Submitted as: methods for assessment of models 07 Jan 2021

Submitted as: methods for assessment of models | 07 Jan 2021

Review status: this preprint is currently under review for the journal GMD.

A climatology of tropical wind shear produced by clustering wind profiles from a climate model

Mark R. Muetzelfedt1,2, Robert S. Plant1, Peter A. Clark1, Alison J. Stirling3, and Steven J. Woolnough1,2 Mark R. Muetzelfedt et al.
  • 1Department of Meteorology, University of Reading, Reading, United Kingdom
  • 2National Centre for Atmospheric Science, University of Reading, Reading, United Kingdom
  • 3Met Office, Exeter, United Kingdom

Abstract. A procedure for producing a climatology of tropical wind shear from climate-model output is presented. The procedure is designed to find grid columns in the model where the organization of convection may be present. The climate-model output consists of east–west and north–south wind profiles at 20 equally spaced pressure levels from 1000 hPa to 50 hPa, and the Convective Available Potential Energy (CAPE) as diagnosed by the model’s Convection Parametrization Scheme (CPS). The procedure begins by filtering the wind profiles based on their maximum shear, and on a CAPE threshold of 100 J kg−1. The filtered profiles are normalized using the maximum wind speed at each pressure level, and rotated to align the wind at 850 hPa.

From each of the filtered profiles, a sample has been produced with 40 dimensions (20 for each wind direction). The number of dimensions is reduced by using Principal Component Analysis (PCA), where the requirement is that 90 % of the variance must be explained by the principal components. This requires keeping the first seven leading principal components. The samples, as represented by their principal components, can then be clustered using the K-Means Clustering Algorithm (KMCA). 10 clusters are chosen to represent the samples, and the median of each cluster defines a Representative Wind Profile (RWP) – a profile that represents the shear conditions of the wind profiles produced by the climate model.

The RWPs are analysed, first in terms of their vertical structure, and then in terms of their geographical and temporal distributions. We find that the RWPs have some features often associated with the organization of convection, such as low-level and mid-level shear. Some of the RWPs can be matched with wind profiles taken from case studies of organization of convection, such as squall lines seen in Tropical Ocean Global Atmosphere, Coupled Atmosphere Ocean Research Experiment (TOGA–COARE). The RWPs’ geographical distributions show that each RWP occurs preferentially in certain regions. Six of the RWPs occur preferentially over land, while three occur preferentially over oceans. The temporal distribution of RWPs shows that they occur preferentially at certain times of the year, with the distributions having mainly one or two modes. Their geographical and temporal distributions are compared with those seen in previous studies of organized convection, and some broad and specific similarities are noted.

By performing the analysis on climate-model output, we lay the foundations for the development of the representation of shear-induced organization in a CPS. This would use the same methodology to diagnose where the organization of convection occurs, and modify the CPS in an appropriate manner to represent it.

Mark R. Muetzelfedt et al.

Status: open (until 04 Mar 2021)

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Mark R. Muetzelfedt et al.

Data sets

Climatology of shear: UM10.9 GA7.0 output data 19880901-19930601 Mark Robert Muetzelfeldt

Mark R. Muetzelfedt et al.


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Short summary
Wind shear causes organized convection in the tropics, producing e.g. squall lines. We have developed a procedure for producing a climatology of sheared wind profiles in a climate model and demonstrated that the profiles are linked with organized convection, both in terms of their structure and their spatio-temporal distribution. The procedure could be used to diagnose organization of convection in a climate model, which could lead to improvements in the model's representation of convection.