Analysis of wind shear for the ExoMars missionusing ultra-high resolution Large Eddy Simulation

T. Bertrand, A. Spiga, A. Colaïtis

Mission contextExoMars ESA mission 2016:

- TGO- EDM

Landing point: Meridiani Planum ̴ 1.88°S / 353.87E

Objectives Evaluate wind shears at ExoMars landing site

Definition:

A strong wind shear at an atmospheric level results either from large differences of wind amplitude or contrasts in wind direction, or both, over a small vertical extent.

Objectives

Wind shear

Evaluate wind shears at ExoMars landing site

Possible effect of a wind shear on EDM’s landing system:

Large Eddy Simulation:

400 levels between 0 and 5 km altitudeVertical resolution =12.5 mLCT = 1 p.m until 1.45 p.m

Horizontal resolution = 12 m

Wind shear characterization

Wind vector

U

V

θUV (alt = n+1)

UV (alt = n)

Δ(θ)

D(UV)

D(UV) enables to evaluate the combined effects of angle and amplitude. It is a good indicator of wind shears.

√𝑈 ²+𝑉 ²

U

V

Vertical discretization

12.5 m12.5 m12.5 m

UV (m/s)

Altitude (m)

1 3 6 7

In order to constrain the mission, we are concerned with wind shears occuring over a vertical distance of 20-30 m

with UV and θ profiles

Exemple of wind shears

Altit

ude

(km

)

Δ(UV) Δθ(°)

Wind shear with high Δ(UV)

Wind shear with high Δθ

Δ(UV) and Δ(θ) at 4 different times and at two different locations (distant from 750m)

Between 0-5 km and 0100PM-0145PM

Maximal values of D(UV)

D(UV) [ s-1 ]

Δ(UV) Δθ

Extreme cases of wind shear profiles

Red curve: sheared caseBlue curve: smooth case

Altit

ude

(km

)

D(UV)

High Δθ

D(UV)= 0.42 s-1 (15.7 m.s-1 over 37,5m)

Probability of wind shearConsidering maximal values of D(UV) only

(s-1)

max

Probability of wind shearConsidering all values of D(UV) from LES

(s-1)

max

Conclusions• Wind shears diagnostic D(UV) … combining gradient of wind

amplitude and wind direction.

• Most plausible ExoMars landing profiles have very moderate shears (< 3 m.s-1 over a vertical distance of 37.5 m).

• A majority of maximal shears are between 0.15 and 0.25 s-1.

• Extreme wind shears of about 15 m.s-1 are found over a vertical distance of 37.5 m (but low probability: P < 0.01).

• On the one hand, the probability of having wind shears less than 0.1 s-1 is quite high when considering all times and altitudes available. When considering only maximal wind shears over the same area, a majority of wind shears between 0.15 and 0.25 s -1 are found. In a general way, over a vertical distance of 37.5 m, variations of wind amplitude of 3 m.s -1 combined with angle variation of about 60° are relatively frequent.