Andrew J. Oliphant

Observed Impact of Atmospheric Aerosols on the Surface Energy Budget

AbstractAtmospheric aerosols scatter and potentially absorb incoming solar radiation, thereby reducing the total amount of radiation reaching the surface and increasing the fraction that is diffuse. The partitioning of incoming energy at the surface into sensible heat flux and latent heat flux is postulated to change with increasing aerosol concentrations, as an increase in diffuse light can reach greater portions of vegetated canopies. This can increase photosynthesis and transpiration rates in the lower canopy and potentially decrease the ratio of sensible to latent heat for the entire canopy. Here, half-hourly and hourly surface fluxes from six Flux Network (FLUXNET) sites in the coterminous United States are evaluated over the past decade (2000–08) in conjunction with satellite-derived aerosol optical depth (AOD) to determine if atmospheric aerosols systematically influence sensible and latent heat fluxes. Satellite-derived AOD is used to classify days as high or low AOD and establish the relationship...

The Lake Tekapo Experiment (LTEX): An Investigation of Atmospheric Boundary Layer Processes in Complex Terrain

A research program on atmospheric boundary layer processes and local wind regimes in complex terrain was conducted in the vicinity of Lake Tekapo in the southern Alps of New Zealand, during two 1-month field campaigns in 1997 and 1999. The effects of the interaction of thermal and dynamic forcing were of specific interest, with a particular focus on the interaction of thermal forcing of differing scales. The rationale and objectives of the field and modeling program are described, along with the methodology used to achieve them. Specific research aims include improved knowledge of the role of surface forcing associated with varying energy balances across heterogeneous terrain, thermal influences on boundary layer and local wind development, and dynamic influences of the terrain through channeling effects. Data were collected using a network of surface meteorological and energy balance stations, radiosonde and pilot balloon soundings, tethered balloon and kite-based systems, sodar, and an instrumented light aircraft. These data are being used to investigate the energetics of surface heat fluxes, the effects of localized heating/cooling and advective processes on atmospheric boundary layer development, and dynamic channeling. A complementary program of numerical modeling includes application of the Regional Atmospheric Modeling System (RAMS) to case studies characterizing typical boundary layer structures and airflow patterns observed around Lake Tekapo. Some initial results derived from the special observation periods are used to illustrate progress made to date. In spite of the difficulties involved in obtaining good data and undertaking modeling experiments in such complex terrain, initial results show that surface thermal heterogeneity has a significant influence on local atmospheric structure and wind fields in the vicinity of the lake. This influence occurs particularly in the morning. However, dynamic channeling effects and the larger-scale thermal effect of the mountain region frequently override these more local features later in the day.

A note on the height variation of foliage clumping: comparison with remote sensing retrievals

Clumping index (CI), quantifying the level of foliage grouping within distinct canopy structures relative to a random distribution, is a key structural parameter of plant canopies and is very useful in ecological and meteorological models. In this letter, we report on validating the global foliage clumping map derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) data at 500 m resolution using new information about vertical profiles of foliage clumping in a wide range of forest type stands. We report that in moderate to dense forest stands with developed undergrowth layer, in situ measurements near the ground surface may considerably underestimate the overall canopy-level clumping effect. This is because the large gaps between tree crowns at upper levels of the canopy may not be all measured near the ground due to obscurity by lower vegetation of branches. This information about height variation of CI is shown to be important for correct estimating and validating the foliage clumping from airborne/satellite remote sensing.

Microclimate and mass fluxes of debris-laden ice surfaces in Taylor Valley, Antarctica

Abstract This study investigates the microclimate and hydrology of debris-laden ice surfaces in the Taylor Valley, Antarctica, in early summer, focusing on the onset of melt. Measurements of energy and mass fluxes were made on an outwash fan and in moraines near the terminus of Taylor Glacier. The surface microclimate was strongly controlled by absorbed solar radiation, with a low albedo of 0.17. Seasonal warming of the substrate led to an abrupt shift in thermal and hydrological patterns as temperatures exceeded freezing point. Within a week the Bowen ratio switched from 2.05 to 0.48 and mass losses to the atmosphere increased four-fold from 0.39 to 1.6 mm d-1. Melt onset also produced complex ground temperature patterns with strong diurnal damping below the freezing front. These patterns were caused by phase changes in the freezing front, coupled with an abundant water supply from local runoff. Of secondary importance to the surface energy balance and mass fluxes was the effect of local winds on boundary layer characteristics. This resulted in larger mass losses during the more turbulent, warmer and drier down-valley flows.

An Evaluation of Semiempirical Models for Partitioning Photosynthetically Active Radiation Into Diffuse and Direct Beam Components

Berkeley Water Center; Lawrence Berkeley National Laboratory; Microsoft Research eScience; Oak Ridge National Laboratory; University of California‐Berkeley; University of Virginia; U.S. National Science Foundations Geographical and Spatial Sciences Program. Grant Number: 19002200; U.S. National Science Foundation Division of Environmental Biology. Grant Number: 1552976; U.S. Department of Agriculture Hatch Project. Grant Number: 228396