Heinz H. Lettau

Amazonia's Hydrologic Cycle and the Role of Atmospheric Recycling in Assessing Deforestation Effects

Abstract Atmospheric and hydrologic phases of the water cycle in Amazonia are modeled coherently by subdividing the basin (which extends from about 47.5° to 77.5° west longitude) into six adjacent regions each 5° wide. Due to long-distance travel of the air masses, the recycling of vapor evaporated from upwind land surfaces is important. For example, it is found that 88% of the water precipitating on the westernmost region falls at least a second time from the air. The modeling of the longitudinal profiles of precipitable water in the atmosphere as well as exchangeable moisture in the soil is supplemented by the modeling of the complementary longitudinal profiles of annual mean surface and air temperature. In a climatonomical experiment, the forest cover in central Amazonia (i.e., in the two regions between about 57.5° and 67.5°W) is assumed to be reduced to one-half of the original value, thereby increasing the evaporative flushing rate of soil moisture in the two partly deforested regions. Computations ...

Wind and temperature profile prediction for diabatic surface layers including strong inversion cases

The micrometeorological research program in Antarctica has provided extensive data on wind and temperature profile structure under strong to extreme inversion conditions (Dalrymple et al., 1966; Lettau et al., 1977). The basic similarity hypotheses and limiting conditions for prediction of diabatic surface layer profiles are summarized. The model by Businger et al. (1971) for dimensionless shear and temperature gradients is revised to conform with the new results for strong stability. A novel similarity hypothesis is introduced to complete the step from shear and gradient prediction to prediction of absolute wind speed, wind energy, and temperature on the basis of prescribed external factors of surface layer structure. The physics of interactions between predicted profile ‘tilting’ and ‘curving’ are discussed and used to explain several micrometeorological paradoxes, including that of the ‘elevated minimum of air temperature’ observed occasionally near the active surface when the energy budget is of the nocturnal type.

Variangular wind spirals

The term ‘variangular’ is introduced to emphasize a significant difference between the present and certain earlier solutions to the problem of organized airmotion within the planetary boundary layer. The latter belong to the family of equiangular wind spirals and have the characteristic that the angle (ψ) formed by the vectors of shearing stress and geostrophic departure is invariant with height; it is shown that in this spiral-family, parabolic height-dependency of the effective (eddy) diffusivity (K) alone is permitted, including the asymptotic case of constant K; the famous Ekman spiral as well as the Rossby spiral are two prominent members of the family of equiangular wind spirals. The new variangular theory, as the name implies, permits variation of ψ with height (z) and produces more versatile profiles of wind and stress due to less restraint in K (z). As an example of comparison with observed data, monthly mean wind profiles obtained at Plateau Station, Antarctica, are selected since they exhibit a noteworthy degree of ‘variangularity’, in relatively satisfactory agreement with properties of the new theoretical model for wind spirals.

Climatonomical modeling of temperature response to dust contamination of Antarctic snow surfaces

Monthly averages of the surface energy balance are parameterized, resulting in a reduced solar forcing function and a non-dimensional time scale for computing the thermal response at the air/snow interface by numerical forward integration. The climatonomic transform of the balance equation serves to assess surface-temperature perturbations resulting from parameter modifications which simulate effects of dust contamination of a snow surface. Three climatonomical model experiments permit the following conclusions: (1) an albedo reduction increases primarily the summer temperatures; (2) an emissivity decrease raises the temperature of all months nearly uniformly; (3) the thermally induced feedback on submedium structure (if summer melting is instigated) increases the storage capacity and reduces spring and summer temperatures with compensating rise in autumn and winter temperature. Quantitative results are exemplified by assumed modification of conditions known to exist at the South Polar Plateau.

The O'Neill experiment of 1953

The official report on this first comprehensive field experiment in boundary-layer meteorology was published 1957 in two volumes, edited by Lettau and Davidson (hereafter L&D). The official report is supplemented in this paper by relevant pre-history developments and a discussion of some selected post-history interpretations and follow-up experiments.

Semi-diurnal wind variation in the friction layer above the tropical ocean

Conventional theory predicts that in the layer of frictional influence, the semi-diurnal forcing by pressure-gradient oscillations should produce responses in air motion which show reduced amplitude and phase-leading, in comparison with frictionless conditions in the free troposphere. Wind-profile data obtained by anemometers on a floating mast during the ‘1965-Atlantic Meteor Expedition’ at an anchor station in the SE-trade-wind regime near the equator showed that the amplitude of the west component at the 4-m level practically equalled the fr ictionless value, while other measurements gave evidence of frictional effects, such as phase-leading, and vertical shear from 1 to 10 m. The observed wind response is explained by dynamic coupling as a consequence of forcing by the horizontal pressure gradient oscillation and resulting semi-diurnal variation of surface stress and therefore of eddy diffusivity.

A diagnostic study of wangara wind profiles in quasi-steady and near-neutral cases

Under the assumption of a geostrophic wind varying exponentially with height, profiles of four turbulence characteristics (Reynolds stress, eddy diffusivity, momentum mixing length, and energy dissipation rate) are evaluated for three cases of quasi-steady wind profile observations under near-neutral boundary layer conditions. These cases occurred during the Wangara Experiment organized by CSIRO at Hay, N.S.W., Australia, in 1967. It is shown that mean vertical motion and surface friction velocity significantly influence the evaluated profiles of the turbulence characteristics.

New Hypothesis for the Relationship between Eddy and Mean States

A three‐dimensional Eulerian vector of eddy displacements is formally defined which relates fluctuations locally to the mean states. In comparison with previous phenomenological theories, three‐dimensionality of turbulent velocity is fully considered, even for one‐dimensional mean motion. The existing tendency towards adaption of transferred fluid property along eddy trajectories is expressed by an equivalent Eulerian concept, in supplement of the tendency towards conservation as singled out in the classical Eulerian concepts of Prandtl and Taylor. Differences in transfer structure between free and wall turbulence are interpreted by the varying role played by a longitudinal versus a lateral length‐scale of turbulence. Both lengths are rigorously defined by Eulerian variances and covariances of eddy displacement components. The latter can satisfy conditions of isotropy while eddy velocity components fail to do so.

Vegetation zones and evaporable water in tropical climates

SummaryKöppens definition of the climates of tropical savanna and forest is validated for Amazonia. A new validation is also found for the rainfall regimes which delimit tropical-subtropical vegetation zones of rainforest, monsoon forest, savanna and desert in India, as originally conceived by Walter. The climatologic definition of Köppen is reconciled with the climatographic definition of the ecologist Walter. The ratio-nalization is based on numerical climatonomic evaluation of the minimum in the annual course of EVAPORABLE WATER within tropical-subtropical rainfall regimes.

Modeling of the Annual Cycle of Soil Moisture

Primary productivity depends strongly on seasonal variations of soil moisture and evaporation. It is also known that the amount of soil moisture and its rate of exchange, both indispensable factors of any plant-climate model, are difficult to assess. For example, Lemon et al. (1971) mention specifically that soil moisture variations and evaporation processes are not very successfully simulated by the so-called “SPAM” model with which he describes the “Sun’s Work in a Cornfield.” This steady state model is an example of the microstructure approach with empha sis on the complexities of a specific plant canopy. On the other end of the spectrum are global-scale models as exemplified by Lieth and Box (1972), who simulate world-wide relationships between primary productivity and annual averages of evapotranspiration in the “Thornthwaite Memorial Model.”