Hone compositing procedure on NICAM 7km run (lots of cold pools to choose from). Using Sato et al 2009 criterion (1.5 C/1.5 h), I found n=698 CP events in western equatorial Pacific over 30-day simulation. Signal is pretty strong in SWD and OLR. I love this diagnostic because it selects for cases with strong downdrafts that penetrate to surface, it allows for easy comparison with ASCAT, and it builds on classic buoy studies (Addis et al, Young et al, Weller and Anderson). The strong relationship between SWD and SAT is presumably due to the robust physical link between the two, which we also see in the SAT snapshots with SWD overlaid (nicam_ascat_compare_1.pptx). Also note that using SAT as our benchmark will presumably allow comparison between clear-sky and cloudy regions. Clear-sky downdrafts should be visible to Seawinds.

We will look at composite maps of div, SAT, OLR for the “virtual buoys” shown above (NICAM grid points). Virtual buoy spacing is 3°×3°. 78 virtual buoys in total. Locations chosen based on Fig. 12 in Sato et al 2009 (CP event occurrence map).

n=698 CP events identified based on Sato et al. SAT criterion (1.5 C/1.5 h); composite buoy temp. shown at right. Max (blue) and min (red) surface divergence within radius of 50km, 80km, 100km, 150km of CP event (occurs at t=0). Divergence computed on 50km squares for direct comparison to ASCAT. Note that we compute the max and min separately (rather than averaging) because strong div + conv often found within radius. Clear spikes in both SWD and SWC. r=80km might be best signal. Note that filtering is not necessary (MMM had to remove background div).

Composite of minimum OLR within radius. We see a nice drop, with minimum actually at t=1.5 h.

Evolution of surface divergence. Red = convergence, blue = divergence. Contour levels are 2×10 -5 s -1. Peak anomaly is ~10×10 -5 s -1, roughly 30x stronger than MMM. Time scale of 3-6 hours, compared to 6-9 h in MMM. Note narrow spatial scale (~1°) of downdraft, compared to 4° in MMM. W-IGW propagation clear. Easterly trade regime clear. Downdraft a little stronger than preceding updraft, consistent with previous plot. I should check composites for 3.5km run as well.

SAT evolution. Contours mark 0.25° intervals (28.5 marked in white). Peak SAT depression exceeds 1.5°. Note the warm anomaly prior to cold pool, indicative of W-IGW propagation. WISHE? Surely we see moistening as well. This is one plot we cannot reproduce from observations, but it does show the strong link between surface divergence and the cool SAT.

OLR evolution. Contours mark 10K intervals; white=220 W m -2. Note the larger spatial scale relative to the divergence anomalies.

Example CP from 156E, 3S. Multiple cold pools are visible in frame, but the virtual buoy clearly records the ΔT from local SWD, roughly km across.

Later CP at same location. Here we see that CP is not centered on “buoy.” But by averaging over many CP events, CP is (roughly) centered in the composite.

Rms surface wind divergence is between 5×10 -5 s -1 and 9×10 -5 s -1 in the most active region. Much of this variance is probably due to up/downdrafts. Note that I computed div with 50km boxes for comparison with ASCAT, which surely has much smaller std(div). I wonder if 3.5km run is comparable.

Distribution of mean(max(div)) and std(max(div)) within 100 km of virtual buoy, for all times (not just CP events). Std(max(div)) is ~10, which gives us some context for the composite, which shows an increase of ~15. Std(min(div)) is ~7, so peak SWC in composite does not stand out as much as peak SWD.

Maps of std(max(div)) and std(min(div)), both computed for div within 100 km radius of virtual buoy. Units = s -1. Max > min overall, and largest std(max) is in southern hemisphere.

Now at 6°N.