Presentation on theme: "GEO3020/4020 Lecture 1: Meteorological elements"— Presentation transcript:
1 GEO3020/4020 Lecture 1: Meteorological elements Repetition
2 Weather is determined by the energy and mass transport at the surface: Meteorological variables are used to describe the weather and to calculate the components of the energy and water balance equation.
6 Daily clear-sky solar radiation Important contributor to the energy balance at the earth surface;Difficult to measure;Method for estimating it on a horisontal and slope surface at an arbritary place is given in Appendix E.
7 Summary = Extraterrestrial Radiation on a sloping plane = Extraterrestrial Radiation on a horizontal plane= Extraterrestrial Radiation on a sloping plane= Total daily clear sky incident radiation on a horizontalplane at the earth surface= global short wave radiation at the earth surface= backscattered radiation (= )and
8 Empirical adjustments The total daily clear sky radiation flux at the surface , are derived fromEmpirical relationships (adjusting for the effect of clouds and vegetation) have been developed, e.g.where global short wave radiation on the surface and , Extraterrestrial Radiation, n = actual sunshine hour and N= max sunshine hour (can be read from table for a given location and season).
9 Relation between K, Kin, Kcs, KET KET = Extraterrestrial (potential) solar radiationKcs = clear sky short wave radiation flux on a horizontal surface on earthKin = adjusted Kcs for slope, aspect, clouds and vegetationK = net flux of solar energy entering the surface, e.g. snowpackNormally K < Kin < Kcs < KETKET = extraterrestrial (potential) solar radiationKin = measured solar radiation
10 Structure of the atmosphere CompositionVertical structurePressure-temperature relation (Ideal gas law)Adiabatic lapse rate (dry & wet)VapourVapour pressure, eaSat. vapour pressure, ea*Absolute humidity, ρvSpecific humidity, q = ρa/ρvRelative humidity, Wa = ea/ea*Dew point temperature, TdThe mass concentration of water vapour in a volume of air (vapour density) [kg m-3]
11 Meteorological variables Measurements PrecipitationRadiationAir temperatureAir humidityWindAir pressureSelve hytten er hvitmalt for å reflektere mest mulig solstråling, og skal plasseres i et åpent område, helst ikke i le av eller for nær bygninger og lignende hindringer som gir skygge.Underlaget under hytten skal være kortklipt gress. For eksempel kan asfalt ett sted og en fuktig myr et annet sted gi helt forskjellige målinger, særlig på varme, solrike sommerdager.Veggene i hytten er ikke helt tett, slik at det er godt ventilert og luften kan bevege seg fritt gjennom hytten.Termometeret er plassert inne i hytten, to meter over bakken.Inni hytten er termometeret beskyttet slik at solen ikke skinner direkte på det og varmer det opp, og det er også skjermet mot nedbør
12 GEO3020/4020 Lecture 2: I. Energy balance Lena M. TallaksenChapter & 7.3.4, Appendix D.4; Dingman
15 Shortwave radiation input, K Net solar radiation, KK = Kin - Kout = Kin·(1-a) (5-27)Where K is net flux of solar energy entering the bodyKin – flux of solar energy incident on the surface (= global radiation)Kout – reflected fluxa – albedo, depends on the properties of the surfaceK can be measured by pyranometers, but more common to estimateKcs represents the clear-sky shortwave radiation fluxKin, adjusted for the slope and aspect:Λ, latitude, J day of year,b, slope inclination angle, a, slope azimuth angleC, fraction of sky covered with cloudsF, fraction of sky obscured by forest canopy,are functions
16 Shortwave radiation input, K The function is derived using the concept of equivalent latitude,The function , the effect of cloud cover can be estimated using empirical relations like,orThe function effect of forest canopy, an example for pine,The albedo, a, changes with location, season, vegetation, ….
