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Generalized Asymmetric Holland Model

The Generalized Asymmetric Holland Model (GAHM) is a parametric hurricane vortex model developed in ADCIRC for operational forecasting purpose. Based on the classic Holland Model, the GAHM removes the assumption of cyclostrophic balance at the radius of maximum wind, and allows for a better representation of a wide range of hurricanes. Another important feature of the GAHM is the introduction of a composite wind method, which when activated enables the usage of multiple storm isotaches in reconstructing the spatial pressure and wind fields, while only one isotach is used in the HM.

The Classic Holland Model

The Holland Model (HM, 1980) is an analytic model that describes the radial pressure and wind profiles of a standard hurricane. To begin with, Holland found that the normalized pressure profiles of a number of hurricanes resemble a family of rectangular hyperbolas and may be approximated by a hyperbolic equation, which after antilogarithms and rearranging yields the radial pressure equation:

$\displaystyle P(r) = P_c + (P_n - P_c)e^{-A/r^B} \quad$ (1)

where $\displaystyle P_c$ is the central pressure, $\displaystyle P_c$ is the ambient pressure (theoretically at infinite radius), $\displaystyle P(r)$ is the pressure at radius $\displaystyle r$ from the center of the hurricane, and $\displaystyle A$ and $\displaystyle B$ are shape parameters that may be empirically estimated from observations in a hurricane.

Substituting (1) into the gradient wind equation, which describes a steady flow balanced by the horizontal pressure gradient force, the centripetal acceleration, and the Coriolis acceleration for a vortex above the influence of the planetary boundary layer where the atmospheric flow decouples from surface friction (Powell et al. 2009), gives the radial wind equation of a hurricane:

$\displaystyle V_g(r) = \sqrt{AB(P_n - P_c)e^{-A/r^B}/\rho r^B + (\frac{rf}{2})^2} - \frac{rf}{2} \quad$ (2)

where $\displaystyle V_g(r)$ is the gradient wind at radius $\displaystyle r$ , $\displaystyle \rho$ is the density of air, $\displaystyle f$ is the Coriolis parameter. In the region of the maximum winds, if we assume that the Coriolis force is negligible in comparison to the pressure gradient and centripetal force, then the air is in cyclostrophic balance. By removing the Coriolis term in (2) we get the cyclostrophic wind

$\displaystyle V_c(r) = \sqrt{AB(P_n - P_c)e^{-A/r^B}/\rho r^B} \quad$ (3)

By setting $\displaystyle dV_c/dr = 0$ at radius to the maximum wind $\displaystyle r = R_{max}$ , it is obtained that

$\displaystyle A = (R_{max})^B \quad$ (4)

Thus the ($\displaystyle R_{max}$ ) is irrelevant to the relative value of ambient and central pressures, and is solely defined by the shape parameters $\displaystyle A$ and $\displaystyle B$ . Substituting (4) back into (3) to get rid of $\displaystyle A$ , we get an estimate of $\displaystyle B$ as a function of the maximum wind speed

$\displaystyle B = (V_{max})^2\rho e/(P_n - P_c) \quad$ (5)

It was notable that the maximum wind speed is proportional to the square root of $\displaystyle B$ and irrespective of the ($\displaystyle R_{max}$ ), given a constant pressure drop. It was also reasoned by Holland that a plausible range of $\displaystyle B$ would be between 1 and 2.5 for realistic hurricanes. Substituting (4) and (5) back into (1) and (2) yields the final radial pressure and wind profiles for the HM

$\displaystyle P(r) = P_c + (P_n - P_c)e^{-(R_{max}/r)^B} \quad$ (6)
$\displaystyle V_g(r) = \sqrt{(V_{max})^2e^{1-(R_{max}/r)^B}(R_{max}/r)^B + (\frac{rf}{2})^2} - \frac{rf}{2} \quad$ (7)

The HM was implemented in the ADICRC as a wind module with NWS = 19. When sparse observations of a hurricane are given, estimates of the $\displaystyle R_{max}$ and shape parameter $\displaystyle B$ may be estimated by fitting data into the radial wind equation, which in turn allow us to compute $\displaystyle P(r)$ and $\displaystyle V_g(r)$ along the radius $\displaystyle r$ of the hurricane. However, discrepancies between wind observations and computed winds were sometimes found, and were negatively correlated to the Rossby number at $\displaystyle r = R_{max}$ , defined as

