Factors Governing the Total Rainfall Yield from Continental Convective Clouds

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  • 1 Department of Atmospheric Sciences, The Hebrew University of Jerusalem, Israel
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Abstract

Several important factors that govern the total rainfall from continental convective clouds were investigated by tracking thousands of convective cells in Israel and South Africa. The rainfall volume yield (Rvol) of the individual cells that build convective rain systems has been shown to depend mainly on the cloud-top height. There is, however, considerable variability in this relationship. The following factors that influence the Rvol were parameterized and quantitatively analyzed: 1) Cloud base temperature—it is shown that when other factors are fixed, a 50% increase in the absolute humidity of the cloud base will nearly double the Rvol. 2) Atmospheric instability—cells in a more unstable atmosphere will rain much less (up to a factor of 5) than cells which are forced to grow to a similar maximum height in a more stable atmosphere. We suggest that more stable cells rain more because they grow more slowly, so that there is enough time for the cloud water to be converted into precipitation particles. 3) The extent of isolation of the cell—it is shown that isolated cells precipitate only about one-third of the Rvol of highly clustered cells, having the other factors be identical.

It is also shown that a strong low level forcing increases the duration of Rvol of clouds reaching the same vertical extent.

Abstract

Several important factors that govern the total rainfall from continental convective clouds were investigated by tracking thousands of convective cells in Israel and South Africa. The rainfall volume yield (Rvol) of the individual cells that build convective rain systems has been shown to depend mainly on the cloud-top height. There is, however, considerable variability in this relationship. The following factors that influence the Rvol were parameterized and quantitatively analyzed: 1) Cloud base temperature—it is shown that when other factors are fixed, a 50% increase in the absolute humidity of the cloud base will nearly double the Rvol. 2) Atmospheric instability—cells in a more unstable atmosphere will rain much less (up to a factor of 5) than cells which are forced to grow to a similar maximum height in a more stable atmosphere. We suggest that more stable cells rain more because they grow more slowly, so that there is enough time for the cloud water to be converted into precipitation particles. 3) The extent of isolation of the cell—it is shown that isolated cells precipitate only about one-third of the Rvol of highly clustered cells, having the other factors be identical.

It is also shown that a strong low level forcing increases the duration of Rvol of clouds reaching the same vertical extent.

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