observing the cryosphere: what do we measure and why? · in situ measurements vs. remote sensing...

Observing the cryosphere: What do we measure and why?

Andrew Hoffman

Today’s goals

What are the variables of interest in cryospheric research?

How are these variables measured now, and how have they been historically? What are the advantages of different observation methods?

What is the basis for remote sensing, and what are the current remote sensing techniques used in cryospheric research?

What do we measure and why?

What controls ice flux through a horizontal gate in our glacier?

H

Motivating flux problem

Motivating flux problemWhat do we measure and why?

dudz

= A(T )τ 3

u = A(T )τ 3H

H

Model


dudz

= A(T )τ 3

u = A(T )τ 3H

uH = A(T )τ 3H 2

H


Model


dudz

= A(T )τ 3

u = A(T )τ 3H

uH = A(T )τ 3H 2

H


τ = ρgHα

Model

Approximation

dudz

= A(T )τ 3

u = A(T )τ 3H

uH = A(T )τ 3H 2

τ = ρgHα

uH = A(T )ρgαH 5

H


Model

Approximation



What controls ice thickness and ice-thickness change when we can ignore glacier sliding?

H



What controls ice thickness and ice-thickness change when we can ignore glacier sliding?

External forcing and glacier geometryH


In Situ Measurements vs. Remote Sensing

Directly collecting information about a system at a point of interest. This requires that the instrumentation be located directly at a point of interest for the system and be in contact with the point of interest.

Collecting information about a system at a distance. This requires transmission of information from the system to the instrument without

direct contact.


Accumulation

To derive total accumulation, you measure change in snow thickness + deposited snow density


0

100

200

300

400

500

600

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Snow thickness (m)

Ele

vatio

n (m

asl

)

Y = X * 0.002962 + 0.58


MeltWhat do we measure and why?

To derive total melt in the ablation zone, you just have to measure changes in ice thickness.

Glacier-thickness changeWhat do we measure and why?

DEM courtesy David Shean

Glacier-thickness changeWhat do we measure and why?

Menunos et al 2019

Ice-shelf thickness change


Ice-shelf thickness change

Dutriuex et al 2014

Shean et al 2016

Motivating flux problemWhat do we measure and why?

What controls ice thickness and ice-thickness change?

External forcing and glacier geometryH

How do we measure ice thickness?What do we measure and why?

Radar-system timescales

Fast Time

Slow Time

1e-4 s[milliseconds/microseconds]

1e-9 s[nanoseconds]


How do we measure ice thickness?What do we measure and why?

Ice-sheet thickness observationsWhat have we measured and why?

In Situ Measurements vs. Remote Sensing

Directly collecting information about a system at a point of interest. This requires that the instrumentation be located directly at a point of interest for the system and be in contact with the point of interest.

Collecting information about a system at a distance. This requires transmission of information from the system to the instrument without

direct contact.

The Physics of Observation

Gravity

! = #$% ∗ $'('

Newton’s Law of Universal Gravitation:F: The force due to gravityG: Universal Gravitational Constantm: Masses of the objectsd: Distance between their centers of mass

Wave Theory

!"#!$" = &"'"#

The Wave Equation:u: The propagating perturbationt: Timec: The wave speed

1D Solution to the Wave Equation:u: The propagating perturbationt: Timex: Distance in the propagation directionk: Wavenumberf: Frequencyc: The wave speed

# $, ) = sin -) − 201 ∗ $ + 4& = 56 - = 75

Wave ReflectionR: Reflection Coefficient

Z: Electric / Acoustic Properties8 = 9" − 97

9" + 97

Black Body Radiation

!" =2ℎ&'() *

+",- − 1

01

!" ≈234&)()

Planck’s Law:L: Radiance (outgoing energy)h: Planck’s Constantf: Frequencyc: Speed of Lightk: Boltzmann’s ConstantT: Temperature

Brightness Temperature Equations:45: Brightness Temperature6: Emissivity 47: Surface Temperature

(Microwave Approximation)

45 = 648

Remote Sensing

Ice Thickness / Material Properties Changes in Ice Mass

Sea Ice Presence / Skin Temperature

Surface Changes and Ice Flow Speeds

Return to silly-putty modelWhat do we measure and why?

dudz

= A(T )τ 3

u = A(T )τ 3H

Model

Importance of sliding

Radar interferometry

t=t1


t=t2


Time series courtesy Nick Holschuh

observing the cryosphere: what do we measure and why? · in situ measurements vs. remote sensing...

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