$\displaystyle R_o = \frac{Nonlinear Acceleration}{Coriolis force} ~ \frac{V_{max}^2/R_{max}}{V_{max}f} = \frac{V_{max}}{R_{max}f} \quad$ (8)

By definition, a large $\displaystyle R_o (\approx 10^3)$ describes a system in cyclostrophic balance that is dominated by the inertial and centrifugal force with negligible Coriolis force, such as a tornado or the inner core of an intense hurricane, whereas a small value $\displaystyle (\approx 10^{-2} \sim 10^2)$ signifies a system in geostrophic balance where the Coriolis force plays an important role, such as the outer region of a hurricane. As a result, the assumption of cyclostrophic balance at $\displaystyle R_{max}$ made in HM is mostly valid for describing an intense and narrow (small $\displaystyle R_{max}$ ) hurricane with a large $\displaystyle R_o$ , but not applicable for a weak and broad hurricane with a small $\displaystyle R_o$ . This intrinsic problem with the HM calls our intention to develop a generalized model that will work consistently for a wide range of hurricanes, which theoretically can be accomplished by removing the above cyclostrophic balance assumption and re-derive the radial pressure and wind equations (6)&(7).

Derivation of the GAHM

The GAHM also starts with the same radial pressure and wind equations (1)&(2) with shape parameters $\displaystyle A$ and $\displaystyle B$ as in the HM. Without assuming cyclostrophic balance at $\displaystyle R_{max}$ , we take $\displaystyle dV_g/dr = 0$ at $\displaystyle r = R_{max}$ to get the adjusted shape parameter $\displaystyle B_g$ as

$\displaystyle B_g = \frac{(V_{max}^2 + V_{max}R_{max}f)\rho e^\varphi}{\varphi(P_n - P_c)} = B \frac{(1+1/R_o)e^{\varphi - 1}}{\varphi} \quad$ (9)

where $\displaystyle {\varphi}$ is a scaling parameter introduced to simplify the derivation process, defined as

$\displaystyle \varphi = \frac{A}{R_{max}^B} \quad$ or $\displaystyle \quad A = \varphi R_{max}^B \quad$ (10)

and later derived as

$\displaystyle \varphi = 1 + \frac{V_{max}R_{max}f}{B_g(V_{max}^2+V_{max}R_{max}f)} = 1 + \frac{1/R_o}{B_g(1+1/R_o)} \quad$ (11)

Thus, the $\displaystyle R_{max}$ in the GAHM is not entirely defined by the shape parameters $\displaystyle A$ and $\displaystyle B$ as in the HM, but also by the scaling factor $\displaystyle {\varphi}$ , as Equation (11) indicates that $\displaystyle {\varphi} \ge 1$ . Numerical solutions for $\displaystyle B_g$ and $\displaystyle {\varphi}$ can be solved iteratively in the model using Equation (9)&(11). Figure 1 illustrates how $\displaystyle B_g/B$ and $\displaystyle \varphi$ vary with $\displaystyle \log_{10}R_o$ given different $\displaystyle B$ values. It is evident that values of both $\displaystyle B_g/B$ and $\displaystyle \varphi$ remain close to 1 when $\displaystyle \log_{10}R_o$ is within the range of [1,2], but increase noticeably as $\displaystyle \log_{10}R_o$ decreases below 1, and the smaller the value of $\displaystyle B$ , the bigger the changes.

Fig 1. Profiles of $\displaystyle B_g/B$ (left panel) and $\displaystyle \varphi$ (right panel) with respect to $\displaystyle \log_{10}R_o$ , given different $\displaystyle B$ values as shown in different colors.

