demonstrated in the context of analyses of the Dansgaard-Oeschger oscillation of Marine Isotope Stage 3 (MIS 3; see Peltier and Vettoretti, 2014)). In the University of Toronto version of CCSM4 water column diapycnal diffusivity is represented using the KPP parameterization of Large et al (1994). This includes explicit contributions due to double diffusion processes which demonstrably play an important role in determining the period of the D-O oscillation itself.</p><p>                                             </p><p>We have developed a DNS-based methodology to test the accuracy of the doubly diffusive contributions to KPP. High-resolution turbulence data sets have been produced based upon two different models: the “unbounded gradient model” and the “interface model” with depth-dependent temperature and salinity gradients. By fitting the vertical fluxes in the unbounded gradient model (for equilibrium states) as a function of density ratio (the governing non-dimensional parameter) we derive a functional form on the basis of which KPP can be revised.  By applying the revised scheme to the interface model we demonstrate that the local fluxes predicted agree well with those from the numerical simulations. The difference between this new parametrization scheme and KPP demonstrates that KPP may significantly overestimate the diffusivities for both heat and salt at low-density ratio.</p>

Sensitivity of the ocean circulation model to the k–omega vertical turbulence parametrization

Ocean general circulation model (OGCM) of the INM RAS with embedded k turbulent model is developed. The solution of the k model equations depends on the frequencies of buoyancy and velocity shift which are generated by the OGCM. The coefficients of vertical turbulence in OGCM depend on k and omega. The numerical algorithms of both models are based on the splitting method for physical processes. The model equations are split into two stages, describing the three-dimensional transport-diffusion of the kinetic energy of turbulence and frequency and their local generation-dissipation. The system of ordinary differential equations arising at the second stage is solved analytically, which ensures the efficiency of the algorithm. Analytical solution also written for the vertical turbulence coefficient equation. The model is used to study the sensitivity of the model circulation of the North AtlanticArctic Ocean to variations in the parameters of vertical turbulence. Experiments show that varying the coefficients of the analytical model solution can improve the adequacy of the simulation.

Verification of the regional atmospheric model CCLM v5.0 with conventional data and Lidar measurements in Antarctica

The non-hydrostatic regional climate model CCLM was used for a long-term hindcast run (2002–2016) for the Weddell Sea region with resolutions of 15 and 5 km and two different turbulence parametrizations. CCLM was nested in ERA-Interim data. We prescribed sea-ice concentration from satellite data, and used a thermodynamic sea-ice model. The performance of the model was evaluated in terms of temperature and wind using data from Antarctic stations, AWS over land and sea ice, operational forecast model and reanalyses data, and lidar wind profiles. For the reference run we found a warm bias for the near-surface temperature over the Antarctic plateau. This bias was removed in the second run by adjusting the turbulence parametrization, which results in a more realistic representation of the surface inversion over the plateau. Differences in other regions were small. A comparison with measurements over the sea ice of the Weddell Sea by three AWS buoys for one year showed small biases for temperature around 1 K and for wind speed of 1 m s−1. Comparisons of radio soundings showed a model bias around zero and a RMSE of 1–2 K for temperature and of 3–4 m s−1 for wind speed. The comparison of CCLM simulations at resolutions down to 1 km with wind data from Doppler Lidar measurements during December 2015 and January 2016 yielded almost no bias in wind speed and RMSE of ca. 2 m s−1. Overall CCLM shows a good representation of temperature and wind for the Weddell Sea region. These results encourage for further studies using CCLM data for the regional climate in the Antarctic at high resolutions and the study of atmosphere-ice-ocean interactions processes.

Coupling a new turbulence parametrization to RegCM adds realistic stratocumulus clouds

To model stratocumulus clouds in the regional climate model, RegCM4.1, the University of Washington (UW) turbulence parametrization has been coupled to RegCM. We describe improvements in RegCM's coastal and near-coastal climatology, including improvements in the representation of stratiform clouds. By comparing output from a 27-yr (1982–2009) simulation of the climate of western North America to a wide variety of observational data (station data, satellite data, and aircraft in situ data), we show the following: (1) RegCM-UW is appropriate for use in general regional climate studies, and (2) the UW model distinctly improves the representation of the marine boundary layer in RegCM. These model–data comparisons also show that RegCM-UW has a slight cold bias, a (wet) precipitation bias, a systematic low bias in the vertically-integrated liquid water content near the coast, and a high bias in the fractional cloud coverage. The model represents well the diurnal, monthly, and interannual variability in low clouds. These results show RegCM-UW as a nascent mesoscale stratocumulus model that is appropriate for stratocumulus investigations at scales ranging from hourly to decadal. The source code for RegCM-UW is publicly available, under the GNU license, through the International Centre for Theoretical Physics.