Category: Uncategorized (Page 7 of 7)

Northern California is currently in the midst of its first significant rainfall of the season.  The plume of moisture currently advecting on shore can be traced all the way back to a weakened typhoon in the western Pacific.  I’ve heard a lot of chatter that this is an example of an “atmospheric river“, a strong, steady plume of moisture advected poleward out of the moist tropics.  We have come to appreciate recently that atmospheric rivers are responsible for many of the significant rainfall events here in California and on the western coasts of several continents.   But, I have to ask myself when an atmospheric river is really an atmospheric river and when it isn’t.    The TPW loop shows that this moisture is not arriving from the deep tropics, but rather, from a ex-tropical system.  The original definition of an atmospheric river makes no mention of the origins of the moisture, and as such, it is perfectly general.  But, the connotation has generally been one in which the moisture originates from the TPW maximum around the equator.  The precise definition becomes important as those of us in CA focus research efforts on the formation mechanism of atmospheric rivers and try to determine how they will change under global warming scenarios.  Therefore, I would like to propose that “rivers” such as the one currently impacting CA simply be grouped in with normal warm frontal processes and we reserve the use of “atmospheric rivers” to those features extracting moisture directly from the deep tropics.

I wanted to mention one thing about a recent paper which used the ICON general circulation model to simulate states of radiative convective equilibrium over ocean.  The authors ran the model on 5 different domains of varying total area.  The largest simulation was 256 times larger than the smallest.  That’s an impressive range!  The authors convincingly show that the domain size affects many aspects of the simulated atmosphere, but the one I want to mention is the physical morphology of the aggregated, high moisture regions (their figure 6).  The larger the domain size, the more structure there is in the moisture field.  I’ve done a lot of work recently on the links between total moisture and cloud behavior, and I can say that all that variation in the moisture field is critical for clouds.  We convective theorists have based a lot of recent thinking about the interaction of convection with it’s environment on the results of RCE simulations, but very few people have thought about the sensitivity of these ideas and theories to basic simulation properties (like domain size or shape) on the kinds of scales these authors do.  I agree with the authors that their results suggest a few new avenues of inquiry that could be rather revealing.

Source: Radiative convective equilibrium as a framework for studying the interaction between convection and its large-scale environment – Silvers – 2016 – Journal of Advances in Modeling Earth Systems – Wiley Online Library

OK, here goes a first post.  I thought I would start off by talking a little bit about a recent paper on which I was a coauthor.  The paper can be found here: Ubiquity and impact of thin mid-level clouds in the tropics.  In this paper as part of his graduate work at Stockholm University, lead author Quentin Bourgeois used the CALIPSO lidar to look for a predominantly overlooked cloud type in the tropical convective atmosphere: optically and physically thin clouds above the freezing level but below the peak outflow layer from deep convection.  Quentin found that clouds meeting these criteria were surprisingly common in the tropics both over ocean and over land.  I think there are two main takeaways here.  The first is that the cloud population in the tropics is much more complex than we often give it credit for.  Because these thin clouds do not rain, they don’t show up on radar, and because of their ethereal nature, they are difficult to simulate.  This has made them easy to overlook.  The second is that simple methodologies, like the one employed in this paper, can be incredibly powerful.  We learned a lot about these clouds without obscuring what we were examining.

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