Predicting Martian Dune Characteristics Using Global and Mesoscale MarsWRF Output Claire Newman working with Nick Lancaster, Dave Rubin and Mark Richardson.

Predicting Martian Dune Characteristics Using Global and Mesoscale MarsWRF Output

Claire Newmanworking with

Nick Lancaster, Dave Rubin and Mark Richardson

Acknowledgements: funding from NASA’s MFR program, and use of NASA’s HEC facility right here at Ames.

Overview of talk

• Motivation (why dunes?)

• Dune features we can compare with

• Some dune theory

• Modeling approach

• Preliminary results: Global

• Preliminary results: Gale Crater

Motivation: how do dunes provide insight into recent climate change on Mars?

• Higher obliquity should produce stronger circulations & surface stresses, hence might expect more saltation and dune formation

• Features (i.e., dune orientations, migration directions, etc.) in disagreement with predictions for current wind regime may indicate inactive dunes formed in past orbital epochs

Also –

• In absence of near-surface meteorological monitoring, predicting characteristics of active dunes can help us confirm that we – have the current wind regime right – understand dune formation processes

Dune features we can compare with

Dune features we can compare with• Locations

Bourke and Goudie, 2009

• Locations• Bedform (crest) orientations

Images: NASA/JPL/University of Arizona

Dune features we can compare with

• Locations• Bedform (crest) orientations

Images: NASA/JPL/University of Arizona

Dune features we can compare with

• Locations• Bedform (crest) orientations• Inferred migration directions (crater dunes)

Hayward et al., 2009

Dune features we can compare with

• Locations– Use a numerical model to predict saltation over a Mars

year, assuming a range of saltation thresholds

• Bedform (crest) orientations– Apply ‘Gross Bedform-Normal Transport’ theory

• Inferred migration directions (crater dunes)– Assume correlated with resultant (net) transport direction

Dune features we can compare with

Some dune theory

2 key issues

• Dunes form in the long-term wind field

• Dune orientations are not determined by net transport

Gross Bedform-Normal Transport

A simple example to illustrate a point

Dune crest

Rubin and Hunter, 1987

A simple example to illustrate a point

Dune crest

First wind direction

Gross Bedform-Normal Transport

A simple example to illustrate a point

Dune crest

First wind direction

Second wind direction

Gross Bedform-Normal Transport

A simple example to illustrate a point

Net transport = 0

Dune crest

First wind direction

Second wind direction

Gross Bedform-Normal Transport

A simple example to illustrate a point

Net transport = 0

But both wind directions shown cause the bedform to build

Dune crest

First wind direction

Second wind direction

Gross Bedform-Normal Transport

A simple example to illustrate a point

Net transport = 0

But both wind directions shown cause the bedform to build

=> rather than net transport, we are interested in gross transport perpendicular to the bedform, regardless of the ‘sense’ of the wind (i.e., N-S versus S-N)

Dune crest

First wind direction

Second wind direction

Gross Bedform-Normal Transport

Bedform orientation – the theory

A = NET TRANSPORT OF SAND

A

Bedform orientation – the theory

Bedform orientation – the theory

B + C = GROSS BEDFORM-NORMAL SAND TRANSPORT

B

C

Bedform orientation – the theory

• Basic concept: dunes form due to sand transport in both directions across bedform

• Bedforms align such that total transport across dune crest is maximum in a given wind field

• where total transport = Gross Bedform-Normal Transport [Rubin and Hunter, 1987]

B + C = GROSS BEDFORM-NORMAL SAND TRANSPORT

B

C

Bedform orientation – the theory

Modeling approach (1)

• Run numerical model to predict near-surface winds at all times for a long time period (at least 1yr) to capture the long-term dune-forming wind field

• Choose a saltation threshold and calculate sand fluxes in all directions [0, 1, …359° from N]

• Consider all possible bedform orientations [0, 1, …179° from N]

• Sum the gross sand flux perpendicular to each orientation over the entire time period

• Find the orientation for which the total gross flux is maximum

• NB: secondary maxima indicate secondary bedform orientations

Bedform orientation – the approach

The numerical model: MarsWRF

• Mars version of planetWRF (available at www.planetwrf.com)

• Developed from Weather Research and Forecasting [WRF] model widely used for terrestrial meteorology

• Multi-scale 3D model capable of:– Large Eddy Simulations– Standalone mesoscale– Global– Global with nesting

• Using global and global with nesting for these studies

Version of MarsWRF used here includes:

• Seasonal and diurnal cycle of solar heating, using correlated-k radiative transfer scheme (provides good fit to results produced using line-by-line code)

• CO2 cycle (condensation and sublimation)

• Vertical mixing of heat, dust and momentum according to atmospheric stability

• Sub-surface diffusion of heat

• Prescribed seasonally-varying atmospheric dust (to mimic e.g. a dust storm year or a year with no major storms) or fully interactive dust (with parameterized dust injection)

• Ability to place high-resolution nests over regions of interest

Results

Present day global dune results comparing with Mars Global Digital Dune Database (MGD3, e.g. Hayward et al., 2009):

1. Dune centroid azimuth

– compare with GCM-predicted resultant transport direction

Present day global dune results comparing with Mars Global Digital Dune Database (MGD3, e.g. Hayward et al., 2009):

