Longitudinal instabilities and secondary flows in the planetary boundary layer: A review

Abstract

Within the past decade, satellite pictures have shown persistent cloud patterns which indicate that the flow in the atmospheric planetary boundary layer is often organized into helical secondary circulations aligned parallel to the mean flow. Theory and observation agree that both convection in the presence of shear and the dynamic inflection point instabilities of the Ekman layer lead to these flow patterns. The observatio•ns and theory are reviewed with emphasis on the dynamics-dominated flow. cal flows developed slowlye:marked by two perceptive and diligent observers, Langmuir and Woodcock, and then grew rapidly as a result of the remote sensing technology developed in the past 20 years. Langmuir [1938] had inferred the existence of helical flow in the top layer of the ocean from observations of long rows of floating seaweed parallel to the wind. His experiments on Lake George, using oil slicks, leaves, cork, and submerged umbrellas buoyed wit h lamp bulbs, established quantitative values for the scale and velocities in these circulations which stand today. The term 'windrows' was applied early by mariners to the long lines of flotsam or suppressed capillary waves appearing along the wind direction. Garbage dumped by the state of New York has been observed drifting back toward New York in the sea breeze in neat rows. In all of these cases the rows are nearly parallel to the wind and, by virtue of characteristic wavelength, appear to be related to the Langmuir circulations in the water. However, the question of the generating mechanism has not been resolved and has been discussed by Welander [1963], Faller [1964], Scott et al. [1969], Craik and Leibovich [1976], and Faller and Caponi [1978]. Woodcock [1942] used seagull soaring patterns correlated with wind and temperature measurements to suggest cellular and lineal convective patterns in the atmosphere. The banded cloud regions appear in broad scale views of the earth obtained from satellites (Figure 1). Lineal cloud bands occur in hurricanes and consistently in the arctic regions, where cold Pape•r number 80R0121. 683 684 BROWN: LONGITUDINAL INSTABILITIES AND SECONDARY FLOWS IN THE PBL

The measurement of the synoptic scale wind over the ocean

“…There may be long cylindrical roll vorticies oriented roughly parallel to the wind under some circumstances as described in detail in a review by Brown (1980). The presence, or absence, of any quasi-organized motions within an area scanned by the radar cells 1S difficult to demonstrate.…”

Analysis of a Small, Vigorous Mesoscale Convective System in a Low-Shear Environment. Part I: Formation, Radar Echo Structure, and Lightning Behavior

“…These vigorous cumuli were arranged in short ''transverse bands'' oriented about normal to the west-southwest mean flow in the ABL. These features appear to be Rayleigh-Bernard rolls (Houze 1993, 61-64) that develop parallel to the shear vector (normal to the mean flow) under even stronger surface heating and lower gradient Richardson number (Brown 1980), calculated at approximately 0.2 from the 1730 UTC RSA sounding. Their wavelength is 8-10 km, somewhat longer than a maximum of 7 km predicted by theory, assuming an ABL height of 1.7 km and an aspect ratio of 4.…”

Three-Dimensional Radar Echo Structure of a Snow Band Formed on the Lee Side of a Mountain

“…Many observational and theoretical studies concerning the cloud bands have been conducted (e.g., Angell et al, 1968;Asai, 1970;Kuettner, 1971;Brown, 1970Brown, , 1972Brown, , 1980Kelly, 1982Kelly, , 1984LeMone, 1973;LeMone and Pennell, 1976;Miura, 1986). The necessary condition for the formation of cloud bands has been identified as a convectively unstable layer having significant vertical wind shear.…”