Changes in vegetation condition and surface fluxes during the North American Monsoon Experiment (NAME) 2004

Russell Scott, USDA-ARSChris Watts, U. of Sonora

Motivation

The NAM results in large changes to surface cover (i.e., It gets green!)

But, few observations have ever quantified this effect in terms of changing surface energy balance, especially south of the border (where things are really happening!)

September

Tucson

Surface energy balance

Rn = SW(1-albedo) + LWn= λE + H

EH

G

Goals and Methodology

0 5 10 15 20

-4

-2

0

2

4

6

8

Vertical Wind Speed

Water vapor density

Eddy covariance used to determine patch- or ecosystem-scale heat, water and carbon dioxide flux

hour

Use micrometeorological techniques to quantify changes in surface energy balance over a variety of representative biomes within the core NAM region in NAME 2004 experiment

Desert Scrub + BuffelgrassSubTropical Shrubland

Santa Rita Mesquite SavannaKendall Desert Grassland

Lucky Hills Shrubland

Charleston Mesquite WoodlandTropical Deciduous Forest

Metflux sites (eddy fluxes, basic met, soil moisture profiles)

Observations - Results

Big Picture • Vegetation response

Site Details: • Precipitation

• Vegetation

• Albedo

• Surface temperature

• Rn/Rs

• Evaporation

• Evaporative fraction

JuneJulyAugustSeptemberSPOT NDVI

10-day compositesTucson

Core of the veg changes

Southern sites greater and more intense precipitationBut all sites still had dry periods

Tropical Cyclones

Mean- 350 mm, 60% in July-Sept10016090160 mm

430210170 mm

Enhanced Vegetation Index

Much larger increases in VI observed for Sonora sites

Albedo Albedo Tesopaco TDF 9 12

Rayon STS 15 17

CM riparian woodland 10 8

SR savannah 15 14

KN grassland 18 17

LH desert shrub 18 17

Pre PostMonsoon

• 2-fold difference in broadband albedo

• But monsoon changes are very smallIncreased absorption in visible bandsCountered by decreases in near-infrared

Daytime LST from MODIS-Aqua

Sonoran sites LST drops by 25 ºC with Monsoon onset, much more than for the Arizona sites

Pre-monsoon data limited

s

n

s

n

R

LWalbedo

R

R1

Energy partitioning

Cooler, wetter surface, Rn/Rs

Larger changes down south, but considerable water

stress still evident at the rainiest of sites

Evaporation

Evaporative Fraction

λE + H

λE

Water limitation still evident even at rainiest site

SummarySummary

We developed a network of sites across a gradient of NAM forcing:

1) to better understand the land surface response in terms of energy, water and CO2 exchange

2) to help improve the understanding of the land-atmosphere feedback in the NAM region