17 Practical considerations Empirical equation for Shortwave radiationC = fraction of sky covered with cloudsKCS = total daily clear sky incident radiation on a horizontal plane at the earth surface
18 Longwave radiation exchange, L Net longwave radiation – equals to incoming atmospheric longwave radiation minus the portion of that reflected and the radiation flux emitted by the surfaceL = Lin – Lout = Lat – (1-es)Lat - Ls (7-28)where:Lat - incoming atmospheric longwave radiation flux,Ls - outgoing radiation from the surfacees - emissivity of surfaceL, Lat, Ls can be measured by pyranometers, but more common to estimate using equations like:eat - emissivity of the atmosphere and canopyes - emissivity of the surfaceσ - Stefan-Boltzmann constantTat ,Ts – temperature of the atmosphere and water surface
19 Longwave radiation exchange, L Combine the two equations (5.35 and 5.36) we get:wheres, Stefan-Boltzmann constant (4.90×10-9 MJ m-2 day-1 K-4)Tat, effective radiating temp of atmosphere and canopy (˚C)Ts, temperature of surface (˚C)Table D-1 for es values, the question remains how to calculate eat.
21 Longwave radiation exchange, L Clear sky, no forest canopyCloudy sky, no forest canopyCloudy sky, forest canopy (general equation)where ea is near surface saturation atmospheric vapor pressureand Ta is air temperature in °C, F is the ratio of the horizontally projected area of forest canopy to the total area of interest, C is degree of cloud cover
23 Sensible heat, H Sensible-heat exchange by turbulent transfer, H: and from equation (D-49)wherera = density of air;Ca = heat capacity of air;k = 0.4;zd = zero plane displacementheightz0 = surface-roughness height;za = height above ground surfaceat which va & Ta are measured;va = windspeed,Ta = air temperatures andTs = surface temperatures.
25 Latent heat, LE Latent heat exchange by turbulent transfer, LE and from equation (D-42)wherera = density of air;λv = latent heat of vaporization;P = atmospheric pressurek = 0.4;zd = zero plane displacementheightz0 = surface-roughness height;za = height above ground surfaceat which va & ea are measured;va = horisontal windspeed,ea = air vapor pressurees = surface vapor pressure (measured at z0 + zd)
26 Exchange of (sensible) heat with the ground, G Positive or negative depending on the temperature of the air and soil surface (often negligible compared to other terms).If soil temperature is increasing downward (due to thermal energy stored during the summer) heat is transferred upwards at a rate:kG is the thermal conductivity of the soil (E L-1 T-1 q-1), depends on soil texture, soil density, and moisture content and vary widely with season and place.
27 Water-advected energy, Aw (lakes) Net water-advected energy Aw [E L-2 T-1] is found from:where:rw density of water [M L-3]cw – specific heat of water [E M-1 θ-1]w – average precipitation rate [L T-1]SW and GW – surface water and ground water inflows and outflowsTs – temperatures of the respective inflows and outflows [θ]Heat input by rain, R (snowpack)rainwater is first cooled to the snow temperature (Eq. 5-47a)if the snow is below zero, then freezing may occur and latent heat realised (Eq. 5-47b)
28 Change in stored energy, ΔQ Energy can be stored in e.g. a snowpack, lake or soilSnow (warming phase)Lakewhere:hm snow water equivalentci heat capacity of iceV lake volume,TL average lake temperature,AL lake area.subscripts 1 and 2 designate values at the beginning and end of Dt
29 Energy balance equation where:K net shortwave radiationL net longwave radiationLE latent heat transferH sensible heat transferG soil fluxAw advective energyΔQ/Δt change in stored energyUnits: [EL-2T-1]
30 Calculation of evaporation using the energy balance method Evaporation can be calculated solving for LE:where:LE has units [EL-2T-1]E [LT-1] = LE/ρwλvLatent Heat of Vaporization :lv= (2.361 × 10-3) Ta
31 Lena M. Tallaksen Chapter 7. – 7.1.2, 7.3.6; Dingman GEO3020/4020 Lecture 2: II. Evapotranspiration - Definitions - Governing factors - MeasurementsLena M. TallaksenChapter 7. – 7.1.2, 7.3.6; Dingman
32 Global water balance76511411411255490275mm/year
33 Water balance for Norway, 1931 -60 Otnes & Ræstad, 1978
38 Evaporation and evapotranspiration SummaryEvapotranspiration is a collective term for all the processes by which water in the liquid or solid phase at or near the earth’s surface (rivers and lakes, bare soils, and vegetative surfaces) becomes atmospheric water vapor.Evapotranspiration is a second largest term in the global water balance; about 62% of precipitation that falls on the continents is evapotranspired.Evapotranspiration is the term that links earth surface’s water balance and energy balance.It is much more difficult to measure evapotranspiration than to measure precipitation and streamflow.There are numerous methods/models available in calculating evapotranspiration, of which the most well-known methods will be discussed in the class.
