Evaluation of Hurricane Wind Speed Analyses in a Simulation of Hurricane Earl (2010) Using Low-Order Wavenumbers

Abstract

To determine how well a low-order wavenumber representation describes a hurricane wind speed field, given its natural variability in space and time, low-order wavenumber representations were calculated for hourly “snapshots” of the 10-m wind speed field generated by the current operational hurricane model. Two distinct periods were examined: the first when the storm is in a reasonably steady state over 7–8 h and the second where the storm is changing its internal structure over a similar time interval. Observing system sensitivity experiments were also performed using wind speed field time series obtained from interpolation of the model snapshots for each of the two periods. The time series were sampled along the flight legs of a typical “figure four” aircraft flight pattern to simulate the surface wind data collection process to ascertain the effects of the wind speed field’s temporal and spatial variability upon the low-order wavenumber analyses. The comparison between the model wind speed field at any time and the wavenumber representations during the “steady state” period shows that the essential features of the wind speed field are captured by wavenumbers 0 and 1 and that including up to wavenumber 3 practically reproduces the model field. However, in the “nonsteady” period the wavenumber 0 and 1 representation is frequently unable to capture the essential characteristics of the wind speed field. The observing system sensitivity experiments suggest that when the primary circulation is rapidly changing in amplitude and/or structure during the data collection period, the low-order wavenumbers analysis of the wind speed field will only represent the temporal mean structure.

Relating Observations of Gradient Nonbalance at the Top of Hurricanes With Their Warm Core Structures

“…The "clean" data were interpolated along the pressure coordinates to fixed 10-hPa intervals and analyzed in two manners: First, we produce vertical cross sections of the storm along the flight path to examine the location of the warm core with respect to the storm center. Second, we estimate the 3-D structure of the storm using a wavenumber decomposition method at each vertical level, following Soukup and Marks (2017), described here in short:…”

“…The convergence of the interpolated field shown in Soukup and Marks (2017) allows to approximate the true field using only Wavenumbers 0 through 3, given that the center of interpolation is close enough to the center of the storm in lower levels.…”

“…In this work data from the HS3 and TCI are analyzed in light of the findings of C17 to examine if, and under what conditions, high-pressure regions develop at the top of observed hurricanes, how they are related to the warm core structures, and whether the flow around these high-pressure regions are expected to violate gradient wind balance. For this purpose, we produce both 2-D vertical slices and 3-D structure of these storms; the latter analysis is done using the wavenumber decomposition method outlined in Soukup and Marks (2017).…”