Atmospheric Modelling

Course info
Lectures, exams, practical sessions
Docent	Nicole van Lipzig
Type of course	Lectures and practicals
Type of exam	Written, open book
Achtergrond
ECTS	6?
When?	1st master, 1st semester

Half of the course marks are based on practical reports you submit throughout the semester, the other half is based on the exam. Of the exam part, 20% is based on the exercises.

2025

18th of January

The entire exam was open book - you could bring everything.

Theory part - open book (1.5hr)

Evolution of cloud droplets
1. What determines the maximum height above cloud base a droplet will reach?
2. Draw the evolution of a cloud droplet's radius after formation on the graph to the right (x-axis = radius, y-axis = height above cloud base)
3. What mechanisms contribute to the growth of cloud droplets? Explain them and indicate where on the graph they are of importance.
4. When does the droplet reach maximum height?
Surface pressure tendency
1. Which equation can you use to evaluate changes in surface pressure?
2. Is this a prognostic or a diagnostic equation? Explain.
3. On which conservation law is the equation based?
4. What do the different terms of the equation stand for? Explain the variables involved.
5. What is the physical meaning of the last term? Why does this contribute to changes in surface pressure?
Paper: in one paper, the probabilistic outcomes of different models are evaluated based on an error metric specifically designed for this purpose.
1. Which paper and which graph am I talking about?
2. Explain how the considered models work.
3. Which models underperform and why?

Exercise part - remaining time

Given a map with isopleths (constant geopotential height) and isotherms for the 500 hPa level.
1. Is there warm or cold air advection at point A? Explain.
2. Estimate the magnitude of the geostrophic wind at point A.
3. Calculate the thermal wind at point A. You can leave the coriolis parameter as is.
Given the horizontal velocity vector V = 10 + 200cos(x)sin(y) - 0005y (example values).
1. Give the horizontal advection term for ..? (i think? you had to calculate k x nablaV thats for sure)
A droplet is suspended in a cloud chamber with initial drop radius 10^-8m, vapour pressure of 23 hPa, pressure of 1000 hPa, temperature of 20 degrees Celsius, diffusion coefficient of 10^2 m^2/s and an supersaturation of 4%.
1. If you leave the droplet for one minute, what size will it be?

2024

Different teacher!

- Draw a graph of the evolution of a raindrop through a cloud, height on the y-axis, drop radius on the x-axis

- Under what circumstances are raindrops very large? Link with equations

- When will a raindrop reach the top of a cloud? On what factors does it depend?

- Some questions about radar reflectivity and the radar reflectivity factor.

2018

Oral:

1. Assume two air masses, one is clean with not much CCN, the other one is polluted and has a lot of CCN. Both air masses are rising (upward motion of the atmosphere) en will form cloud droplets. Make a sketch for both situations and show the change of the droplet's radius and the supersaturation in time. What is the essential difference between both situations with clean and polluted air?

2. What is the Marshall-Palmer relationship? Explain how this relation is used to derivate rainfall rate. What is the error made when unusual large rain drops are formed which aren't comparable to the Marshall-Palmer relationship? Will the algorithm over- or underestimate the rainfall?

3. Precipitation can be convective or stratiform. How does the precipitation intensity between these two systems differ? What kind of effect does the type of precipitation (convective/stratiform) have on the relationship between temperature and precipitation intensity?

Excercices:

1. A map is shown with lines of equal geopotential height and isothermes. Is cold or warm advection occurring at point A? How can you quantify the magnitude of this advection term?

2. Takehome 17 a&b with different numbers

3. An air parcel has a vapor pressure of 24.33 hPa and a temperature of 20°C. The water vapor diffusion coefficiënt of air is given by:

D=2.10^(-2).T^(0.81). 1/P

Where P is the pressure and T is the temperature. Calculate how much time it takes in this case for a droplet to grow from 10 to 20 µm by the process of water difussion only.

4. The divergence in the atmospheric column in between 1000-500 hPa is 3.10^-7 /s. Above 500 hPa there is covergence until a pressure level of 200 hPa of 2.10^-7 /s. What is the value of ws=dps/dt?

2015-2016

Augustus

1. Papers: IPCC gebruikt vaak de term 'stralingsforcering'. Waarom is dit misleidend? Illustreer aan de hand van gelezen papers.

2. Geef formule die evolutie luchtdruk met tijd weergeeft. Welke atmosferische variabelen komen voor in deze formule? Leg uit waarom deze atmosferische variabelen een invloed hebben op de oppervlaktedruk.

