Buoyancy¶
ERF has three options for how to define the buoyancy force. Even in the absence of moisture these expressions are not equivalent.
Density of the mixture¶
The total density in a cell containing air, water vapor, liquid water and precipitates is given by
where \(m_a\) is the mass of dry air, \(m_v\) is the mass of water vapor, \(m_c\) is the mass of liquid water, and \(m_p\) is the mass of precipitate. From the definitions of the mass mixing ratio (ratio of mass of a component to mass of dry air), we have for any component
Using this we can write
where \(\rho_d \equiv \cfrac{m_a}{V}\) is the density of dry air.
Type 1¶
One version of the buoyancy force is expressed simply as
where the total density \(\rho_{total} = \rho_d(1 + q_v + q_c + q_p)\) is the sum of dry and moist components and \(\rho_0\) is the total density for the background state. For eg., a usual scenario is that of a background state that contains only air and vapor and no cloud water or precipitates. For such a state, the total background density \(\rho_0 = \rho_{d_0}(1 + q_{v_0})\), where \(\rho_{d_0}\) and \(q_{v_0}\) are the background dry density and vapor mixing ratio respectively. As a check, we observe that \(\rho^\prime_0 = 0\), which means that the background state is not buoyant.
Type 2¶
The second option for the buoyancy force is
To derive this expression, we define \(T_v = T (1 + 0.61 q_v − q_c − q_i - q_p)\), then we can write
Starting from \(p = \rho R_d T_v\) and neglecting \(\frac{p^\prime}{\bar{p}}\), we now write
and define
where we have retained only first order terms in perturbational quantities.
Then
where the overbar represents a horizontal average of the current state and the perturbation is defined relative to that average.
Again keeping only the first order terms in the mass mixing ratios, we can simplify this to
We note that this reduces to Type 3 if the horizontal averages of the moisture terms are all zero.
Type 3¶
The third formulation of the buoyancy term assumes that the horizontal averages of the moisture quantities are negligible, which removes the need to compute horizontal averages of these quantities. This reduces the Type 2 expression to the following:
We note that this version of the buoyancy force matches that given in Marat F. Khairoutdinov and David A. Randall’s paper (J. Atm Sciences, 607, 1983) if we neglect \(\frac{p^\prime}{\bar{p_0}}\).
Type 4¶
This expression for buoyancy is from khairoutdinov2003cloud and bryan2002benchmark.
The derivation follows. The total density is given by \(\rho = \rho_d(1 + q_v + q_c + q_p)\), which can be written as
This can be written using binomial expansion as
Taking log on both sides, we get
Taking derivative gives
Using \(- 0.61 q_v + q_c + q_p \ll 1\), we have
Since the background values of cloud water and precipitate mass mixing ratios – \(q_c\) and \(q_p\) are zero, we have \(q_c' = q_c\) and \(q_p' = q_p\). Hence, we have