Additional Processing and Validation

To calculate wind characteristics of the HBL based on dropwindsonde measurements, the use of the composition methodology is unavoidable since an individual dropwindsonde profile provides only a little information about the entire measured wind field. Because one individual dropwindsonde drop only provides a profile reflecting the instantaneous wind velocity variation with height in a small region of the whole hurricane wind field, the calculation of the turbulence information from dropwindsonde measurements should be based on a compositional approach taking numerous dropwindsonde profiles together. For that reason, the same composition strategy, as in the analysis of the mean wind pro- file detailed in section 4.2, is followed to calculate the HBL turbulence in this section. In addition, since the turbulence is commonly recognized as a local phenomenon, which in- dicates the horizontal measurement position is not important in the calculation, only the MBL wind speed and gradient wind are used as grouping indicators. Naturally, the resid- uals of raw measurements, left by eliminating the mean wind profile, should be adequate to represent the turbulent component of the measured wind velocity, and therefore the calculation should be based on the composition of these remains. However, as discussed in section 4.2 about the mean wind profile from different regions in a hurricane, there is a systematical difference of the mean wind profile from different hurricane regions. Thus, direct compositing the residuals includes the variations beyond that introduced by the turbulence.

To reduce this inherent deficiency of the composition methodology, a high-pass filter is applied to the remains left over by eliminating the mean wind profile calculated as detailed in section 4.2. The high-pass filtering process is employed because it is obvious that the variations introduced by the systematical difference shown in Fig. 4.8 have a spatial scale much larger than turbulence length scale. The simple moving average filter is revised and applied to do the high-pass filtering. Since the moving average procedure filters out high frequency fluctuations of signals to smooth curves, the part of the measured wind velocity filtered out by the moving average filter is taken as the turbulent component of the wind measurement. This process is illustrated in Fig. 4.10. When using the

revised moving average filter, the only parameter that needs to be determined for the filter design is the length scale within which fluctuations are taken as turbulence. With no other references, a subjectively decided scale of 800mis used, because it is a reasonable assumption of the turbulence influencing scale. It should be noted that the analysis of reasons introducing the fluctuations of the dropwindsonde measured wind velocity does not exhaust all possibilities, and therefore variations introduced by other methodology deficiencies may have a spatial scale similar to that introduced by turbulence. As a result, the composition methodology and the use of the high-pass filter should be validated by the turbulence calculated based on other observations. More importantly, the arbitrarily chosen cut-off scale, 800m, lacks a sound theoretical base. Although a sensitivity analysis shows that the turbulence intensity composition results are not that sensitive to the filter scale given the scale varies within the order of magnitude of∼100m, the error brought by this arbitrarily chosen cut-off scale can only be detected by comparing the composition turbulence results to that calculated based on other observations.

The turbulent wind velocity profile calculated based on the CBLAST observational data are suitable for validating the additional processing procedures used to find turbu- lent component of dropwindsonde wind measurement articulated above. The profile of the turbulent wind velocity, defined as the square root of the measured wind velocity variations, composited from dropwindsonde measurements is compared to the turbulent wind velocity found in the CBLAST experiment (J. Zhang 2011, personal communica- tion). The comparison is shown in Fig. 4.11. It should be noted that the turbulent wind velocity is calculated based on the tangential wind, decomposed from the total wind mea- surement using storm tracks found earlier, because it is close to its counterpart in the CBLAST data, the along-wind turbulent wind velocity. Furthermore, only two groups, corresponding to the MBL wind velocity ranging from 20m/s to 23m/sand from 23m/s

to 26m/s, are shown in the comparison because their measured wind field has a strength comparable to that observed in the CBLAST experiment. The qualitative agreement shown in the comparison indicates the validity of the composition methodology and the use of the high-pass filter, including the subjectively chosen cut-off scale.

Figure 4.10: Sketch illustrating the process to calculate the turbulent wind from instan- taneous dropwindsonde wind measurements. The arrow in the lower figure indicates the turbulent component produced by the high-pass filter. Profiles shown in the figure are calculated based on a real dropwindsonde profile.

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Figure 4.11: Comparison of the turbulent velocity for the tangential wind with the results from CBLAST experiment, only first two groups corresponding to MBL wind speed

20m/s− 23m/s and 23m/s− 26m/s are shown.