Overview of research in the Desai Lab

Our group researchs a wide variety of phenomena related to surface-atmosphere coupling and carbon/water cycles, focusing on the roles of land cover variability, disturbance, and management, in collaboration with scientists from many disciplines. Please visit individual research project pages for more details. Some of the major research techniques and advances we employ in our research are described below.

Observing surface-atmosphere exchange with eddy covariance methods

The eddy covariance method takes advantage of high-frequency measurements of atmospheric turbulence and fluctuations in scalars to directly observe surface-atmosphere trace-gas and energy exchange on scales of 1-10 km. The measurements can be related to biophysical state parameters, remotely-sensed radiative quantities or meteorological variables. Surface layer micrometeorology theory can be used to define flux footprint boundaries and quantify non 1-D advective flows. We have instrumented sites in northern Wisconsin and Michigan as part of the Chequamegon Ecosystem-Atmosphere Study.

We have also worked on developing standardized algorithms to decompose the observed net fluxes into their components: respiration and photosynthesis via creating and testing missing data gap-filling routines.

Assimilating in-situ and remotely sensed observations into regional land-atmosphere models

Through collaborations within a number of flux tower field projects, we have worked on improving biogeochemical models of forested, wetland, and regional ecosystems. This work relies on applying the mathematical technique of data assimilation so as to optimally estimate model parameters for a given set of known observations and their uncertainty. Remote sensing, meteorological, and flux data are all required to drive or optimize these models and scale local observations up to the regional and global level. We have applied these techniques to many sites and are in process of extending this methodology to CO2 and CH4 fluxes from wetlands in northern watersheds.

A unique model we have used is the Ecosystem Demography (ED) model. ED is a size and age structured ecosystem model that allows for explicit inclusion of stochastic and imposed disturbance, mortality, forest harvest and multiple vegetation types within a grid cell (Moorcroft et al, 2001). Time rate of change equations for mean-moment state variables are conditioned on stand age since disturbance. Multiple cohorts, each with similar height and vegetation type, grow, die and reproduce within patches of different age. We have used this model to understand ecosystem trajectories in heterogeneous land managed regions.

Using trace gas observations and transport models to infer regional fluxes

A mesoscale tower-based network of carbon dioxide mixing ratio sensors and/or observations of aircraft and column CO2 can be used to assess regional carbon budgets using top-down mass-balance budgets and inversion techniques. These techniques have been applied in ChEAS? during the Ring of Towers study in 2004 and also for the Airborne Carbon in the Mountains 2007 field experiment in the Rocky Mountains and the Cycling of Carbon in Lake Superior (CyCLes? ) project. Ground-based Fourier-transform spectrometers can also be used in these efforts to provide additional constraint on top-down budgets.

Constraining of regional land-atmosphere fluxes with top-down and bottom-up methods

Estimating fluxes at the regional scale remains a major research hurdle and priority. By combining the above mentioned techniques at the bottom-up (flux tower, remote sensing, plot level observation) with top-down (tracer, tall tower, aircraft) data using various budgeting and data assimilation techniques, we can arrive at optimal estimates of ecosystem state and parameters so as to improve future prediction of ecosystem fluxes. We are working on using this in the Chequamegon Ecosystem-Atmosphere Study to constrain wetland and whole region carbon fluxes, the Airborne Carbon in the Mountains 2007 study to constrain ecosystem carbon fluxes in complex terrain, and the Cycling of Carbon in Lake Superior study to constrain air-sea fluxes of CO2 from Lake Superior.

Developing prognostic tools to assess atmospheric boundary layer structure

Convective boundary layer development is strongly controlled by the surface exchange of energy and water vapor. The variability of surface energy fluxes, in turn, is related to terrestrial state variables, such as soil moisture, vegetation type and cover. We have worked in several field campaigns including Southern Great Plains 1997 (SGP97), International H₂O Project 2002 (IHOP) and the Airborne Carbon in the Mountains Experiment (ACME07) to examine this coupling between land and atmosphere.

One of our studies of a precipitation dry-down event revealed strong coupling between soil moisture as measured by airborne microwave remote sensing and surface virtual potential temperature (buoyancy) flux as measured by in-situ eddy covariance and Bowen ratio systems (Desai et al., 2006).