Seminar: Madalina Surcel and Dominik Jacques

Madalina Surcel and Dominik Jacques have combined efforts to bring you a
special student seminar tomorrow at 14:35. Please join us for cookies and coffee before at
2:15 preceding the talk.

Abstract

For our student seminar, we have prepared something special. When
discussing our respective talks, we realized that we were touching many
similar concepts from different points of view. Instead of working out a
bridge between our presentation, we have interlaced them in order to
make a story. We think this will make it easier (and more interesting)
to understand the concepts presented. For us, it make a nice change from
the more formal conference presentations that we are accustomed to.

In the same spirit, here is our common abstract,

*Mesoscale prediction of precipitation: current status and future work*

It is widely accepted that whereas the performance of Numerical Weather
Prediction (NWP) models is continuously improving, precipitation still
remains very difficult to forecast and quantitative precipitation
forecasting (QPF) skill is generally low. This talk discusses the
ability of current generation mesoscale models (with dx~1km) to forecast
rainfall. Emphasis is put on the main factors affecting forecast quality
and on the methods that could improve QPF.

Through the evaluation of high-resolution ensemble
precipitation forecasts it is shown that models have generally very
little skill in forecasting rainfall at scales lower than 100km.
Furthermore, while ensemble methods can increase predictability at
scales larger than 100km, for small scales, the spread is too large to
provide useful forecasts.

At storm scales, assimilation of radar observations has the potential to
improve model predictions. So far, demonstrating the improvements
brought by assimilation has proven very challenging as forecasts show
great sensitivity to small errors in initial conditions. This is
especially true for humidity, which is not corrected significantly
through assimilation. As a solution to this problem, an alternative
method for the assimilation of radar observations based on a combination
of variational techniques and statistical analysis of model output is
discussed here.

Another pathway for the improvement of forecasts is through the use of
more accurate model physics. However, sensitivity tests show that
despite large dependence of results on various model parameters at small
scales, no single parameter explains the largest forecast errors.

The work presented here seems to indicate the existence of
a critical spatial scale situated around 100km. Above this scale,
forecasting results are satisfactory, while below it QPF skill is poor.
The effect of radar data assimilation is limited to scales smaller than
the critical scale such that improvements due to assimilation are
expected to be short-lived.

Invigoration of shallow cumuli by mesoscale ascent

Prof. Daniel Kirshbaum gives us another research spotlight in the field of cloud dynamics.

Simulated fields of trade-wind convection impinging on an idealized island ridge

These images show simulated fields of trade-wind convection impinging on an idealized island ridge with a height of 500 m. Conditions for these cases are derived from field campaigns (BOMEX and RICO) over the western Atlantic Ocean. The ocean is depicted in blue, the island in green, and the cloud field (the 0.1 g/kg isosurface) in gray. The clouds clearly become more vigorous and deeper over the island ridge. Note also that individual cloud complexes are apparent with much larger sizes than any oceanic clouds. These larger clouds are the main precipitation producers over the island.

Cumulus cloud fields are typically studied in idealized environments that are horizontally homogenous over large areas. In reality, however, these clouds fields typically exhibit substantial variability on the mesoscale (scales of 1-1000 km), which appears to violate the assumption of horizontal homogeneity. Rather than forming in random locations, clouds often cluster in specific regions. This variability may be associated with feedbacks from the clouds onto the larger-scale flow (e.g., cold pools, gravity waves) and/or external forcings imposed on the cloud field. This study investigates the latter mechanism, specifically the impact of mountainous islands on preexisting cloud fields. This is carried out through an analysis of radar, rain-gauge, and aircraft observations over the Caribbean island of Dominica and through idealized large-eddy simulations. The observations document an intense enhancement in cloud coverage and precipitation over Dominica, which is large in virtually all synoptic environments. The simulations provide a physical basis for interpreting the observations. They reveal two mechanisms for the large enhancement in cloud vigour: (1) an increase in cloud buoyancy as moist air ascends alongside dry air with different adiabatic lapse rates and (2) an increase in the mean size of cumulus clouds, which weakens the fractional entrainment of environmental air. Although both mechanisms increase the buoyancy and vertical velocity of convective cores, the latter is potentially more important due to the increase in cloud liquid water, which stimulates faster accretional growth of precipitation particles. For more information see the following references:

  • Kirshbaum, D. J. and A. L. M. Grant, 2012: Invigoration of cumulus cloud fields by mesoscale ascent. Q. J. R. Meteorol. Soc., in press, DOI: 10.1002/qj.1954
  • Smith, R. B., J. R. Minder, A. D. Nugent, T. Storelvmo, D. J. Kirshbaum, R. Warren, N. Lareau, P. Palany, A. James, and J. French, 2012: Orographic precipitation in the tropics: the Dominica Experiment. Bull. Amer. Meteor. Soc., 93, 1567-1579.
  • Kirshbaum, D. J. and R. B. Smith, 2009: Orographic precipitation in the tropics: large-eddy simulations and theory. J. Atmos. Sci., 66: 2559-2578.

About the author

Dr. Kirshbaum is an Assistant Professor in the Atmospheric and Oceanic Sciences Department at McGill. His research focuses on the mechanisms and predictability of atmospheric convection and other mesoscale phenomena.

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