Atmospheric processes (precipitation)

Transcription

1 Atmospheric processes (precipitation) Interception Two lectures precipitation process (formation, measurement, characterisation, ) storm (extreme) rainfall Precipitation Lecture content 1 1 Precipitation formation 2 Global and local distribution of precipitation 3 Measurement and measurement errors 4 Areal rainfall estimation MeteoSchweiz Precipitation 1

2 Atmospheric circulation solar radiation different at low and high latitudes energetic deficit between low (heat source) and high latitudes (heat sink) warm air from the tropics moves to the polar regions belts characterised by different atmospheric pressure (H vs L) differences in pressure, temperature and humidity induce the transport of air masses Mechanisms of precipitation formation warm front Frontal Precipitation warm air cools because of upforcing mechanism du to the presence of cold air front cold front cold air front forces up warm air, which cools Precipitation 2

3 Frontal precipitation [ Mechanisms of precipitation formation Convective Precipitation temperature or mass gradients generate local convective clouds water vapour rises, cools, condenses into clouds leading to rain Orographic lifting moist air travelling to the land is forced by orography to higher elevations where it cools and condenses leading to rain Precipitation 3

4 Convective precipitation (rainfall) [ Orographic lifting [ Precipitation 4

5 Hydrology I Autumn Semester 2015 Mean annual p recipitation - global no d ata mm/a Mean annual p recipitation - CH Precipitation 5

6 Winter vs Summer precipitation - CH 1000 mm 500 mm 0 mm Winter (%) Summer (%) Precipitation and temperature regimes - CH 500 P T Precipitation 6

7 Precipitation regimes - World Precipitation regimes are typical of climate regions and are controlled by atmospheric circulation patterns equatorial à 2 rainfall maxima at the equinoxes tropical à 1 rainy season, 1 dry mediterranean à 2 rainfall maxima in spring and autumn continental à evenly distributed with summer maximum high latitudes à predominantly summer maximum monsoon à hot and wet season, cold and dry season Precipitation and temperature regimes - World 500 P T Precipitation 7

8 Hydrology I Autumn Semester 2015 Precipitation measurements The standard measurement of precipitation is the precipitation depth in a given temporal interval ΔT (e.g. one minute, one hour, one month, one year, ) The units are [L], typically [mm] or [l m- 2 ] The r ainfall intensity is defined as à depth/δt Precipitation can be measured by means of raingauges (recording vs non recording) à temporal evolution of precipitation at a point in space be estimated from weather radars or satellites à space- time evolution of precipitation Non recording raingauges Measurements a re o btained from periodical reading of the g raduated b ottle (e.g. d aily, monthly) UK Metoffice totalisator Precipitation! 8

9 Hydrology I Autumn Semester 2015 Non recording raingauges Measurements a re o btained from periodical reading of the g raduated b ottle (e.g. d aily, monthly) Hellman raingauge (standard totalisator MeteoSwiss) Recording raingauges Mechanisms: Tilting siphon Tipping bucket Weighing Measurements a re o btained through a utomatic recording o f the rainfall depth o ver a chart rotating on a d rum mm! typical chart resolutions: 2 0 minutes, 1 0 mm Precipitation 9

10 Hydrology I Autumn Semester 2015 Recording raingauges Tipping bucket!! A s ignal is recorded when the bucket is filled. The s ignal corresponds to an amount of p recipitation equal to the volume of the b ucket. Typical resolutions of a b ucket: 0.25 mm, 0.2 mm, 0.1 mm Temporal resolution d ictated b y tips à need to resample to obtain regular temporal resolution Modern d evices u se an electronic s torage s ystems in p lace of the d rum- chart mechanism Recording raingauges Weighing raingauge The signal is recorded b y a s cale, which w eighs c ontinuously the rainfall accumulated in the collecting b ottle. Typical resolutions: 0.01 mm Temporal resolution: 1 Precipitation 10

11 Measurement errors Precipitation measurements are subject to 2 type of errors systematic due to obstacles due to location / height of the instruments with respect to the ground due to change of instrument and/or change of instrument location random due to reading errors due to evaporation (only for non recording, esp. in monthly totalisators) due to wind and/or snow Measurement errors Double mass curve technique allows to detect a change of instrument and/or location CUMULATIVE ANNUAL PRECIPITATION AT GAUGE CUMULATIVE ANNUAL PRECIPITATION FOR N GAUGES (N-STATION MEAN) curve departure indicates a change Precipitation 11

12 Measurement errors Wind induced undercatch (random) precipitation wind raingauge precipitation Wind and snow shield MeteoSchweiz Obstacle induced undercatch (systematic shading effect) From measurements to precipitation data hyetograph max rainfall depth for 30, 1h and 2h cumulative hyetograph Precipitation 12

13 Hydrology I Autumn Semester 2015 Effect o f aggregation at coarser temporal resolution hourly 3 hours loss of information instantaneous 6 hours 12 hours Variability of p recipitation in s pace and time precipitation estimates from ETH radar images cumulative precipitation in 5 Precipitation 13

14 Variability of precipitation in space and time precipitation fields a) estimated from radar b) estimated from raingauge interpolation a) b) c) simulated by a stochastic model d) simulated by a fractal model c) d) Estimation of areal rainfall (basin precipitation) Problems: - precipitation variability in space - low density of raingauge stations Technical solutions - for small areas à basin precipitation = homogeneous, measured at raingauge - for large basins à computation of average rainfall - - arithmetic mean - - Thiessen polygons - - isohyetal maps - - geostatistical interpolation methods (e.g. inverse distance, kriging) To consider precipitation type, # of raingauges, orography, temporal resolution Precipitation 14

15 Estimation of areal rainfall (basin precipitation) r, radius; A area a i = weighing factors à Arithmetic mean weighing factors a i = 1 Estimation of areal rainfall (basin precipitation) Thiessen polygons a i = weighing factors weighing factors a i = γ i n = number of polygons Precipitation 15

16 Estimation of areal rainfall (basin precipitation) Isohyethal method a i = weighing factors weighing factors a i = ΔA i ΔA i = surface between two consecutive isohyets Estimation of areal rainfall (geostatistical methods) Kriging Goestatistical method is based on the spatial correlation structure of the precipitation field (i.e. correlation among raingauges) Highly influenced by the number of raingauges It provides typically estimates on a regular grid Optimal linear interpolation technique à with a i such that Var[ε 2 ]=min, ε=p i, comp - P i,obs Inverse distance Geostatistical method is based on the spatial correlation structure of the precipitation field (i.e. correlation among raingauges) as represented by the distance from the estimation point Highly influenced by the number of raingauges Typically provide estimates on a regular grid a i weighing factors depending on distance among stations à a i =1/d i 2 Precipitation 16

17 Hydrology I Autumn Semester 2015 Precipitation example of application of knowledge Engineering Problem: Identify regions favourable to rainfed agriculture Solution Characterise the (annual/seasonal) p recipitation c limatology o f the region Method Isohyethal map i nterpolated from point measurements Precipitation 17