Horizontal Grid Length Research Articles

Observations and Numerical Model Simulation of a Partialy Trapped Lee Wave over the Welsh Mountains

Abstract A large-amplitude lee-wave event detected in radiosonde ascents during a field experiment in the Welsh mountains is described and wave characteristics are deduced. The synoptic-flow pattern accompanying this wave field is used to initialize the U.K. Meteorological Office's nonhydrostatic mesoscale model (with a 3-km horizontal grid length) in an attempt to simulate the gravity-wave response over the Welsh mountains. After 4 h of integration time the resulting flow held is quasi-study and exhibits a fairly regular wave field with a dominant wavelength at about 22 km and nearly vertical phase lines. Verification is achieved by comparing rate-of-ascent variations with model vertical velocity variations along the trajectory of a radiosonde: the agreement is very good. The vertical structure equation for linear gravity waves is solved using the smoothed wind and temperature fields obtained from the sondes. A “leaky” resonance is identified at the dominant wavelength of the model simulation. Limited co...

A spectral model for investigation of amplifying baroclinic waves

A spectral quasi-geostrophic model with vertical derivatives approximated by finite differences is used to study the effect of different vertical resolutions on the behaviour of baroclinic waves. The effect of the small terms in the complete vorticity and thermodynamic equations is also investigated. These terms are treated separately and modify a first order growing wave and the induced zonal field. The baroclinic instability with respect to first order perturbations is computed for different x- and y-wave numbers. Numerical experiments indicate that a low vertical resolution is most serious when the horizontal scale is small. High resolution models have a significantly higher instability in the short wave interval 1 000–2 000 km and the instability is split into two separate maxima. The secondary maximum at the shortest wavelengths is very sensitive to changes in the static stability and is also present in a linearized primitive model. A necessary spacing needed to resolve this instability is of the order of 5 vertical levels and 150 km horizontal gridlength. The effect of the vertical resolution on the induced zonal and second order field was found to be very small. The contribution from the small terms in the complete equations is mainly a deepening of the cyclones and a weakening of the anticyclones and an increased cyclonic circulation in the eastern parts of the upper troughs and ridges. The relative importance of each of the small terms is studied and it is concluded that it is important to keep them all. DOI: 10.1111/j.2153-3490.1974.tb01621.x

A 10‐level atmospheric model and frontal rain

AbstractA 10‐level primitive equation model suitable for studying the dynamics of fronts and frontal rainfall is described. The atmosphere is assumed to be hydrostatic and inviscid and the effects of friction and topography are ignored. Latent heat due to evaporation and condensation is incorporated in the thermodynamic equation. No distinction is made between the ice and water stage and the atmosphere is assumed to be dry above 300 mb. The horizontal grid length is 40 km. The results of one 24‐hr integration are described in detail.