Lecture 4: Rain (measuring and statistics) Flashcards
Mention two types of climate models.
General (global) circulation models (GCM’s)
Regional climate models (RCM’s)
Explain the GCM model (spatial and temporal scale)
Daily time scale, 100-200 km space scale
Explain the RCM model (spatial and temporal scale)
Daily time scale and 25-50 km space scale
Mention three types of precipitation
Convection, convergens and front passage (cold front/hot front)
How do we measure rain (tools)?
Raingauge
Sensors
Radar
Why do we see differences in the results from the different measuring methods?
Different types/technologies for measuring rain intensity gives slightly different results due to the fact that they measure different things. The tipping bucket measures in 1 point while the radar measures over a large area.
How can we compare different technologies and their differencies?
A way to compare the different technolgies and the differences are to calculate the bias (bias basically means intensity/amounts measured with one technology devided by the intensisty/amounts measured by the second technology – xmm/ymm
What is nowcasting?
Modelling the functionality of the sewer system and being able to forecast how the rain will influence and what measured can be taken to manage the water. Doing this in real time is called nowcasting.
What do Intensity Duration Frequency (IDF) curves describe?
IDF curves describe the relationship between rainfall intensity, rainfall duration, and return period.
What is a Chicago Design Storm (CDS) and what are the advantages/disadvantages?
- An artificial storm event designed to simulate different durations in one simulation.
Advantages:
- Short simulation time!
Disadvantages:
- Impossible to find the right combination of intensity and duration which gives flooding in the system.
What are the limitations with constructed rain series (IDF and CDS)?
It is assumed that the statistical properties of the rain is assumed to be the same as the hydraulic statistics (e.g. flooding)
o This is not always true!
For the following return periods 1, 5, 20 and 100 years: Consider the minimum number of years of rain measurements which is necessary to estimate the correct IDF rain intensities. Discuss why this is possible/not possible to acquire in practice.
For DK it is possible to estimate IDF for 1 and 5 years rain events because that would need 10 and 50 years of data (we need 10x years of data to call it a x amount of year rain event). For 20 and 100 year we would need 200 and 1000 years of data and this is not possible because we only have weather data from 1873.
Describe the possibilities of using weather radars and nowcast in water management. Discuss differences between urban and rural applications.
Weather radars and nowcasting can be used to decrease failures in the water management system by preparing the network for the volume of water and the location where most of the water will precipitate. Comparing this information of amount and location to the properties of the water management system can give an advantage fighting the effects of flooding.
In rural areas the lack of sealed surfaces and high availability of percolation reduces flood risk, while in urban areas the given factors increase the risk. In rural areas open water storage capacities, like lakes, also decrease the impact of floodings. However, the damage on the agricultural fields cannot be neglected, which, in case of a major rain event, must be redirected to decrease harm. Weather radars and nowcasting can help there.
Explain the mechanism of the tipping-bucket rain gauge
Tipping-bucket rain gauge –> Measures the amount of rainfall by collecting the rain in a container, and then measuring the amount of water that is discharged through a narrow outlet, or “bucket,” at the bottom of the container.
The bucket is divided into two compartments of equal size, and when one compartment fills with water, it tips and empties its contents into the outlet, while the other compartment fills up with water. The frequency at which the buckets tip is used to determine the rate of rainfall, and the total amount of water that is discharged from the gauge over a period of time is used to determine the total amount of rainfall that has fallen. This type of gauge has a good accuracy and it is relatively inexpensive, easy to install and maintain. Only measures in one point.
