IMPLICIT NONLINEAR NORMAL MODE INITIALIZATION FOR A BAROTROPIC PRIMITIVE EQUATION LIMITED AREA MODEL

A simple version of implicit nonlinear normal mode initialization is applied to a limited area one-level primitive equation model over a tropical domain. The model formulation is based on shallow water equations in spherical co-ordinate and potential enstrophy conserving finite difference scheme is employed. The model is used for predicting the movement of a typical monsoon depression formed over the Bay of Bengal. The above scheme is found to be very effective as it requires only three iterations for attaining balance between the mass and wind tields. However this model is not able to predict the movement of the depression very ac-curately due to the limitations of such a one-level model.

Tropical cyclone simulation with Emanuel’s convection scheme

ABSTRACT. A new convection parameterization scheme proposed by Emanuel (1991) is used to simulate the evolution of tropical cyclone. The numerical model used for this study is a 19 level axi-symmetric primitive equation, hydrostatic model in a z co-ordinate system. The vertical domain ranges from 0 to 18 km and the horizontal domain ranges upto 3114 km with a resolution of 20 km. in the central 400 km radius and with increasing radial distance thereafter. The evolution of an initially balanced vortex with an initial strength of 9 m/sec is studied. It is shown that Emanuel's convection scheme is successful in simulating the development of the initial vortex into a mature, intense cyclonic storm. At the mature stage, a minimum surface pressure of 930 hPa is attained with the associated low level maximum tangential wind speed of 70 m/sec. The simulated circulation features at the mature stage show the formation of an intense cyclone. Two different sensitivity experiments were performed. A set of experiments with the variation of sea surface temperature (SST) from 300.5° to 302° K in steps of 0.5° K have shown that the intensity of model cyclone increases with the increase of SST. Another set of experiments with variation of latitude has shown that the cyclonic storm is more intense at lower latitudes.

Prediction of tropical cyclones by numerical models A review

ABSTRACT. This paper contains a review of some past and recent developments in cyclone track prediction problem by dynamical models. The early attempts aimed at predicting tropical cyclone motion by using simple barotropic models based on vertically integrated vorticity tendency equation. Barotropic models are still used operationally in some centres due to their simplicity. However, current emphasis is on advanced primitive equation models incorporating physical processes, like cumulus convection, which are necessary to account for a major component of the cyclone movement. An important aspect of cyclone prediction by dynamical models is prescription of a correctly analysed synthetic vortex in the initial fields for running a forecast model. Several approaches developed by various groups for generating synthetic vortex are discussed. Examples of some cases of track prediction by limited area model in IMD and by global models are illustrated.

Bedymo: a combined quasi-geostrophic and primitive equation model in sigma coordinates

<div> <div> <div> <p>We introduce the idealised atmospheric circulation model Bedymo, which combines the quasi-geostrophic approximation and the hydrostatic primitive equations in one modelling framework. The model is designed such that the two systems of equations are solved as similarly as possible, such that differences can be unambiguously attributed to the different approximations, rather than the model formulation or the numerics. Using either approximation, Bedymo successfully simulates a mid-latitude atmospheric storm track and the stationary wave response to orographic forcing or diabatic heating.</p> <p>In addition to the atmospheric core, Bedymo also includes a slab ocean model and passive tracer module that could provide the basis for an idealised parametrisation of moisture and latent heat release. Further, Bedymo has a graphical user interface, making it particularly useful in teaching.</p> <p>In contrast to most other quasi-gestrophic models, Bedymo is using sigma-coordinates in the vertical. This is unique as it ensures mass continuity within the model domain and allows a more direct inclusion of orography. We point out several insights and potential pitfalls when deriving quasi-geostrophy in sigma-coordinates and show that it is possible to obtain a self-consistent set of equations.</p> </div> </div> </div>

At the dawn of global climate modeling: the strange case of the Leith atmosphere model

