WindSat Observations of Soil Moisture and Vegetation Water Content Associated with the European Heat Waves and Indian Monsoon Onset
Abstract
The 2003 European summer heat wave was an extreme climatic anomaly. Understanding its evolution and the underlying physical mechanisms are important for improving climate seasonal forecast and developing early warning systems. Regional climate model experiments suggest that land-atmosphere coupling played an important role in the evolution of the heat wave. Extremely low soil moisture and severe vegetative stress conditions reduced the latent heat cooling and created a positive feedback effect for soil moisture-temperature interactions. This extended the duration of the heat wave and accounted for a majority of the number of hot days. However, only limited ground observational data are available to corroborate the dry soil conditions and ensuing soil moisture dynamics. Measurements from the passive microwave WindSat instrument provide simultaneous retrievals three key parameters for studying the evolution of a heat wave: 1) surface soil moisture, 2) vegetation water content and 3) land surface temperature. Using these measurements we developed four-year climatology (2004 - 2007) and then examined differences between the climatology and the 2003 heat wave. We show that spatial and temporal variations in these 3 parameters capture features of the extreme temperature of the 2003 heat wave and agree well with regional climate simulations in terms of the soil moisture evolution and anomaly. The annual cycle of the Asian summer monsoon carries the passage of onset isochrones (contours of constant time-of-travel) of precipitation. Model simulations and data analysis suggest that the soil moisture impacts the motion of the onset isochrones from Kerala to New Delhi. To the immediate north of the isochrone, non convective anvil rains enhance soil moisture of once very dry surfaces and generate large buoyancy ahead of the isochrones due to strong solar heating to these surfaces. Newer convective elements are formed as towering cumulus and cumulonimbus clouds that are advanced by the divergent circulations of the parent isochrone to the north and eventually to the northwest. As the older clouds die; the newer ones grow and slowly replaces the parent isochrone behind it. In this way, the soil moisture pulls the isochrones forward. To verify this theory, we classified the isochrones into two advancing isochrones when the isochrones are moving, and stationary ones when they are not. Then we binned the Windsat soil moisture data ahead of isochrones according to its distance to the isochrones. We found that there is a soil moisture enhancement ahead of advancing isochrones (solid line in the lower right figure), and no soil moisture enhancement ahead of stationary isochrones (dash line). This is the first evidence to support this theory of monsoon onset isochrone movement.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.H11N..06L
- Keywords:
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- 1631 GLOBAL CHANGE / Land/atmosphere interactions;
- 1817 HYDROLOGY / Extreme events;
- 1855 HYDROLOGY / Remote sensing;
- 1866 HYDROLOGY / Soil moisture