1. Dune centroid azimuth

– compare with GCM-predicted resultant transport direction

2. Slipface orientation

– compare with normal to GCM- predicted bedform orientation

Present day global dune results comparing with Mars Global Digital Dune Database (MGD3, e.g. Hayward et al., 2009):

1. Dune centroid azimuth

– compare with GCM-predicted resultant transport direction –agreement => within 45° of dune centroid azimuth direction

2. Slipface orientation

– compare with normal to GCM-predicted bedform orientation – agreement => within 45° of normal to slipface

Comparison of predicted and inferred migration direction for present day using saltation threshold=0

Green => agreement between predicted and inferred migration directionBlue => no agreementRed => no comparison possible

Comparison of predicted and inferred migration direction for present day using saltation threshold=0.007N/m2

Green => agreement between predicted and inferred migration directionBlue => no agreementRed => no comparison possible

Comparison of predicted and inferred migration direction for present day using saltation threshold=0.021N/m2

Green => agreement between predicted and inferred migration directionBlue => no agreementRed => no comparison possible

Comparison of predicted and measured bedform orientation direction for present day using saltation threshold=0

Green => agreement between predicted and inferred bedform orientationBlue => no agreementRed => no comparison possible

Comparison of predicted and measured bedform orientation direction for present day using saltation threshold=0.007N/m2

Green => agreement between predicted and inferred bedform orientationBlue => no agreementRed => no comparison possible

Green => agreement between predicted and inferred bedform orientationBlue => no agreementRed => no comparison possible

Comparison of predicted and measured bedform orientation direction for present day using saltation threshold=0.021N/m2

Comparison of predicted and inferred migration direction for obliquity 35° using saltation threshold=0.007N/m2

Green => agreement between predicted and inferred migration directionBlue => no agreementRed => no comparison possible

Comparison of predicted and inferred migration direction for present day using saltation threshold=0.007N/m2

Green => agreement between predicted and inferred migration directionBlue => no agreementRed => no comparison possible

Modeling approach (2)

One or more high-resolution ‘nests’, placed only over regions in which increased resolution is desired

Nesting in MarsWRF

20°E 25°E 30°E 35°E

5°N

5°S

0°

15°E

Mother [global] domain (only a portion shown)

Domain 2(5 x resolution of domain 1)

Domain 3(5 x resolution

of domain 2; 25 x resolution

of domain 1)

20°E 25°E 30°E 35°E

5°N

5°S

0°

15°E

Mother [global] domain

Domain 2

Domain 3

2-way nesting => feedbacks between domains

1-way nesting => parent forces child only

Sample results for Gale at Ls ~ 0° using mesoscale nesting in global MarsWRF

Gale dune studies – early results (~4km resolution)

Gale Crater: predicted

(a) resultant transport direction [black arrows]

(b) dune orientations [white lines]

for saltation threshold = 0

Gale dune studies – early results (~4km resolution)

Gale Crater: predicted

(a) resultant transport direction [black arrows]

(b) dune orientations [white lines]

for saltation threshold = 0.007N/m2

Gale dune studies – early results (~4km resolution)

Gale Crater: predicted

(a) resultant transport direction [black arrows]

(b) dune orientations [white lines]

for saltation threshold = 0.021N/m2

Gale dune studies – present day MarsNo saltation threshold Saltation threshold = 0.007N/m2

No saltation threshold Saltation threshold = 0.007N/m2

Gale dune studies – present day Mars

No saltation threshold Saltation threshold = 0.007N/m2

Gale dune studies – present day Mars

From Hobbs et al., 2010

Gale dune studies – present day Mars

• Simply changing the assumed saltation threshold greatly improves the match to observed bedform orientations

• Effect of large dust storms not yet examined, but likely will also impact winds hence orientations

• Orbital changes and impact on circulation widen parameter space even further!

Gale dune studies – obliquity 35°

Gale Crater: predicted

(a) resultant transport direction [black arrows]

(b) dune orientations [white lines]

for saltation threshold = 0

Gale dune studies – obliquity 35°

Gale Crater: predicted

(a) resultant transport direction [black arrows]

(b) dune orientations [white lines]

for saltation threshold = 0.007N/m2

Gale dune studies – obliquity 35°

Gale Crater: predicted

(a) resultant transport direction [black arrows]

(b) dune orientations [white lines]

for saltation threshold = 0.021N/m2

Gale dune studies – obliquity 35°No saltation threshold Threshold = 0.007N/m2 Threshold = 0.021N/m2

Conclusions (1)

• Bedform orientations and dune migration directions may be related (roughly) to ‘gross’ and ‘net’ sand transport by long-term (~multi-annual) wind regime

• Typical global model resolutions (2°-5°) are too low to resolve topographic features influencing dunes

• Mesoscale numerical modeling spanning entire year can be used to predict long-term wind regime and sand transport

• Rubin and Hunter [1987] Gross Bedform-Normal Transport theory provides clean predictions of bedform orientations

Conclusions (2)

• Recent studies (e.g. Bridges et al., Nature, in press) indicate currently active dune fields that can be used to validate our approach for present day Mars

• Possibly inactive regions with strong discrepancies not explainable by varying saltation threshold, dust loading, etc. may imply formation during past orbital epoch

• Future study regions include Proctor Crater, a North Polar dune field, …

• Lots more to do!