Models
2022-03-24 08:48:22.0 [tstevens] insert AltLabel (id: null category: primary text: Three-Dimensional Trajectory Model language code: en); insert Definition (id: null text: Parcel trajectories are often calculated to obtain an appreciation of the history of air masses (e.g. Fuelburg et al., 1996). The direction and length of trajectories are useful in diagnosing processes that may affect particular air masses under certain conditions. In this regard it is important that parcel trajectories are determined accurately. The three-dimensional (3D) algorithm presented here (traj3d) builds on the two-dimensional (2D) model of Law (1993) and reported in application by Perrin and Simmonds (1995). From a specified parcel location in the atmosphere, xn at time n, a finite integral is solved to advect the parcel and generate the trajectory path. Given the 3D wind v(xn) the governing prognostic equation for the trajectory path over a short time interval Δt is xn+1 = xn + v Δt . The wind at a given point is found by cubically interpolating from a spatial grid then linearly in time. For back trajectories the wind direction is reversed. The finite integral is solved using a fourth-order Runge-Kutta scheme to obtain an estimate of the wind. This method is considerably more accurate for trajectory calculations than a simple first-order approach. For mathematical details about the 3D algorithm see Noone and Simmonds (1999) and Barras and Simmonds (2009). We present an implementation of the 3D algorithm in Fortran 77 that is suitable for any platform given an appropriate compiler. A precompiled Linux version is also available. The input to the program is a simple binary format referred to as CMP. At present the input data (usually 3D winds) are limited to constant pressure levels (e.g. 1000, 925, 850 hPa etc) as is the case with the commonly available reanalysis products e.g. NCEP reanalysis. The output of the program is a NetCDF file (and therefore NetCDF libraries need to be accessible for compilation) and this may be imported into many software packages e.g. Matlab, NCL, for subsequent analysis and plotting. We include some utilities to convert from NetCDF (a common format for 3D meteorological fields like winds) to CMP as well as plotting the trajectories based on NCAR Graphics/NCL. Although intended for computing 3D trajectories the algorithm is equally applicable to 2D trajectory studies e.g. 10 m winds, or a single pressure level e.g. 500 hPa. Finally, it is possible to interpolate auxiliary variables e.g. air temperature, to the trajectory positions and include these in the output NetCDF file. language code: en);
Parcel trajectories are often calculated to obtain an appreciation of the history of air masses (e.g. Fuelburg et al., 1996). The direction and length of trajectories are useful in diagnosing processes that may affect particular air masses under certain conditions. In this regard it is important that parcel trajectories are determined accurately. The three-dimensional (3D) algorithm presented here (traj3d) builds on the two-dimensional (2D) model of Law (1993) and reported in application by Perrin and Simmonds (1995). From a specified parcel location in the atmosphere, xn at time n, a finite integral is solved to advect the parcel and generate the trajectory path. Given the 3D wind v(xn) the governing prognostic equation for the trajectory path over a short time interval Δt is xn+1 = xn + v Δt . The wind at a given point is found by cubically interpolating from a spatial grid then linearly in time. For back trajectories the wind direction is reversed. The finite integral is solved using a fourth-order Runge-Kutta scheme to obtain an estimate of the wind. This method is considerably more accurate for trajectory calculations than a simple first-order approach. For mathematical details about the 3D algorithm see Noone and Simmonds (1999) and Barras and Simmonds (2009). We present an implementation of the 3D algorithm in Fortran 77 that is suitable for any platform given an appropriate compiler. A precompiled Linux version is also available. The input to the program is a simple binary format referred to as CMP. At present the input data (usually 3D winds) are limited to constant pressure levels (e.g. 1000, 925, 850 hPa etc) as is the case with the commonly available reanalysis products e.g. NCEP reanalysis. The output of the program is a NetCDF file (and therefore NetCDF libraries need to be accessible for compilation) and this may be imported into many software packages e.g. Matlab, NCL, for subsequent analysis and plotting. We include some utilities to convert from NetCDF (a common format for 3D meteorological fields like winds) to CMP as well as plotting the trajectories based on NCAR Graphics/NCL. Although intended for computing 3D trajectories the algorithm is equally applicable to 2D trajectory studies e.g. 10 m winds, or a single pressure level e.g. 500 hPa. Finally, it is possible to interpolate auxiliary variables e.g. air temperature, to the trajectory positions and include these in the output NetCDF file.
TRAJ3D