AN INTRODUCTION TO THE STUDY OF AIR MASS ANALYSIS By JEROME NAMIAS IV. THE ROSSBY DIAGRAM—INTERPRETATION1 In the preceding article the me- following will serve as an outline for chanics of plotting the Rossby dia- those who have a limited knowledge gram were discussed, and it may be of mathematics and physics. said here that the curve joining the In the Rossby diagram the char- individual points is called the char- acteristic curve for an unsaturated acteristic curve for the air column. particle of air, A (see fig. 4) having The present article deals with the a given potential temperature (6) significance of various curves on the and specific humidity (q) and being diagram and the changes in appear- displaced vertically, is represented as ance of these curves as the more a point. Owing to the fact that its common atmospheric processes, such potential temperature must remain as vertical displacement, occur. At constant, the point A cannot be dis- this point it is only fair to remind placed from the horizontal line repre- the reader that the following dis- senting its potential temperature. cussion is, from the standpoint of a Furthermore, the specific humidity quantitative analysis, rather unsatis- remains constant during the adiabatic factory in that it is an attempt to process with unsaturated air, and thus describe processes which are better the point A cannot be displaced from expressed in more exact mathematical the vertical line representing constant terms. Yet it is hoped ;th\at the specific humidity. It is evident, then, that the characteristic curve of a dry aThi» is the fourth article of the series particle of air which is undergoing begun in the Aug.-Sept. BULLETIN, and con- tinued in the October and December BUL- adiabatic transformation reduces to LETINS. Reprints of the preceding articles a point on the diagram. For a given are available at 5c each from the Secretary. FIG. 4. CHARACTERISTIC CURVES ON THE ROSSBY DIAGRAM Unauthenticated | Downloaded 10/02/21 05:34 AM UTC element of air this point will be rep- that what has been said refers to one resented by the same coordinates (0 stratum of air, no new strata being and q) until the level of condensa- introduced or removed during the tion has been reached. From this process. The line element CD may level on it is assumed that the water then be considered as the charac- produced by condensation drops out teristic curve for the given layer. immediately—in other words, the If an entire aerological sounding is process is considered pseudoadiabatic plotted on the Rossby diagram, it (pseudo, since there is a small amount may be considered as the charac- of heat removed from the air by the teristic curve of the air column falling water). From the definition through which the sounding has been of equivalent-potential temperature made. (0E) it is clear that the characteristic In the definition of an air mass, curve of the saturated mass of rising horizontal homogeneity was stressed. air is a line of constant equivalent- Soundings, then, made at different potential temperature. This amounts places within a source region and to saying that the lines, of constant remaining within the same air mass 0E are also the saturation adiabats. should exhibit nearly identical char- In a rough fashion it is evident that acteristics; the characteristic curves the 0E lines must slope as they do; should be similar. Frequently the the potential temperature increasing air masses, even at the source regions in the saturated air particle because and particularly after travelling of the realized (latent) heat of con- some distance, are subjected to forces densation, and the specific humidity which lead to appreciable vertical dis- falling because of the condensation placements. When this occurs un- and removal of the liquid water. equally in different sections of the The fact that an adiabatic process air mass the homogeneity with re- with unsaturated air is represented spect to level tends to be destroyed. by a point makes the Rossby diagram The surfaces of constant potential particularly adaptable to modern temperature and constant specific synoptic analysis. A thin layer of humidity become curved instead of air having a uniform distribution of horizontal, and plots of temperature temperature and moisture may be or moisture against elevation appear represented on the diagram as a markedly dissimilar. It follows that straight line. Let this line be CD it is difficult to identify and follow in fig. 4. If the entire layer CD be air masses by means of these dia- raised or lowered the temperature grams. The characteristic curves on and relative humidity of each particle the Rossby diagram, on the other of air within the layer will be hand, will have overlapping parts for changed because of the adiabatic ex- the same column of air regardless of pansion or compression. But the po- the extent of the expansion or com- tential temperature and specific hu- pression, providing no condensation, midity of each particle of air will evaporation, or introduction of a new remain unchanged, so long as conden- air mass has taken place. The reader sation or evaporation do not take may refer to papers published by the place, and for this reason the layer, Meteorology Course of the Massachu- compressed or expanded, will be rep- setts Institute of Technology and to resented on the diagram by the same Harvard Meteorological Studies, No. line CD. It is important to note 2, for examples of this characteristic Unauthenticated | Downloaded 10/02/21 05:34 AM UTC quality for various air masses on the adiabat, or 1 C deg. per 100 m. If Rossby diagram. there is an increase in potential tem- Fig. 4 illustrates characteristic perature of 10 deg. in 1000 m the winter curves for two air masses: lapse rate is isothermal. In general, one having its source over Northern the greater the increase in poten- Canada (Pc—Polar Canadian) the tial temperature with elevation the other from the Gulf of Mexico (Tg greater the stability. If the potential —Tropical Gulf). The numbers, be- temperature decreases with elevation, sides points on the curve, represent the lapse rate is superadiabatic. elevations in meters above sea level. The second type of stability, that The Pc curve exhibits the pronounced within a layer which is being dis- coldness, dryness, and stability of the placed vertically, is best treated by Polar Canadian air. The Tg curve the Rossby diagram. From the slope shows the warm and moist character of the characteristic curve relative of the tropical maritime air. to the slope of the lines of constant The Rossby diagram is helpful in 0E, one can determine whether the the treatment of stability. Thermo- layer in question is convectively un- dynamic diagrams, for the most part, stable (sometimes called potentially deal with the stability or instability unstable). This condition is defined of a particle of air with respect to as one in which the equivalent-poten- its surroundings. This is of import- tial temperature decreases with ele- ance in penetrative convection, such vation, and is indicated on the Rossby as in cumulus cloud and thunderstorm diagram by a line which possesses a formation. However, in the more im- slope more horizontal than the lines portant types of convection, that is, of constant 0B, yet not so horizontal in the case when a layer of warm as the lines of constant 0• In other air of large horizontal extent is words the line representing convective forced over an underlying cold air instability is one which lies between wedge or a mountain range, the the potential and the equivalent-po- classical energy diagrams fail to give tntial isotherms. The importance of a measure of the potential energy this particular distribution of tem- available in the layer. It is here that perature and moisture lies in the fact the Rossby diagram is invaluable. that lifting of the entire layer brings It is true, however, that in regard to about a more unstable condition, in particle stability the classical dia- fact, if the layer is lifted sufficiently grams are more useful than the it will eventually become unstable Rossby diagram, but even in this with respect to dry (unsaturated) type of stability it is not difficult to air. The following illustration will apply the equivalent-potential tem- serve to show this increasing in- perature diagram of Rossby. This stability. Suppose we have the layer may be done with the aid of the lines CD, which is by definition convectively of equal potential temperature (the unstable, having the equivalent-poten- horizontal lines) and the elevations tial temperature 320°A at the base of the points which may be conveni- of the layer and 315°A at the top. ently indicated by figures beside the If the layer is now lifted pseudo- individual points of the sounding. adiabatically to the top of the atmos- Thus if there is no increase in po- phere, the base of the layer will have tential temperature through a layer, an equivalent-potential temperature the lapse rate is equal to the dry greater than that at the top of the Unauthenticated | Downloaded 10/02/21 05:34 AM UTC layer by 5 C deg., the additional heat air at those temperatures, while the being supplied to the bottom of the upper part of the layer cools at the layer by the extra heat of condensa- adiabatic rate for dry air. It is tion given to it by the greater amount obvious that the base of the layer is of water vapor at the base compared becoming warmer and warmer rela- with the top of the layer.