Is the "Stratiform Precipitation" Associated with

a Mesoscale Convective System Really Stratiform?

by

Charles A. Doswell III

NOAA/National Severe Storms Laboratory#

Norman, Oklahoma

and

Jan Krácmar

Czech Hydrometeorological Institute

Prague, Czech Republic

 


#Current affiliation: Cooperative Institute for Mesoscale Meteorological Studies, Norman, OK

This essay has not been, nor is it likely to be, submitted anywhere for publication. Rather, it is being put out on the Web as a means of gauging reactions to this admittedly nit-picky point. Comments sent via Internet e-mail to me at: cdoswell@hoth.gcn.ou.edu will be welcomed, even if they are simply flames. I have included an update, based on some recent feedback, for your interest and enjoyment.

Last update: 17 May 1996


It has been common terminology to refer to the precipitation within a mesoscale convective system (MCS) as comprising "convective" and "stratiform" components. The use of this terminology is quite widespread (see e.g., Loehrer and Johnson 1995; Rutledge et al. 1988; Smull and Houze 1985; and many others), with a schematic echo structure illustrating these two precipitation components shown in Fig. 1 of Loehrer and Johnson (1995). Although it is apparent that the mechanisms producing the radar echoes in these two areas within an MCS are almost certainly not the same, it is not obvious that the term "stratiform" is appropriately descriptive. The Glossary of Meteorology (Huschke 1959, p.545) describes "stratiform" as pertaining to clouds of extensive horizontal development, in contradistinction to "cumuliform." Insofar as this refers to clouds, one certainly could argue that the precipitation falling from the trailing anvil of an MCS is of extensive horizontal development. When discussing stratus clouds, the Glossary (op. cit., p. 547) notes that stratus clouds generally produce little or no precipitation and what little might fall consists of minute particles (e.g., drizzle).

The extensive area of precipitation produced from an MCS' trailing anvil does not typically fall as drizzle. Rather, the rainfall can assume a moderate intensity (~10 mm hr-1) with the intensity flucutating markedly, giving it a showery character. Further, there are measurements of the vertical motion within MCS anvils (Smull and Houze 1987) indicating vertical motions within the anvil of several meters per second. These characteristics are not very compatible with what one calls to mind when thinking of "stratiform" precipitation. To emphasize this point, consider Figs. 1 and 2. These images from the 3 cm radar at Prague in the Czech Republic reveal quite clearly the difference between the so-called "stratiform" precipitation region of a small MCS and truly stratiform precipitation.

The MCS on 19 May 1993 is shown at about 1500 UTC to the west-northwest of the radar. The RHIs shown in Fig. 1a and Fig. 1b reveal a distinctly cellular character to the anvil, in spite of the fairly uniform field of maximum reflectivity at low levels beneath the trailing anvil (Fig. 1c). This view of the anvil certainly suggests a nonhomogeneous anvil.

Figure. 1. Radar depiction of an MCS on 19 May 1993 from the radar at Prague in the Czech Republic, showing (a) range-height indicator (RHI) cross section of reflectivity along the 290 deg radial at 1505 UTC, (b) RHI cross section along the 260 deg radial at 1504 UTC, and (c) a plan-position indicator (PPI) view of the maximum reflectivity in the column above the surface, along with cross sections of the maximum row and column reflectivity as a function of height on the sides of the plan view. Reflectivity in dBz is indicated by the color scale on the left.

On the other hand, the truly stratiform activity on 20 July 1993 reveals a dramatically different nature on radar. The cross section (Fig. 2a) reveals mainly the bright band with relatively little horizontal variation in intensity, while the plan view shows some slight fluctuations with mainly very weak reflectivity (generally < 40 dBz, indicative of small precipitation particles). In fact, the plan view of the truly stratiform precipitation in Fig. 2b seems to show as much or even more horizontal variation than the so-called "stratiform" region of the MCS; the real difference in the character is shown most dramatically in the cross sections.

Figure 2. As in Figure 1, except showing stratiform precipitation on 20 July 1993: (a) RHI cross section along the 329.7 deg radial at 0024 UTC, and (b) PPI view of maximum reflectivity at 0032 UTC, with row and column maxima on the sides of the plan view.

It appears to us that the term "stratiform" is misapplied in this context. Although this might be considered to be a minor semantics point, nevertheless terminology can convey implications that are not intended. To view the area of precipitation associated with the anvil as "stratiform" might lead one to believe that the horizontal variations within the anvil are of little or no importance, whereas it appears that substantial structure exists in the anvil. We believe the "stratiform precipitation" nomenclature ought to be changed to something more descriptive; perhaps to something like "trailing anvil precipitation."

Acknowledgments. The lead author's trip to Prague where these radar images were first seen was supported by the Joint Czechoslovak-United States Science and Technology Program, Project No. 94067.


REFERENCES

Huschke, R.E., 1959: Glossary of Meteorology. Amer. Meteor. Soc., 638 pp.

Loehrer, S.M., and R.H. Johnson, 1995: Surface pressure and precipitation life cycle characteristics of PRE-STORM mesoscale convective systems. Mon. Wea. Rev., 123, 600-621.

Rutledge, S.A., R.A. Houze, Jr., M.I. Biggerstaff, and T. Matejka, 1988: The Oklahoma-Kansas mesoscale convective system of 10-11 June 1985: Precipitation structure and single-Doppler radar analysis. Mon. Wea. Rev., 116, 1409-1430.

Smull, B.F., and R.A. Houze, Jr., 1985: A midlatitude squall line with a trailing region of stratiform rain: Radar and satellite observations. Mon. Wea. Rev., 113, 117-133.

______, and ______, 1987: Dual-Doppler radar analysis of a midlatitude squall line with a trailing region of stratiform rain. J. Atmos. Sci., 44, 2128-2148.