A critical stage in the development of our ability to model and project climate change occurred in the late 1950s–early 1960s when the first primitive-equation atmospheric general circulation models (AGCMs) were created. A rather idiosyncratic project to develop an AGCM was conducted virtually alone by Cecil E. Leith starting near the end of the 1950s. The Leith atmospheric model (LAM) appears to have been the first primitive-equation AGCM with a hydrological cycle and the first with a vertical resolution extending above the tropopause. It was certainly the first AGCM with a diurnal cycle, the first with prognostic clouds, and the first to be used as the basis for computer animations of the results. The LAM project was abandoned in approximately 1965, and it left almost no trace in the journal literature. Remarkably, the recent internet posting of a half-century-old computer animation of LAM-simulated fields represents the first significant “publication” of results from this model. This paper summarizes what is known about the history of the LAM based on the limited published articles and reports as well as transcripts of interviews with Leith and others conducted in the 1990s and later.

Seasonal Variability of Mesoscale Instabilities in the Azores Current System

<p>The Azores Current-Front system coincides with the northern limit of the subtropical gyre in  the Eastern North Atlantic. The mean zonal jet is positioned south of the Azores archipelago  and extends from west of the mid-atlantic ridge to the Gulf of Cadiz, where it partially  turns south. North of the main jet, a sub-surface counter-current is found, flowing westwards. The associated thermal front separates the warm subtropical waters from the colder subpolar waters. The instantaneous flow in the Azores Current/Front system is characterized by the presence of meandering currents with length scales of 200 km that regularly shed anticyclonic warm water and cyclonic cold water eddies to the north and south of the mean jet axis, respectively, due to vortex stretching and the planetary beta effect. The time scale of eddy shedding is 100-200 days. On the meandering arms of the current, downwelling <br>and upwelling cells are found and sharp thermal gradients are formed and a residual poleward heat transport is observed. The instability cycle that originates the mesoscale meanders and the eddies is well-known from quasi-geostrophic and primitive equation models initialized from a basic baroclinic state: a first phase of baroclinic instability feeds on available potential energy to raise eddy kinetic energy levels, that, in a second phase feed the mean kinetic energy by Reynolds stress convergence. The cycle repeats itself as long as the APE reservoir is filled at the end of each cycle.</p><p>However, seasonal variability of the zonal jet dynamics has not been addressed before and it can provide valuable insights in to the variations of the Eastern North Atlantic between the subtropical and subpolar gyres. We use a primitive equation regional ocean model of the Eastern Central North Atlantic with realistic climatological wind and thermal forcing to study the yearly cycle of meandering, eddy shedding and restoration of the mean jet in the Azores/Current system. We observe an semi-annual cycle in the jet's kinetic energy with maxima in Summer/Winter and minima in early Spring/Autumn. Potential energy conversion by baroclinic instability occurs throughout the year but is predominant in the first half of the year. The mean kinetic energy draws from the turbulent kinetic energy through Reynolds stress convergence in periods of 50 - 100 days, that are followed by short barotropic instability periods. During Winter, Reynolds stress convergence, and thus mean jet reinforcement from the mesoscale eddy field, occurs along the jet meridional extent, in the top 500 m of the water column, but from Spring to Autumn it is observed only in the southern flank of the mean jet axis.</p>

The Influence of Anthropogenic Aerosols on the Aleutian Low

<p>There is an incomplete understanding of the mechanisms that govern the Pacific Decadal Oscillation (PDO), a major mode of climate variability that plays a key role in the evolution of global climate on decadal time-scales. Recent research has suggested that regional anthropogenic aerosol (AA) emissions could modulate the behaviour of the PDO, including the transition to a negative PDO phase starting in the late 1990s (Smith et al., 2016). However, other studies have questioned whether this connection is robust (Oudar et al., 2018). East Asia is a region of particular focus, where AA emissions having increased in recent decades (Bartlett et al., 2017). Here we combine analysis of an ensemble of coupled climate models running idealised AA perturbation experiments and a steady-state primitive equation model (LUMA) forced by diabatic heating anomalies to examine whether AA emissions influence the behaviour of the Aleutian low - a climate feature closely associated with the PDO  - and if so, test the posited teleconnection mechanisms proposed by Smith et al. (2016). We further compare the response of the Aleutian low to well mixed greenhouse gases to examine if AAs and GHGs influence the Aleutian low in a similar manner.</p>