Notes for Convective Weather Refresher

Notes for Convective Weather Refresher - Notes for...

Info icon This preview shows pages 1–3. Sign up to view the full content.

Notes for Convective Weather Refresher Usage recommendation: Print these notes, then view the presentation in "Slide Show" mode using the notes as a guide. The notes here are identical to those attached to each slide in the Powerpoint file. Slide, topic Explanation (self-explanatory pages and self-evident main points omitted) 4 Background Here is an example of what happens when the convective parameterization fails to relieve instability fast enough. On the far left is a model sounding showing a saturated layer with a lapse rate steeper than moist-adiabatic. This layer is produced by strong mesoscale convergence and lift in the resolved grid-scale motions and is a characteristic signature of such lift in a moist environment. The plot next to it shows vertical velocity at the same time in green, an hour later in blue, and two hours later in magenta. The saturated unstable layer is at the top of the region of strong ascent. The model continues to lift this region, forming a grid- scale cumulonimbus rising over several hours. The extreme vertical velocities clearly violate the model’s hydrostatic assumption, making the pressure and velocity forecast evolution physically unrealistic. The strong convergence and vertical motion bulls-eye results in a locally-intense vorticity maximum. On the US map, the pre-storm ambient vorticity is shown, with typical values of 3 to 12 x 10 -5 s -1 . The upper zoomed-in area shows 3-hour precip contours and shaded vorticity (same color scale) at the time of the sounding shown in the left-most plot, which was for a grid point to the southeast of the precipitation bulls-eye at that time. The vorticity has now increased to over 20 x 10 -5 s -1 . The lower zoomed-in area shows the same for 3 hours later, with the vorticity maximum exceeding 39 x 10 -5 s -1 co-located with the precipitation bulls-eye near the location of that sounding. Precipitation at that sounding location is listed in the table. The convective parameterization did trigger, but it could not relieve instability fast enough to prevent the model from making a thunderstorm updraft core on the grid scale. Most of the precipitation coming from the grid-scale scheme in a convective situation is a classic characteristic of a model grid-scale blow-up. A grid-scale thunderstorm is desirable in a very high resolution nonhydrostatic model (the storm would grow more rapidly and have a narrower updraft), but is exactly what must be avoided in operational NWP models that are not at high enough resolution to resolve a convective updraft. The purpose of the convective parameterization is to avoid this scenario. 5 Background Note how the sharpness of the trough as well as the wind speeds and CAPE are different in these two model runs. The models were the same except for the convective parameterization. The model convection affected not only the precipitation but also the model mass, horizontal wind, and vertical motion fields.
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

6 Background The skill of model precipitation forecasts in convective situations is poor, even for rain vs. no rain, not only for the amount of rain.
Image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern