NASA LAND SURFACE HYDROLOGY PROGRAM
Current Research Under NRA-98-OES-11

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HEAT AND MOISTURE TRANSPORT IN A COUPLED
LAND-ATMOSPHERE SYSTEM
 

Peter P. Sullivan¹ (P.I.), Edward G. Patton^1,2, and Chin-Hoh Moeng¹

 ¹National Center for Atmospheric Research
²Department of Soil, Water and Climate, University of Minnesota

November 1999
 

Project Summary

The objective of this research is to advance our understanding of the dynamical and hydrological coupling between land surfaces and the cloud-free atmospheric planetary boundary layer (PBL) on the basis of its calculation through a coupled land-surface large-eddy simulation (LES) model. High resolution computational grids will be used that simultaneously capture fine scale turbulence physics but also are adequate to examine the PBL's response to kilometer scale heterogeneous forcing. The specific research will address: 1.) prediction of surface temperature and moisture, their fluxes, and the land-surface atmosphere moisture budget at scales varying from less than 50 meters to more than 30 kilometers; 2.) use of remote sensing and in situ observations as initial conditions and as validation for fully coupled numerical simulations of typical field campaigns; 3.) elucidating the temporal and spatial scales at which land surface heterogeneity influences the surface fluxes of heat and moisture, boundary layer depth, and turbulent motions; 4.) exploring the effect of soil-vegetation properties on the diurnal cycle of the PBL; and 5.) development of better parameterizations for surface fluxes and boundary layer depth in the presence of heterogeneous land surfaces for use in larger scale models and remote sensing. Data from the Southern Great Plains 1997 (SGP97) experiment will be used extensively for model initiation and comparison.
 

 1. Introduction

The importance of the atmospheric planetary boundary layer (PBL) in land-atmosphere interactions is well known. Turbulent processes in the PBL regulate the exchange of momentum and scalars between the land surface and overlying atmosphere. Furthermore, concentrations of the important elements in the surface energy balance, heat and moisture, influence the fluxes themselves, in a feedback loop.

In this work, we will investigate interactions between the atmospheric PBL and land surfaces by developing a fully coupled land-surface atmosphere numerical model. For the atmospheric component, we will use our well-established large eddy simulation (LES) code (Moeng, 1984; Sullivan et al., 1996). LES has become an important tool for investigating turbulent processes in the atmospheric boundary layer. The output of LES is time dependent 3-D turbulence fields which can be used to generate ensemble statistics and to identify organized coherent structures. An example of a recent LES focusing on PBL entrainment is given in the animation in Figure 1.

2. Proposed Research

Historically, LES has focused on basic studies of atmospheric turbulence at spatial scales on the order of tens of meters and temporal scales of a few hours. Experience has taught us that the grid resolution needs to be O(50 m) and less in simulations of the atmospheric PBL to resolve instantaneous local fluxes of heat, moisture and momentum, establish realistic entrainment rates at the PBL inversion, and capture the important coherent turbulent structures in the PBL, like thermal plumes. At the same time, the mixed-layer modeling of McNaughton and Raupach (1996) predicts that the time scale for the response of the PBL to a sudden change in surface conditions is many hours and that the state of the PBL in late afternoon reflects the whole history of the PBL since dawn. Thus, we anticipate that to include heterogeneous land surface forcing we need to consider computational domains and time scales that are large compared to those routinely considered in LES of the PBL. For the purposes of this work, we will use grids with O(6x10⁶) grid points that simultaneously capture relatively fine scale turbulence physics but are also adequate to examine the PBL's response to kilometer scale heterogeneous forcing.

2.1 Development of a Coupled Land-Surface LES

LES studies have traditionally focused only on PBL dynamics that result from prescribed surface temperature and moisture. In this proposal, we will extend our LES and develop a coupled land-surface PBL simulation tool that predicts surface temperature and moisture (and hence surface fluxes), covering scales from tens of meters up to several kilometers. This will then allow us to examine the interaction between heterogeneous land surfaces and turbulent motions, but more importantly, the coupled evolution and response of the PBL to larger scale forcing. In this way, the land surface hydrology and atmospheric dynamics are fully coupled.

We plan to couple our LES with the Oregon State University (OSU) Coupled Atmosphere-Plant-Soil (CAPS) model (Chang et al., 1999). We base our preliminary choice of the OSU CAPS model mainly on a balance between computational efficiency, their inclusion of equations comprehensive enough to represent the physical processes thought to be most important, and the availability of Southern Great Plains 1997 (SGP97) measurements to prescribe required parameters.

