ASCAT observations of downdrafts from mesoscale convective systems

ASCAT observations of downdrafts from mesoscale convective systems Thomas 1, 2 Kilpatrick , Shang-Ping 1, 2 Xie 1. Scripps Institution of Oceanography, La Jolla, CA. 2. International Pacific Research Center, Honolulu, HI. Introduction Mesoscale convective systems (MCSs) produce a large proportion of tropical rainfall and, through latent heating effects, are intimately coupled to the largescale circulation. Downdrafts of air cooled by evaporating raindrops are an essential component of MCSs. However, it has been a challenge for satellite scatterometers to observe these MCS downdrafts because of rain contamination. Here we utilize surface wind observations from the Advanced SCATterometer (ASCAT) and concurrent rain rate observations from the Microwave Humidity Sounder (MHS) to identify MCS downdrafts over the western equatorial Pacific; more than 1300 downdrafts are identified over the observation period (2009– 2014). ASCAT scatterometer MCS (image: Andrew (image: EUMETSAT) Grosse) Leading line/trailing stratiform MCS structure. Adapted from Houze (2004) and Houze (1977). Composite analyses Fig. 2. TRITON buoys Data and method MCS downdrafts are identified from individual snapshots of ASCAT winds (2009 – 2014) as regions of horizontal wind divergence exceeding 10 -4 s-1. We are careful to limit rain contamination by omitting all wind vector cells with rain rates ≥ 3 mm h-1. Concurrent rain rate measurements area available from Metop’s MHS. We validate the ASCAT observations by compositing strong divergence events against independent satellite observations of IR temperature and rainfall rate, and buoy measurements of air temperature. IR temperature Example: two mesoscale downdrafts Rain rate Buoy temp. variance Fig. 3. Composites around ASCAT surface wind divergence events, centered at the locations of the SWDEs. The 95% confidence intervals (twice the standard error) are indicated. Fig. 4. IR temperature (color; K) composite around ASCAT surface wind divergence events. Times (– 18 h, etc. ) are relative to the observed divergence. Composite ASCAT wind vectors and divergence contours (thin=0 s− 1, thick=10− 4 s− 1) are overlaid in the 0 h panel. Summary Fig. 1. (a) Surface winds (arrows) and surface wind divergence (× 10− 5 s− 1; color) measured by ASCAT at 11 -Oct-2009 00: 05 UTC. Rainy areas are stippled, with rain rates ≥ 3 mm h− 1 in darker shade; rain rate measurements are from the concurrent MHS aboard the Metop satellite. (b) Surface air temperature recorded by the TRITON buoys at 147°E 2°N (green) and 147°E 0°N (purple), whose locations are marked by triangles in (a). The UTC day (bottom) and hour (top) are marked along the abscissa. The dashed line marks the time of the wind and rain measurements in (a). • The ASCAT scatterometer detects downdrafts from individual MCSs. The horizontal wind divergence associated with these downdrafts is 1– 2 orders of magnitude stronger than a previous study based on a Quik. SCAT product (Mapes Milliff Morzell 2009). • Observations of OLR, rain rate, and surface temperature corroborate the downdrafts. • ASCAT-observed downdrafts lag the peak convection by 8– 12 h, most likely because ASCAT detects the mesoscale downdrafts in the trailing stratiform region of MCSs. • The global observations by the satellite scatterometer open a new avenue for studying MCSs. References Kilpatrick, T. and S. -P. Xie (2015), ASCAT observations of downdrafts from mesoscale convective systems, Geophys. Res. Letters, in press, doi: 10. 1002/2015 GL 063025. Mapes, B. , R. Milliff, and J. Morzel (2009), Composite life cycle of maritime tropical convective systems in scatterometer and microwave satellite observations, J. Atmos. Sci. , 66, 199– 208.