Saturday, October 22, 2011

The Trees and the Forest and Worms

Since I am a person who tends to takes a large-scale perspective on tropical meteorology (thousands of kilometers and greater), watching the radar all day and observing the small-scale features like clouds that are building blocks for large-scale tropical systems like that MJO has given me a very useful different perspective on the tropical atmosphere. I asked world renowned radar and tropical meteorologist Bob Houze whether there was anything interesting or new he has observed so far during DYNAMO that he has not seen in other regions of the tropics. One thing that was noticed in early October were what Bob termed “worms”. These were very narrow (~5 km diameter) convective features that formed upright vertically towers and poked up through very dry layers of the atmosphere. An example of such structure is shown in this vertical scan of radar reflectivity from October 3.

If we look at the sounding on October 3 (below), dry layers are noted near vertical pressure levels of 700 millibars and 400 millibars (1000 millibars is an approximate surface pressure at sea level in the tropics away from tropical cyclones, and pressure decreases upward away from the surface). However, these “worms” poked up through these dry layers. Typically convective clouds entrain (ingest) air from their environments, and if the environment is dry, evaporation/sublimation of cloud liquid water and ice within the rising cloud parcels cools the rising air, limiting how buoyant the clouds are. Clearly these worm clouds are not strongly feeling the dry environment and shooting right up through these dry layers, possibly indicating that entrainment is weak. The sounding also shows the wind profile in the vertical, indicating that there is very little wind at any height in the atmosphere until about 250 millibars is reached. This level is in the upper troposphere. The cloud top occurs somewhat above this level at 15 km above the surface. Maybe this lack of wind and associated wind shear through much of the troposphere is limiting entrainment? The atmosphere is also relatively unstable on this day, which would tend to foster greater buoyancy of convective air parcels and make them somewhat less sensitive to entrainment of dry air.

One thing I appreciate from watching the radar over the last week and zeroing in on a 150 km radius neighborhood around the radar is that it becomes very easy to lose a bigger picture perspective of what is going on in the Indian Ocean. While my previous posts have expressed some skepticism about the onset of a new MJO precipitation event in the DYNAMO observing region, it should not be forgotten that the Indian Ocean has undergone a dramatic moistening over the last couple of weeks, as can be seen by comparing these plots of vertically-integrated water vapor for October 9 and today. These plots show the amount of water vapor in the atmosphere summed from the surface to top of atmosphere. We hypothesize that a necessary condition for MJO convection to occur is growth of a deep moist layer in the Indian Ocean so that a dry atmosphere does not generally stifle deep MJO convective clouds. This large scale moistening we have observed should aid the growth of an MJO precipitation event. So maybe the conditions are primed for a nice event after all?

If this MJO precipitation initiation does indeed occur, an interesting question will be what generated this large-scale moistening of the Indian Ocean. If we can determine this, then we might have the keys to understanding MJO initiation.

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