 2.2 Initialization and Validation of Coupled Land-Surface LES with Remote and In Situ Measurements

Typical PBL LES relies on specified surface boundary conditions and rarely includes a time-evolution of the surface forcing. Our proposed research takes advantage of remote and in situ measurements from the SGP97 field program as initial conditions and forcing for the simulations. Simulations will focus on measurements taken in both the El Reno and Little Washita watersheds due to the comprehensive measurements available at these sites, which are required to initiate and drive our simulations. The proposed coupling of LES with a soil-vegetation-atmosphere transfer model, combined with measurements of air-soil-vegetation properties, will eliminate the need to specify the surface fluxes in LES and will allow for a realistic prediction of the PBL time evolution.

 2.3 Canonical Studies of the PBL with Coupled Land-Surface LES

We intend to examine the effect of surface heterogeneity on the development of the bulk properties of the PBL including surface fluxes (heat, moisture, and momentum), boundary layer depth and entrainment rates. These bulk parameters and their dependence on land surface heterogeneity are of major interest, but also are the building blocks needed to construct parameterizations for larger scale models. Thus, we anticipate conducting a series of coupled land-surface LES experiments with systematic variations in land surface heterogeneity with the objective of quantifying its effect on the average PBL properties. The heterogeneity can take the form of varying soil type, soil moisture content, and or vegetative cover.

The bulk PBL properties are, however, sensitive to the more detailed aspects of PBL turbulence, like the presence of turbulent coherent structures. It is of importance to identify the effects of spatially varying large scale forcing not only on the average PBL properties, but also on the turbulence. We expect that altering the surface forcing through a heterogeneous land surface can potentially alter the formation, frequency, and strength of thermal plumes, which in turn can modify important PBL dynamics, like the entrainment rate. At the same time, altering the turbulent motions modulates the surface fluxes, leading to a coupled land-atmosphere system that is sensitive to surface heterogeneity. We intend to identify the changes to turbulent structures and hence to net heat and moisture fluxes through our canonical experiments.

2.4 Diurnal Cycling of the PBL with Coupled Land-Surface LES

The development of a coupled land-surface LES code will also allow us to investigate the effects of time varying, including diurnal forcing on the PBL development. Most LESs of the PBL focus on the idealized situation where the large scale forcing is fixed and the turbulence statistics and structures are nearly quasi-steady, which is representative of late morning and early afternoon. Daily diurnal LES calculations of the PBL are not routinely performed for the following reasons: 1.) initial conditions and large scale forcings are not typically documented in sufficient detail; 2.) the calculations are computationally expensive since the time step is small; and 3.) calculation of stable PBLs is very much an emerging technology. We propose to concentrate our efforts on diurnal forcing during unstable and neutral regimes and, depending on the success of these calculations, the computations will be extended into the stable regime.
As a prototype experiment, we have carried out uncoupled LES of the standard Wangara day 33 case with prescribed diurnal cycle of surface heat and moisture fluxes, some results are depicted in Figure 2.

2.5 Parameterizations for Larger Scale Models and Remote Sensing

Building suitable parameterizations for large-scale models requires a precise understanding of how to determine model parameters for cell sizes of O(10 km) or larger from LES calculations at much higher spatial resolution O(50 m). This "upscaling" of the LES results is similar to what needs to be done with remote sensing data which is frequently sampled at much finer resolution than a typical large scale model. This further emphasizes the need to understand landscape heterogeneity and more precisely, to understand the length scale at which that variation significantly impacts bulk PBL properties. The canonical numerical experiments outlined above will form a database from which PBL parameterizations for larger scale models can be built in a systematic manner.

References:

Chang, S, Hahn, D., Yang, C.-H., Norquist, D., and Ek, M.: 1999, Validation study of the CAPS model land surface scheme using the 1987 Cabauw/PILPS dataset. J. Appl. Meteorol. 38, 405-422.

McNaughton, K. and Raupach, M. R.: 1996, Responses of the convective boundary layer and the surface energy balance to large scale heterogeneity. In J. Stewart (ed.), Scaling up in hydrology using remote sensing, Wiley and Sons, Chichester, England, pp. 171-182.

Moeng, C.-H.: 1984, A large-eddy simulation model for the study of planetary boundary-layer turbulence. J. Atmos. Sci.41, 2052-2062.

Sullivan, P. P., McWilliams, J. C., and Moeng, C.-H.: 1996 A grid nesting method for large-eddy simulation of planetary boundary layer flows. Boundary-Layer Meteorol. 71, 247-276.