Calculations of Cumulus Vertical Draft Velocities in Hurricanes from Aircraft Observations

Abstract

In cumulus convective atmospheres the most significant length scale of vertical motion is of cumulus draft size—of horizontal lengths two to four orders of magnitude less than the synoptic scale and of magnitude one to three orders greater. This draft motion on scales between the gust and synoptic motion has been least investigated. The purpose of this paper is to propose a new method of calculating draft velocities from aircraft and present results of calculations with this method in hurricanes. An aircraft Doppler radio navigation instrument which is capable of measuring horizontal wind variations to a space resolution of a fraction of a nautical mile can be employed with other instruments to measure an aircraft's pitch angle changes. From this determination, along with other standard aircraft measurements such as radar and pressure altitude, power setting, etc., it is possible to make determinations of average vertical air motion to space resolutions of 0.4 to 0.7 nautical miles. Calculations of vertical motion are performed along a number of radial leg flight tracks flown by the National Hurricane Research Project B-50 aircraft during the 1958 season with the above method. Typical draft velocity was 5–15 knots and draft widths 1–3 nautical miles. Maximum derived gust velocities within the drafts were typically in the range of 5–15 knots. Comparison of results with those of the Thunderstorm Project is made. Vertical accelerometer data are also presented.