trade wind cumulus clouds over the western Atlantic

Second phase of the Grey Zone Project

A second phase of the Grey Zone Project has been proposed in a WGNE/GASS White Paper. It is envisaged to consist of two parts: one part which focusses on shallow convection over the western tropical Atlantic based on EUREC4A, and one which concentrates on deep convection and shallow to deep transitions over the eastern tropical Atlantic based on GATE.

The EUREC4A field campaign will take place between 20th January and 20th February 2020 east of Barbados over the western tropical Atlantic. The field study investigates how shallow cumulus clouds respond to changes in the large-scale environment, and examines the factors controlling shallow-convective aggregation (see Bony et al., Surveys in Geophysics, 2017).

The Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) was conducted in the summer of 1974 and was one of the largest and most comprehensive meteorological field campaigns in history. The central objective of GATE was to understand the scale interactions between convection and the large-scale atmospheric circulation, and to elucidate the mechanisms of deep-convective organisation. Phase III of GATE took place over the eastern tropical Atlantic and covered the period from 30th August to 18th September 1974.

Questions

The second phase of the Grey Zone Project plans to address the following questions:

  • In what way do model resolution and scale-aware convection and turbulence schemes impact the representation of the interaction between convection and circulation?
  • In what way do model resolution and different formulations of scale-aware model physics determine the representation of convective organisation?
  • What is the effect of the representation of convective organisation on vertical transport of heat, moisture, and momentum?
  • Are there new challenges to the coupling between turbulence scheme and convection parameterisation in the grey zone?
  • How can the effective resolution of a model be determined? What is the impact of model numerics on the characteristics of convective structures? How can small-scale variability be distinguished from numerical noise?
  • How can the resolution-dependent probability distributions of resolved and unresolved vertical velocities and cloud-base mass flux be characterised?

Part I. EUREC4A-GreyZone

The period of the main EUREC4A field campaign will be covered by several 5-day forecasts. Model simulations will be performed with grid lengths of order O(5km), O(1km), and O(100m). Global models, limited area models, and large-eddy models in limited area model setups are invited to participate. The protocol will specify forecast initial times for 5-day forecasts, and definitions of domains for limited area model simulations. Additional sensitivity experiments will be proposed in order to elucidate specific aspects of the grey zone issue.

In order to prepare for the EUREC4A field campaign in February 2020, groups are encouraged to carry out test simulations over periods in February 2017 and February 2018. MODIS satellite images suggest that shallow-convective organisation differed over these two periods. Two domains are specified for limited area model simulations, one for simulations with O(100m) grid lengths (100m to 900m), and one for simulations with O(1km) grid lengths (1km to 9km):

  • The O(100m) grid length model domain extends from 59.5W to 55.5W, and 12N to 15N
  • The O(1km) grid length model domain extends from 67W to 37W, and 0N to 27N
  • The length of each model simulation is 5 days
  • The model start dates are: February 5, 2017, 00:00 UTC, and February 8, 2018, 00:00 UTC
  • The baseline setup is to drive the O(1km) model simulations with ECMWF ERA5 reanalysis, and to drive the O(100m) model simulations with the 1km grid length simulation
  • Vertical resolution is not specified, but it is pointed out that this aspect might matter
  • Of course also global model simulations O(5km) can be performed

First results from the test simulations, and other aspects of the Grey Zone Project, were discussed at informal side meetings at the UCP2019 conference in Berlin in February 2019, and at the ParaCon Convection Workshop 2019 at the Met Office in Exeter in July 2019. Another preparation meeting is planned on June 15, 2020, in the context of the conference on Improvement and calibration of clouds in models in Toulouse, France.

Related to the experimental protocol described above, there are currently plans for a corresponding Lagrangian LES case (contact Peter Blossey at bloss@atmos.washington.edu or Steef Boeing at S.Boeing@leeds.ac.uk) as well as a single column model case (contact Roel Neggers at neggers@meteo.uni-koeln.de).

Part II. Grey Zone Vintage: GATE revisited

The period of phase III of GATE will be covered by several 5-day forecasts. Model simulations will be performed with grid lengths of order O(5km), O(1km), and O(100m). Global models, limited area models, and large-eddy models in limited area model setups are invited to participate. Initial conditions and lateral boundary conditions will be provided by ECMWF ERA5 reanalysis (available by mid 2020). Additional sensitivity experiments will be proposed in order to elucidate specific aspects of the grey zone issue.

Initially every participating group will store their data at their institution, but a list of required output variables will be provided in order to facilitate cross-model analysis.

Scientific committee

The Scientific Committee of the second phase of the Grey Zone Project:

Lorenzo Tomassini, Rachel Honnert, George Efstathiou, Adrian Lock, Stephan de Roode, Pier Siebesma

Contacts

  • lorenzo.tomassini@metoffice.gov.uk
  • rachel.honnert@meteo.fr         
  • G.Efstathiou@exeter.ac.uk
  • adrian.lock@metoffice.gov.uk
  • s.r.deroode@tudelft.nl
  • A.P.Siebesma@tudelft.nl
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