39 Governing factors of evaporation I. Meteorological situationEnergy availabilityHow much water vapour can be receivedTemperatureVapour pressure deficitWind speed and turbulenceOptimal conditions: ?
40 Governing factors of evaporation II. Physiographic and plant characteristicsCharacteristics that influence available energyalbedoheat capacityHow easily can water be evaporatedsize of the evaporating surfacesurroundingsroughness (aerodynamic resistance)salt contentstomataWater supplyfree water surface (lake, ponds or intercepted water)soil evaporationtranspirationThe wind speed immediately above the surface.The humidity gradient away from the surface.The rate and quantity of water vapor entering into the atmosphere both become higher in drier air.Water availability.Evapotranspiration cannot occur if water is not available.
41 Evapotranspiration Measurements Free water evaporationPans and tanksEvaporimetersEvapotranspiration (includes vegetation)LysimetersRemote sensing
42 DefinitionsPotential evapotranspiration, PE, is the rate at which evapotranspiration would occur from a large area completely and uniformly covered with growing vegetation which has access to an unlimited supply of soil water and without advection or heat-storage effects (i.e. the rate is depedent on the vegetation)Actual evapotranspiration, ET, is the rate at which evapotranspiration occurs (i.e. describes all the processes by which liquid water at or near the land surface becomes atmospheric water vapor).
43 Pan evaporation methods Epan = W – [V2-V1]whereW = precipitation during DtV1 = the storage at the beginning of DtV2 = the storage at the end of DtFor American Class-A pan, Kohler et al. (1955) developed an empirical equation to account for energy exchange through sides of a pan, and adjust daily pan evaporation, Epan, to free water evaporation, Efw [mm day-1] (Equations 7-41 and 7-42).
44 Pan evaporation methods Pan coefficientElake/Epan = kpwherek is a coefficient that varies with seasons and lake.Its annual average over the US is about 0.7
46 Pan evaporation methods Example of pan coefficient in the Yangtze River catchment in China46
47 LysimeterOne of the most reliable way of measuring potential or actual evapotranspiration is to use large containers (sometimes on the order of several metres across) called lysimeters;Evapotranspiration is calculated by subtraction considering the different components of the water balance.A lysimeter is most accurate when vegetation is grown in a large set up which allows the rainfall input and water lost through the soil to be easily calculated from the difference between the weight before and after a given period.
48 Lysimeter for measuring potential evapotranspiration input (Rainfall R and Additional water A) and output (Percolated water P)collected in the receiver, then PE can be estimated from the equation:PE = R + A – PRAP
49 Lysimeter for measuring actual evapotranspiration Figure. Schematic of a weighable gravitation lysimeter.
50 Estimation of evapotranspiration by remote sensing Remote sensing has two potentially very important roles in estimating evapotranspiration (Engman, 1995).First, remotely sensed measurements offer methods for extending point measurements or empirical relationships to much larger areas, including those areas where measured meteorological data may be sparse.Secondly, remotely sensed measurements may be used to measure variables in the energy and moisture balance models of ET, such as as radiometric surface temperature, albedo, and vegetation index.