The Atmospheric model parameters tab is used for describing a model atmosphere by adjusting the parameters in the US Standard Atmosphere, and setting a model wind field. Entries are required in parameter set 1 whereas entries in parameter sets 2 through 5 inherit values from set 1.
Default values in of the atmospheric model parameters can be recovered in parameter set 1 by pressing the button marked
whereas settings in parameter sets 2 through 5 can be cleared by pressing the button marked
The US Standard Atmosphere has a piecewise linear temperature profile with height and a hydrostatic pressure balance. In PlumeRise a model atmosphere can be constructed by adjusting the lapse rates of temperature in the atmospheric layer, the altitude of the atmospheric layers, and the temperature and pressure at sea level. The atmospheric temperature, Ta is given by
Here Ta0 is temperature at sea-level (at z = 0), μ and λ the lapse rates of temperature in the troposphere and stratosphere, respectively, and H1 and H2 are the altitudes of the tropopause and stratosphere, respectively. Note, typically the temperature decreases with height in the troposphere, so μ < 0.
The pressure, Pa, in the model atmosphere is hydrostatic and given by the solution of the equation
where g is the gravitational acceleration, RA is the gas constant of the atmosphere (which may vary with height due to water vapour in the atmosphere), and Pa0 is the pressure at sea-level.
The wind speed V is modelled as linearly increasing in the lower atmosphere (up to the tropopause) with constant wind speed above,
Here V1 is the wind speed in the upper atmosphere, referred to as the stratospheric wind speed. The wind direction is taken to be constant with height.
The table below gives a description of the parameters with typical values and the range of allowed values. Note all parameters must be given values if 'Use modelled standard atmosphere' is selected, and the range that parameters can take is intentionally much larger than the range of expected values.
The table below gives a description of the parameters with typical values and the range of allowed values. Note all parameters must be given values if 'Use modelled standard atmosphere' is selected, and the range that parameters can take is intentionally much larger than the range of expected values.
| Parameter name | Description | Allowed values |
|---|---|---|
| Air temp. at sea level (°C): | Air temperature at sea level. | −100 – 100 |
| Pressure at vent (hPa): | Atmospheric pressure at the altitude of the vent | 100 – 1500 |
| Stratospheric wind (m/s): | Speed of the wind within the stratosphere. Wind speed is assumed to be zero at sea level and linearly increasing through the troposphere. The wind speed is constant through the tropopause and stratosphere. |
0 – 200 |
| Wind direction: | Wind direction is specified by the angle that the wind originates from, in degrees (the bearing). The wind direction is assumed to be constant through the atmosphere. |
−360 – 360 |
| Height of tropopause (km): | The altitude at which the tropopause begins (i.e. the height of the top of the troposphere).
The tropopause is the atmospheric layer between the troposphere and the stratosphere, marked by an abrupt change in the atmospheric lapse rate. The tropopause is more stable than the troposphere below. In the US Standard Atmosphere, the tropopause is at 11 km representing mid-latitude atmospheres. In the tropics the tropopause is typically higher, at around 17 km, whereas the tropopause in polar regions is lower at around 8 km. The height of the tropopause is also affected by the weather. |
2.5 – 20 |
| Height of stratosphere (km): | The altitude at which the stratosphere begins (i.e. the height of the top of the tropopause). The stratosphere has an increasing temperature with height. In the US Standard Atmosphere, the stratosphere begins at is at 20 km representing mid-latitude atmospheres. Typical heights at which the stratosphere begins are 10 km to 20 km. |
5 – 40 |
| Temp. grad. in troposphere (K/km): | The lapse rate of temperature in the troposphere. The temperature in the troposphere decreases with height, so the temperature gradient in the troposphere is negative. The US Standard Atmosphere has a tropospheric temperature gradient of −6.5 K/km. |
−30 – 0 |
| Temp. grad. in stratosphere (K/km): | The lapse rate of temperature in the stratosphere. The temperature in the stratosphere increases with height, so the temperature gradient in the stratosphere is positive. The US Standard Atmosphere has a stratospheric temperature gradient of 1.0 K/km. |
0 – 20 |