3. LWC en detrainment uitleggen

Reeks 1

Oral:

ow does the amount of aerosols influence the rain-forming process? Make a sketch where you plot the evolution of the droplet radius and supersaturation with time for a cloud in clean air and polluted air. A lot of questions asked about figure 6.16 in Wallace & Hobbs, e.g. what is the influence of pollution on the activation on droplets?
In which paper did we see the explanation for the recent slowing down of global warming? How do they explain this and what methods did they use?
How is the surface pressure calculated in an atmospheric model (equation + explain principle + discretization). What is fundamentally different with the calculation of the geopotential in an atmospheric model?

Exercises:

A map is shown with lines of equal geopotential height and isothermes. Is cold or warm advection occurring at point A? How can you quantify the magnitude of this advection term?
A model has a grid spacing of 1 km and a timestep of 30 s. With an upstream FED the model calculates an advection wave with a speed of 100km/hr, what is the value of the numerical amplification factor for this model? (values for p were not given)
Take home exercises 17 A and B but with different values (COSMO model: calculate the time when only autoconversion and accretion is considered)
An aerosol with a thin layer of water around it has a total radius of 3E-8 m. In a labo, experiments are conducted in a chamber where T = 20°C and supersaturation = 4%. Will this droplet grow, evaporate or reach a steady state?

Reeks 2

oral:

1. Given a map (north up) with lines of geopotential thickness (5 km to the lower left to 5.4 km to the upper right) between the 1000 and 500 hPa level. At the surface, there is a southerly wind. In which direction is the wind at the 500 hPa level? Is there cold advection or warm advection? Explain.

2. What is meant with the effective radius of a hydrometeor? In which paper that we discussed did we see this in detail? What were the conclusions of that paper?

3. Explain the CFL criterium.

Excercises:

1. An air parcel has a vapor pressure of 24.33 hPa and a temperature of 20°C. The water vapor diffusion coefficiënt of air is given by:

D=2.10^(-2).T^(0.81). 1/P

Where P is the pressure and T is the temperature. Calculate how much time it takes in this case for a droplet to grow from 10 to 20 µm by the process of water difussion only.

2. Sedimentation in the COSMO model:

Gridbox at height 850 hPa and vertical extend of 500m. There are raindrops in this gridbox, with a total mixing ratio of 3.10^-3 kg/kg. The Tempature is 20°C. How long does it take for the mixing ratio to reduce to 1.5.10^-3 kg/kg? There are no raindrops in the atmosphere column above this gridbox. Will -in this COSMO model- the average size of the drop increase, decrease, or stay the same? Explain why.

3. A collector drop with radius 30 µm catches slightly smaller drops with a radius of 20µm. Will the collection efficiency stay the same, increase or decrease when the collector drop grows? Explain why.

4. The divergence in the atmospheric column in between 1000-500 hPa is 3.10^-7 /s. Above 500 hPa there is covergence until a pressure level of 200 hPa of 2.10^-7 /s. What is the value of ws=dps/dt?

2014-2015

Oral:

Reeks 1

If the number of droplets linearly decreases with height, how would this be implemented in the continuous collection model?

How does an atmospheric model calculates the easterly wind advection? Derive an equation using the primitive equations. Discretisize. (kinda like this: For a given situation, the meridional (v-) component of the wind is zero. Write, from the horizontalequation of motion, an expression for the zonal (u-)component of the wind, and describe what the different terms refer to. )

How does land use change affects the ocean temperature? Use one paper to explain this --> paper about deforestation and climate forcing

Reeks 2

1. Explain the growth of a droplet within subsaturated/supersaturated air + some smaller questions regarding the diagram + what does delta E mean?

2. Effect of warm advection on the formation of extratropical cyclones + some smaller questions regarding extratropical cyclones.

3. Explain the taylor diagram from the paper regarding the Alpine flood. What does it show and what do you learn from it.

Written exercises:

Reeks 1

a. a model has a grid spacing of 1km and a timestep of 30s. With an upstream FED the model calculates an advection wave with a speed of 100km/hr, what is the value of the numerical amplification factor for this model?

b. From a weather card, it can be read that at 850 hPa the temperature increases between 12:00UTC and

15:00UTC by 3K. If you assume that advection played a minor role, what is the average diabatic heating

rate between 12:00UTC and 15:00UTC at the 850hPa level?

c. For the same situation as under b), but now assuming that only the horizontal advection can be neglected,

estimate the average vertical temperature advection and diabatic heating rate at the 850hPa level between

12:00UTC and 15:00UTC. You can assume a vertical lapse rate of 6°C/km, a surface temperature of 20°C

and a vertical velocity at 850hPa of 5·10-3 m/s.

d. A collector cloud drop has a radius of 30micrometers. A second drop has a radius of 20mirometer. Would the Collision Efficiency increase or decrease as the first drop increases in radius?