Vertical Structure of the Atmospheric Boundary Layer in Trade Winds Yumin Moon MPO 551 September 26, 2005

Papers to Present Riehl, H., Yeh T. C., Malkus J. S., and La Seur, N. E., 1951: “The Northeast Trade of the Pacific Ocean”. Quarterly Journal of the Royal Meteorological Society. 77, Augstein E., Schmidt, H., and Ostapoff, F., 1974: “The Vertical Structure of the Atmospheric Planetary Boundary Layer in Undisturbed Trade Winds over the Atlantic Ocean”. Boundary Layer Meteorology, 6,

Riehl 1951 Analyzed observations in the Northeast Pacific Ocean during the dry season (July to October). Three weather ships, Pearl Harbor, Hickam Field, both in Honolulu, HI. Hourly surface observations, two radiosonde observations per day.

Riehl 1951 Wind Steadiness Air above the Inversion Top Inversion Layer Cloud Layer Subcloud layer V m = mean speed

Riehl 1951 Vertical Cross-Section of Wind Speed Mean Vertical Distribution of Wind Speed

Riehl 1951 Vertical Distribution of Divergence Equation of Continuity, assuming steady state

Riehl 1951 “Whereas the inversion ascends downstream, individual columns descend, shrink vertically and spread horizontally. Large masses of air, located above the inversion at 32N have become a part of the cloud layer when they reach Honolulu, HI”.

Riehl 1951 The air that has been incorporated in the inversion layer The air that has been incorporated in the cloud layer The air that has been below the inversion throughout the journey from 32N.

Riehl 1951

Equation of Continuity for Latent Heat in a layer of unit thickness and cross-section and extending over the distance ds VerticalHorizontalTurbulent Exchange Source/Sink Steady-state is assumed. Lateral mixing is neglected compared to vertical mixing. The vertical coordinate is attached to the trajectory of the mean motion  w vanishes everywhere thus the second term (vertical) is dropped out, except at the top where the boundary is a horizontal surface.

Riehl 1951

Rise of the inversion is accomplished through the pickup of latent heat by the trade in the course of its passage over the tropical ocean. The bases of cumulus clouds are nearly uniform height, but the tops are very irregular. The tops of the cumulus clouds break off and evaporate quickly. Moisture is introduced into the lower portions of the inversion layer. Then the air in the inversion layer becomes gradually similar to the characteristics of the cloud layer.

Riehl 1951 Inflow term upstream, downstream Inflow term top, bottom Pressure Term Turbulent term top, bottom Equation of momentum Integrating over a volume bounded by ds, dz, and of unit thickness in the direction normal to s

Riehl 1951 Layer (mb) Inflow Upstream Outflow Downstream NetInflow Top Outflow Bottom NetPressure Term Turbulent Term Bottom Turbulent Term Top In units of 10^8 G CM SEC^-2

Augstein 1974 Analyzed observations collected during the Atlantic Expedition 1965 Sep 12 to Oct 11 and the Atlantic Tradewind Experiment (ATEX) 1969 Feb Three ships, Planet at the northeast, Discoverer at the northwest, and Meteor at the south. 8 radiosondes observations per day, radar wind measurements.

Augstein 1974 Surface Layer – adiabatic temperature gradient, decrease of specific humidity with height, slight statical instability. Mixed Layer – adiabatic temperature lapse rate, nearly constant vertical specific humidity. Transition Layer – nearly isothermal temperature distribution, strong upward decrease of moisture. Cloud Layer – temperature gradient slightly higher than the moist adiabatic lapse rate, upward weak decrease of specific humidity, conditionally unstable. Trade Inversion – increasing temperature, steep decrease of moisture.

Augstein 1974

 Increasing cumulus convection causes an increase of downward flux of inversion air into the cloud layer, thus pushing the inversion upward. This process is combined with downward heat flux which effects a diabatic warming of the cloud layer. Strong convective activity destroys the trade inversion; the organized cloud circulation then transports air parcels with relatively low potential temperature upward and with high potential temperature downward. This process results in a diabatic warming of the lower part of the cloud region and in a diabatic cooling of the upper part

Augstein 1974