cmf::atmosphere Namespace Reference

Contains classes to describe interactions with the atmosphere. More...

Detailed Description

Contains classes to describe interactions with the atmosphere.

Classes
class	aerodynamic_resistance
	Abstract class. Child classes can be used to calculate aerodynamic resistances against turbulent heat fluxes. More...
class	ConstantMeteorology
	A primitive implementation of the Meteorology interface. More...
class	ConstantRainSource
	A simple implementation of RainSource. More...
class	IDW_Meteorology
	Regionalizes meteorological measurements using a simple inverse distance weighted (IDW) method. More...
class	IDWRainfall
	A RainSource using a spatially interpolated rainfall intensity from all stations. More...
class	log_wind_profile
	, A logarithmic wind profile More...
class	Meteorology
	An abstract class, for objects generating Weather records at a specific time. More...
class	MeteoStation
	A meteorological station holding timeseries to create Weather records. More...
class	MeteoStationList
	A list of meteorological stations. More...
class	MeteoStationReference
	A reference to a meteorological station. More...
class	RainfallStation
	RainfallStation describes a rainfall timeseries in mm/day at a certain place. More...
class	RainfallStationList
	A list of rainfall stations. More...
class	RainfallStationReference
	References a single RainfallStation to provide rainfall intensity data. More...
class	RainSource
	An abstract class for different types of rainfall sources. More...
class	TimeseriesRainSource
	A rainsource with a timeseries. More...
struct	Weather

Functions
double	extraterrestrial_radiation (cmf::math::Time t, double longitude=8, double latitude=51, double time_zone=1, bool daily=false)
	Calculates the global radiation in MJ/(m2 day) from the sun position and the sunshine fraction.
double	global_radiation (double Ra, double height, double sunshine_fraction)
	Calculates the global radiation in MJ/(m2 day) from the sun position and the sunshine fraction.
double	MJ_to_watts (double MJ)
	Transforms an energy flux from MJ/day to W.
double	Pressure (double height)
	Returns the average air pressure for a height (m a.s.l.)
double	rH_from_vpd (double T, double vpd)
	Returns the rel.
double	vapour_pressure (double T)
	Returns the saturated vapor pressure in Pa for temperature T [degC].
double	vpd_from_rH (double T, double rH)
	Returns the vapor pressure deficit in Pa for temperature T [degC] and rel.
double	watts_to_MJ (double Watts)
	Transforms an energy flux from W to MJ/day.

Function Documentation

extraterrestrial_radiation()

double extraterrestrial_radiation	(	cmf::math::Time	t,
		double	longitude = 8,
		double	latitude = 51,
		double	time_zone = 1,
		bool	daily = false )

Calculates the global radiation in MJ/(m2 day) from the sun position and the sunshine fraction.

Parameters

t	actual time step
longitude,latitude	Geographical position in degree. Latitude is only taken into acount for subdaily calculation
time_zone	Offset by timezone from GMT, eg. central Europe=1 US west coast = -8
daily	If true, the average radiation for the whole day is given (therefore latitude and time zone ignored), otherwise the average of the current hour is returned

The calculation of the global radiation follows http://www.fao.org/docrep/X0490E/x0490e07.htm#radiation.

The following formula is used:

• \( \phi\) Latitude in \(rad\)
• \( \delta = 0.409 \sin\left(\frac{2\pi}{365}DOY - 1.39\right)\) Declination, DOY is day of year
• \( \omega_s = \arccos(-\tan\phi\tan\delta) \) Sunset angle
• \( G_{sc} = 0.0802 \frac{MJ}{m^2min} \) Solar constant
• \( d_r = 1+0.033 \cos\left(\frac{2\pi}{365}DOY\right) \) Inverse relative distance Earth-Sun
• \( b = \frac{2\pi(DOY-81)}{364}\)
• \( S_c = 0.1645\sin(2b)-0.1255\cos(b)-0.025\sin(b) \) Seasonal correction for solar time
• \( \omega = \frac {\pi} {12} \left(t_h+\frac{(\mbox{geogr. Longitude})^\circ}{15^\circ}-\mbox{Timezone}+S_c-12\right) \) solar time in \(rad\)
• If daily: \( R_a = \frac{24\ 60}{\pi}G_{sc}\ d_r \left(\omega_s \sin\phi \sin\delta + \cos\phi \cos\delta \sin\omega_s\right) \)
• If sub daily: \( R_a = \frac{12\ 24\ 60}{\pi}G_{sc}\ d_r \left(\left(\omega^+ -\omega^-\right) \sin\phi \sin\delta + \cos\phi \cos\delta \left(\sin\omega^+ - \sin\omega^-\right)\right) \)
• \( \omega^+,\omega^- = \omega \pm\frac{\pi}{24} \)

global_radiation()

double global_radiation	(	double	Ra,
		double	height,
		double	sunshine_fraction )

Calculates the global radiation in MJ/(m2 day) from the sun position and the sunshine fraction.

Parameters

Ra	extra terrestrial radiation
height	Height above sea level
sunshine_fraction	Fraction of sunshine hours per potential sunshine duration in h/h

The calculation of the global radiation follows http://www.fao.org/docrep/X0490E/x0490e07.htm#radiation.

The following formula is used:

• \(R_a(t, \phi, \lambda)\) Extraterrestrial radiation (cmf::atmosphere::extraterrestrial_radiation) \(\frac{MJ}{m^2 day}\)
• \(\frac n N\) Fractional sunshine duration
• \(R_s = \left(0.25+\left(0.5+2\ 10^{-5}z\right)\frac{n}{N}\right)R_a \) Global radiation in \(\frac{MJ}{m^2 day}\)
• \(z\): Height a.s.l. in m

rH_from_vpd()

double rH_from_vpd	(	double	T,
		double	vpd )

Returns the rel.

humidity in % for temperature T [degC] and vapor pressure deficit vpd [Pa]

the rel. humidity is calculated from the vapor pressure deficit \(vpd = e_s - e_a\) as:

\[rH = 100 * \frac{e_a}{e_s(T)}, e_a = e_s(T) - vpd\]

The definition is from http://www.fao.org/docrep/X0490E/x0490e07.htm#concepts

Parameters

T	Air temperature in degC
vpd	Vapor pressure deficit in Pa

\(e_s(T)\) is calculated using cmf::atmosphere::vapor_pressure(double)

vapour_pressure()

double vapour_pressure

(

double

T

)

Returns the saturated vapor pressure in Pa for temperature T [degC].

The saturated vapor pressure \(e_s\) is calculated follwing the following formula

\[e_s = 0.6108 \exp{\frac{17.27 T}{T+237.3}}\]

The definition is from http://www.fao.org/docrep/X0490E/x0490e07.htm#concepts

vpd_from_rH()

double vpd_from_rH	(	double	T,
		double	rH )

Returns the vapor pressure deficit in Pa for temperature T [degC] and rel.

humidity rH [%]

The vapor pressure deficit \(e_s - e_a\) is calculated from rel. humidity as:

\[e_s - e_a = (1-rH/100) * e_s(T)\]

The definition is from http://www.fao.org/docrep/X0490E/x0490e07.htm#concepts

Parameters

T	Air temperature in degC
rH	Rel. humidity in %

\(e_s(T)\) is calculated using cmf::atmosphere::vapor_pressure(double)