Location: Hydrology and Remote Sensing Laboratory
Title: Effect of dew on aircraft-based passive microwave observations over an agricultural domain Authors
Submitted to: Journal of Applied Remote Sensing (JARS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 30, 2012
Publication Date: December 1, 2012
Citation: Du, D., Jackson, T.J., Bindlish, R., Cosh, M.H., Li, L., Hornbuckle, B., Kabela, E. 2012. Effect of dew on aircraft-based passive microwave observations over an agricultural domain. Journal of Applied Remote Sensing (JARS). DOI: 10.1117/1.JRS.6.063571. Interpretive Summary: Microwave remote sensing can provide reliable measurements of surface soil moisture. However, some land surface conditions can have a perturbing influence on soil moisture retrievals. In the Soil Moisture Experiments in 2005 (SMEX05), we attempted to contribute to the understanding of the effect of dew using concurrent ground and aircraft observations. Early morning flights were conducted with an airborne microwave radiometer from June 19 to July 2, 2005 in Iowa, USA over an agricultural domain. Results of the experiment indicated that dew had a small but measurable effect on the observed 10.7 GHz (X-band) brightness temperatures. The results indicate that the horizontal polarization emissivity increased 0.015 - 0.04 for the corn sites, 0.014 - 0.02 for soybean, and 0.01 for forest sites as dew evaporated. These results suggest that the presence of dew decreases X-band land surface emissivity slightly and the effect of dew varies with vegetation types. Our findings are consistent with and expand on previous investigations that found that the effect of dew depends on both the type of vegetation and the wavelength of observation, but further studies should be conducted to verify this hypothesis.
Technical Abstract: Dew is the natural deposition of water condensed from relatively warmer air onto cooler surfaces. The formation of dew typically happens in the early morning and depends on a number of factors including relative humidity near the surface, surface wind speeds, the temperature gradient near the land surface, and net radiation. Dew on vegetation increases the overall water content of the vegetation canopy. Vegetation has a significant effect on the microwave emission of terrestrial surfaces. Since the effect of vegetation on microwave emission is primarily due to the liquid water content of the vegetation canopy itself, dew on vegetation raises an important question for satellite-based microwave sensors with early-morning overpass times: “What is the impact of early morning dew on the microwave emissivity and consequently soil moisture retrievals?” This is an important question for current and future satellite missions such as the Special Sensor Microwave Imager (SSM/I), WindSat, Soil Moisture and Ocean Salinity (SMOS) mission, Aquarius and NASA’s future Soil Moisture Active Passive (SMAP) mission. Isolation of the effects of dew is difficult since other factors that influence the brightness temperature/emissivity, such as soil moisture and canopy and soil temperature also change with time. Previous studies have come to different conclusions. Since the observed effects of dew have been rather small, well-designed and controlled experiments must be made in order to further contribute to our understanding of the effects of dew. As part of SMEX05, such an experiment was conducted using aircraft passive microwave measurements with concurrent ground observations over an agricultural domain (Iowa, USA. The results indicated that there can be a small but measurable increase in the 10.7 GHz emissivity as the amount of dew decreases. Interpretations should consider that we used an aircraft based X-band sensor for the specific conditions encountered in SMEX05. These analysis results as well as the experimental techniques are useful for current and future missions include early morning overpass times.