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SUJET : Why Reflectivity?

Why Reflectivity? il y a 3 mois 2 semaines #4

In the first release of XyGrib we have included a new meteorological parameter known as "Simulated Composite Reflectivity". What does all this mean?

Reflectivity is the forecast degree of reflection of the atmosphere to a weather radar beam. Composite means that the entire atmosphere is expressed in a single layer (similar to total cloud coverage). Simulated means that this is not the actual real time weather radar image but the radar image that the numeric model predicts will exist in the future.

Ok, explanation aside, why is this important or valuable?

It's all about forecasting severe convection activity. Convection can be triggered when the atmosphere is unstable. Convection is the phenomena that causes thunder storms or squalls, supercell thunder storms and what is known as Mesoscale Convective Systems. All these bring violent winds, waterspouts and tornadoes.

Forecasting convection is difficult, as the global hydrostatic models by definition are not able of actually simulating convection. They can only imply its existence based on the stability of the atmosphere. We have two options of being informed by a global model (i.e GFS ECMWF) in regards to the instability and potential of convection.
  1. We can download altitude data for temperature and relative humidity in multiple layers. This can then by analysed in a Skew-T thermodynamic diagram. Good for the experts but not for the average user. Also needs big downloads
  2. Alternately we can use composite single layer indicators such as CAPE, CIN or Lifted Index. These are quite straight forward indicators and don't make heavy gribs
The problem is that the above only show us the potential for convection. It may or may not happen. There are a lot of other factors that influence whether convection triggers or not. Movement of masses of air, topography affects - you name it.

Regional (Local Area) models on the other hand, are non-hydrostatic models and they are capable of resolving convection when run at higher resolutions (mostly 4km and under). These models can do a better job than most of us in deciding whether general unstable conditions will actually lead to convection triggering.

The next relevant question is, OK, so it knows but how can it show convection to us? Answer:- With simulated Radar reflectivity. The reflectivity is driven by the amount and type of water in the atmosphere. Hail causes high values of reflection and when and where does hail exist? Where there is severe convection (like in the middle of a cumulonimbus).

Rule of thumb - values of 45 or 50db and up mean thunderstorms or other larger convective events.

At this time it appears that only openWRF Grib files include reflectivity. These can be found at openskiron.org . Following is an image in XyGrib of very severe convection that hit Ibiza and Formentera on the 9-10th of August 2017.


A case study presentation on that event can be found at this link

Feel free to discuss convection, reflectivity and other indicators in this topic
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Dernière édition: par davidgal.

Why Reflectivity? il y a 3 mois 2 semaines #6

Does this tell you more than CAPE?

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Why Reflectivity? il y a 3 mois 2 semaines #8

Frank écrit: Does this tell you more than CAPE?

Hi Frank and a hearty warm welcome to this forum,

In a sense, reflectivity does tell you more than CAPE but it is not a fair apples-to-apples comparison.

CAPE is a summary indicator as to the instability of the atmosphere above given points. It shows the potential of convection triggering but does not directly indicate that it will actually happen and where approximately that might be. An experienced meteorologist with knowledge of local factors would be able to predict the possibility of thunderstorms based on CAPE. CIN, Skew-t diagrams and other factors. For the average sailor - high values of CAPE should just raise a warning flag that there could be thunderstorm activity in the very general area. High values of CAPE will in many cases have no associated convection activity.

A high resolution non-hydrostatic model does very much what the experienced meteorologist does. It makes a more educated guess when and where convection might trigger. A result of the model deciding that convection will trigger can be represented by high values of simulated radar reflectivity.

My feeling is that reflectivity is a valuable parameter for thunderstorms if available but the users should never abandon the risk indicators such as CAPE, Lifted Index and CIN.

David

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Dernière édition: par davidgal.

Why Reflectivity? il y a 3 mois 2 semaines #11

Thanks for that. I have been aware that CAPE is not a predictor. As you say, it is a warming.

Frank

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