The Southern Plains of the U.S. is in the grips of a serious heat wave and drought this summer. Temperatures have been soaring into the 100s and even 110s routinely over Texas and Oklahoma in recent weeks, with little to no widespread rainfall. Recall, however, that last summer’s conditions were quite different with substantial rain and moist conditions from Mexico up to Colorado.
SPoRT has been generating a daily-updated Greenness Vegetation Fraction (GVF) over the Continental U.S., which is derived from MODIS overpasses of Normalized Difference Vegetation Index that measure the coverage of healthy green vegetation from the near-IR and red-visible channels. Figure 1 shows the SPoRT GVF from last summer, valid 1 August 2010, centered on Texas. This image show fairly large values of GVF for the region, especially over west Texas and Mexico. Meanwhile, the SPoRT GVF for 1 Aug 2011 (Fig. 2) depicts markedly different values of vegetation. GVF values are considerably lower over much of Mexico, Texas and Oklahoma. The difference field in Fig. 3 highlights the area of decreased GVF over the region. Some local areas have seen decreases in GVF of over 40%.
Such dramatic variations in GVF from one year to the next can have significant ramifications to numerical weather prediction, because operational models typically employ a climatological GVF that remains the same year-to-year.