Early this morning, Michael Brennan (NHC – Senior Hurricane Specialist) noticed some interesting features in the SEVIRI (Meteosat-9) RGB Air Mass imagery surrounding and interacting with Tropical Storm Nadine in the far eastern Atlantic. The NHC (both the Hurricane Specialist Unit (HSU) and the Tropical Analysis and Forecast Branch (TAFB)) are currently in the midst of the NHC Proving Ground Activities and therefore they are in the process of evaluating the applicability the RGB Air Mass product in operations. Here is an excerpt with an example image below:
“The evolution of the RGB airmass product around Nadine has been quite interesting over the past 6-12 hours. Just before 00Z 9/21 Nadine looked more like a ‘typical’ TC with the airmass imagery showing a moist envelope of air in the vicinity of the TC itself. However, by this morning the upper-level PV anomaly to the northeast of Nadine appears to have helped push some sort of airmass boundary through the northern portion of the circulation (this is also evident in surface observations from the Azores and conventional geostationary imagery). Nadine now appears surrounded by a thin ribbon of reddish/brown color on the periphery of the circulation, indicative of warm moist mid-level air. The mid-level circulation of Nadine has also begun to elongate east-to-west in the imagery. I’m not sure if this helps us diagnose the ‘status’ of Nadine, but it definitely indicates some changes have occurred in the past few hours.”
To follow-up on what Mike posted this morning, I have attached (below) three RGB Air Mass images of Nadine from 12z overlaid with 300 mb absolute vorticity and heights, 500 absolute vorticity and heights, and 700 mb T along with MSLP from the 06z GFS. In training forecasters on the applicability of this product in operations, I have found that relating the red-coloring to vorticity and using the 300 and 500 mb absolute vorticity fields was quite effective. Notice how the red-colors match up best with the 300 mb vorticity, but you still get a signal at 500 mb. This could be explained by the differencing of the two water vapor fields which would give you a layer of approximately 300-500 mb where the dry, stratospheric air acts as a tracer to identify PV anomalies. In this case, the reflection of Nadine at upper-levels is very apparent, but much weaker than the Mid-Latitude low to the north-northeast. I also wanted to include the 700 mb T field again to diagnose the possible connection with the green coloring northwest of Nadine. The signal is weaker this morning, but there is still evidence of a warm layer north-northwest of Nadine (white “ellipse”) that looks separated from Nadine’s circulation and possible due to some overrunning warm air just north of the red-coloring. Another idea is that this might be warm air directly related to Nadine that is mixing with the impinging trough and strong upper ridge (594 dam and 1035 mb sfc according to the 6 hour forecast by the 06z GFS).
I know this is subjective, but I think it’s plausible and requires more research.
Also, when looking at the 500 mb heights and vorticity, does anyone else notice that Nadine appears to be captured by the trough already and that the latest guidance showing a visit to the Iberian Peninsula is looking more likely? This is a very interesting tropical to extratropical transition!
I have included a 16 hour animation below to better illustrate the evolution of Nadine on 09/21/12.