Weather Forecasting Dr. Adel Awad

What is synoptic Meteorology? Historical Background The adjective synoptic comes from the Greek word synoptikos, meaning “ affording a general view of a whole”

Synoptic in context refers to horizontal dimensions and length of time of atmospheric phenomena ; such as Extra-tropical cyclones and Anti-cyclones, Troughs and Ridges and extent to Frontal zones and Jets.

Horizontal-length scales and time scales for the following atmospheric phenomena: A, dust devils ; B, tornadoes and waterspouts ; C, cumulus clouds ; D, downbursts ; E, gust fronts ; F, meso-cyclones ; G, thunderstorms ; H sea/land/lake breezes, mountain-valley circulations, and meso-highs and meso-lows ; I, precipitation bands ; J, coastal fronts ; K, meso-scale convective systems ; L, the low-level jets ; M, dryline; N, “bombs” and tropical cyclone; O, upper level jet; P, surface fronts ; Q, extra- tropical cyclones and anti-cyclones ; and R, troughs and ridges in the barclinic westerlies.

Classification of atmospheric phenomena The atmospheric phenomena classified by two factors, their horizontal dimensions and time scales. Time scale means; the time it takes air to travel completely across the feature. For example; an intense extra-tropical cyclone may form over a 1-2 day period, not in 3 hours, 10 minutes or a month.

There are several common classification schemes for phenomena: Isidoro Orlanski sub-divides phenomena according to the order of magnitude of the horizontal extent of each phenomena, using Greek letters and the prefixes ; “micro”, “ meso “, and “ macro “

Ted Fujita sub-divides phenomena according to the order of magnitude of the horizontal extent of each phenomena relative to the circumference of the earth at the equator.

The width of synotic-scale features such as: troughs ( R ) and ridges ( R ) in the baroclinic westerlies and large extra-tropical cyclones ( Q ) and anticyclones ( Q ) is much grater than their depth; the Dynamics of these features is Hydrostatic.

Although Fronts ( P ) and Jets ( L ) associated with the synoptic scale features are as long as the Troughs ( R ) and Ridges ( R ) in the baroclinic westerlies and have similar time scales, they are much narrower.

Intense Oceanic extra-tropical cyclones, tropical cyclones ( N ), meso-scale convective systems ( MCSs) ( K ), drylines ( M ), and the low- level jet ( L ) are of sub-synoptic scale. With the exception of squall lines, sub-synoptic scale features are Hydrostatic, but with horizontal scale and time scale slight shorter than synoptic phenomena.

Meso-scale features include the following: coastal fronts ( J ), outflow boundaries ( J ), precipitation bands in extra- tropical cyclones ( I ), gravity waves ( H ), sea/land/lake breeze fronts ( H ), mountain- valley circulations ( H ), meso-highs ( H ), and meso-lows ( H ).

Meso-scale features have time scales roughly identical the period of a pendulum day, and horizontal dimension less than the sub- synoptic scale phenomena. Precipitation bands ( J ) and dry-lines ( M ), which are deeper, are not hydrostatic, while the other meso-scale features are.

Thunderstorms ( G ), meso-cyclones ( F ), gust fronts ( E ) and large downbursts ( D ) are storm scale, they have horizontal dimensions on the order of convective storms and time scales larger than that of the Brunt- vaisala period, N -1 ( usually around minutes).

cumulus clouds ( C ), tornadoes ( B ), and waterspouts ( B ) are sub-storm scale. They are contained within the area bounded by convective storms. Their time scale range from 10 minutes to nearly an hour. These features are markedly non-hydrostatic.

finally, sub-vortices within tornadoes ( B ) and waterspouts ( B ) and dust devils ( A ) are micro-scale. They are markedly non-hydrostatic, and have time scales on the order of the typical Brunt- vaisala period and horizontal dimensions smaller than these of the sub-storm scale.

In summery, meteorological features occur over a wide range of space and time scales. It is interesting that phenomena having long space scales have long time scales, and vies versa, with ration between horizontal space and time scale is roughly of the same order of magnitude for all features ( i,.e. 10 ms -1 )

As a postscript we note that, if a feature’s width is much greater than its depth it is Hydrostatic.