Search Results

You are looking at 1 - 10 of 10 items for

  • Author or Editor: H. Lettau x
  • Refine by Access: All Content x
Clear All Modify Search
H. Lettau

Abstract

No abstract available.

Full access
H. Lettau

Abstract

No Abstracts Available

Full access
H. Lettau

Abstract

A previously introduced vorticity-transfer-and-adaptation hypothesis is briefly summarized and supplemented by consideration of certain spatial derivatives of the components of the eddy displacement vector. The conventional length-scale of turbulence, redefined in the preceding note as a lateral co-variance = (xz′¯)½, is supplemented by the new concept of a longitudinal length-scale, defined as the variance L = (x2¯)½). In correspondence to the Karman constant (which concerns the rate of change of l in the direction lateral to the mean flow) there is a new constant which concerns the rate of change of L in the longitudinal direction. It is proposed to refer to this as the Reichardt constant, because the supplements are of direct importance for turbulence in free flows (such as submerged jets), and because the new hypothesis offers an improvement in comparison with “Reichardt's momentum transfer law” or “inductive theory” of jet-diffusion. It is shown that the structure of mean profiles in two-dimensional jets can be very satisfactorily predicted by the new theory. The empirically known discrepancy between lateral distributions of heat and momentum is now definitely explained as the result of vorticity transfer; specifically, by the fact that momentum follows from vorticity transfer, and therefore does not diffuse in the same manner as a scalar property such as heat per unit mass. The ratio of eddy diffusivities for heat and momentum in the two-dimensional jet has the minimum value of 4/3 at the axis. Finally, profiles of cross-stream velocity components in free flow are discussed and a new explanation for an old and thus far unresolved discrepancy between empirical and theoretical results is given.

Full access
H. Lettau

Abstract

No Abstracts Available

Full access

EVAPOTRANSPIRATION CLIMATONOMY

I. A NEW APPROACH TO NUMERICAL PREDICTION OF MONTHLY EVAPOTRANSPIRATION, RUNOFF, AND SOIL MOISTURE STORAGE

H. LETTAU

Abstract

The background and development of a theoretical method to solve the water balance equation for land areas is discussed. A forcing function is considered that is essentially determined by the product of absorbed solar energy multiplied by monthly precipitation; the response function is soil moisture in its month-to-month variations. A very simple parameterization is provided by one nondimensional surface characteristic named the “evaporivity” (which measures the fraction of absorbed insolation utilized during the month in the vaporization of concurrent precipitation) and a characteristic lag-time interval of the order of 2 to 3 mo to express “delayed” evapotranspiration and runoff. The solution is obtained by a closed integration of the water balance equation (rather than employment of regression or correlation methods) and yields a coherent set of data on monthly evapotranspiration, runoff, levels of exchangeable soil moisture, and storage changes. For verification, area averages for the central plains and eastern region of North America are calculated and compared with several years of actual data analyzed and evaluated by Rasmusson. In spite of simplifying tentative assumptions used in the model calculation, the agreement between predicted and observed data is improved in comparison with results of the earlier prediction methods discussed by Rasmusson.

Full access
H. Lettau

Abstract

Full access
H. Lettau
and
J. Zabransky

Abstract

A semi-empirical model for wind profile modification, in airflow from land to water on the 2-km scale, is discussed using 1950 Lake Hefner data. Initial (rough-surface) and final (smooth-surface) profiles are derived and an interpolation model is developed and used to calculate intermediate profiles. A distribution of vertical velocity with height is determined with the aid of the assumption that the divergence in the x direction near the surface is compensated by convergence in the y direction aloft. The rate of growth of the internal boundary layer follows as a direct consequence of the model. Theoretical computation is compared to Super's observations of wind profile variations with fetch on Lake Mendota. A diurnal variation of subsidence found over Lake Hefner is related to the variation in thermal stratification and its effect on the wind profile structure. Momentum budgets between land and lake are constructed and give evidence of a diurnal variation in surface stress over Lake Hefner.

Full access
Heinz H. Lettau

Abstract

No Abstract Available.

Full access
H. H. LETTAU
and
M. W. BARADAS

Abstract

The model of evapotranspiration climatonomy is expanded by the incorporation of feedback to account for parameter dependencies on soil moisture. The concept is applied to climatic and hydrologic data for the 46-km2 Mabacan River watershed in Laguna, Philippines, in a humid tropical climate where average runoff (about 1.2 m/yr) exceeds evapotranspiration (about 0.7 m/yr) significantly. Of primary concern was the development of methods for parameter evaluation or watershed calibration. The numerical model requires input of mass and solar energy (monthly precipitation and global radiation) to predict monthly runoff, soil moisture storage, and evapotranspiration. For the investigated 12-yr period, averages of monthly runoff and the root-mean-square value of departures from the annual mean are 100 ± 22.3 mm from observations and 100 ± 20.5 mm from model simulation, with a linear correlation coefficient of 0.89. Computed monthly evapotranspiration was comparable to empirical data obtained at the International Rice Research Institute (about 10 km north of the watershed and about 65 km southeast of Manila). Weaknesses of the parameterization are discussed with the aid of model-simulated runoff for each month from January 1965 to December 1968, a sequence of a “dry”, a “wet”, and a “normal” year.

Full access
Heinz H. Lettau
and
Edward J. Hopkins

Abstract

The present study is the third in a sequel by Lettau and Baradas. The Evapoclimatonomy I model has been discussed and applied by various authors including Hare, Hay, Kutzbach, Pinker, and Corio. In the present study the semiempirical method of “watershed calibrations” proposed in Evapocilmatonomy II is replaced by a less subjective diagnostic scheme. Water storage on land areas is defined in terms of “evaporable water.” The climatic mean annual course of balanced monthly evaporation is reconciled with that of monthly runoff, where both are defined as functions of evaporable water.

Model application is exemplified by three case studies, one each for a continental, a maritime, and a tropical climate. Two types of modifications are investigated., 1) the monthly balance of evaporable water in response to a simulated summer drought, for a continental or summer-rain climate in comparison with the less severe long-term reactions in a maritime or winter-rain climate; 2) the monthly balance of evaporable water and runoff reduction as caused by rainforest depiction in tropical Panama.

Evapoclimatonomy III is compared with two other methods for climatic water budget evaluation: 1) the Thornthwaite-Mather method using data for Wilmington, Delaware; and 2) the Penman method in a “monthly reconciliation of meteorological and hydrological variables” for Barro Colorado, Panama, by Dietrich et al.

Full access