Substituting (9)&(11) back into (1)&(2) yields the final radial pressure and wind equations for the GAHM

$\displaystyle P(r) = P_c + (P_n - P_c)e^{-\varphi(R_{max}/r)^{B_g}} \quad$ (12)

$\displaystyle V_g(r) = \sqrt{V_{max}^2(1+1/R_o)e^{1-(R_{max}/r)^{B_g}}(R_{max}/r)^B_g + (\frac{rf}{2})^2} - \frac{rf}{2} \quad$ (13)

Influence of the Coriolis force on the radial pressure and wind profiles are evidenced by the presence of $\displaystyle R_o$ and $\displaystyle \varphi$ in (12)&(13). A special case scenario is when we set $\displaystyle f=0$ , which corresponds to an infinitely large $\displaystyle R_o$ , then (12)&(13) in the GAHM reduce to (6)&(7) in the HM. However，for a hurricane with a relatively small $\displaystyle R_o$ , the influence of the Coriolis force can only be addressed by the GAHM. It meets our expectation that the GAHM’s solution approaches to that of the HM’s when the influence of Coriolis force is small, but departs from it when the Coriolis force plays an important role in the wind system.

The above reasoning can be demonstrated by the 3D plots in Figure 2, which show the normalized gradient winds of the HM (left panel) and the GAHM (right panel) as functions of the normalized radial distances $\displaystyle r/R_{max}$ , the Holland $\displaystyle B$ parameter, and $\displaystyle R_o$ . In both panels, each colored surface represents the normalized gradient winds corresponding to a unique Holland B value. By definition, we get $\displaystyle V_g = V_{max}$ at $\displaystyle r = R_{max}$ , which means all the surfaces in each panel should intersect with the plane of $\displaystyle r/R_{max} = 1$ on the plane of $\displaystyle V_g/V_{max} = 1$ , no matter what values of $\displaystyle R_o$ . However, the line of intersection (shown by the black line) shown in the left panel deviates from the plane of $\displaystyle V_g/V_{max} =1$ as $\displaystyle \log_{10}R_o$ decreases from 2 to close to 0 ($\displaystyle R_o$ decreases from 100 to 1), while remains on the plane regardless of how $\displaystyle R_o$ changes in the right panel, demonstrating that the GAHM is mathematically more coherent than the HM.

Fig 2. The normalized gradient wind profiles of the HM (left panel) and the GAHM (right panel) as functions of the normalized radial distances and $\displaystyle \log_{10}R_o$ , given different Holland $\displaystyle B$ values.

To have a dissective look of the surface plots in Figure 2, we draw slices perpendicular to the axis of $\displaystyle \log_{10}R_o$ at three different values 0, 1, 2, and plot the lines of intersection with each surface in Figure 3. It is evident that we get $\displaystyle V_g = V_{max}$ at $\displaystyle r = R_{max}$ consistently in the right panel for the GAHM regardless of the value of $\displaystyle R_o$ . The HM in the left panel, however, generates distorted wind profiles with underestimated maximum winds skewed inward towards the storm center, espeically when $\displaystyle \log_{10}R_o < 1$ . As a results, when both models being applied to real hurricane cases, the GAHM will perform more consistently than the HM.

Fig 3. Slices of the normalized gradient wind profiles (as shown in Figure 2) at $\displaystyle \log_{10}R_o =0, 1, 2$ (or correspondingly $\displaystyle R_o =1, 10, 100$ ).

Calculation of the Radius to the Maximum Wind

Both the HM and the GAHM use processed forecast advisories (during active hurricanes) or best track advisories (post-hurricanes) from the National Hurricane Center (NHC) in ATCF format as input files, which contain a time series of storm information such as storm location, storm movement, central pressure, 1 minute averaged maximum wind, radii to the 34-, 50-, and/or 64-kt storm isotaches in 4 storm quadrants (NE, SE, SW, NW), etc. As a standard procedure, the $\displaystyle B_g$ and $\displaystyle R_{max}$ for all data entries (usually at 6-hour intervals) at all available isotaches in 4 storm quadrant in the input file are pre-computed in the ASWIP program (an external FORTRAN program developed by Flemming et al. and further developed in this study to accommodate the GAHM) and appended to the input file prior to running an ADCIRC simulation forced with the GAHM wind model.

First, the maximum sustained wind and the 34-, 50-, and/or 64-kt isotaches in NHC’s forecast or “best track” advisories, normally reported at 10 meter height, must be scaled to the gradient wind level to remove the influence of the boundary layer effect. Practically, the maximum gradient wind can be directly calculated as

$\displaystyle V_{max} = \vert \frac{\overrightarrow{V_M} - \gamma\overrightarrow{V_T}}{W_{rf}} \vert = \frac{V_M - \gamma V_T}{W_{rf}} \quad$ (14)

where $\displaystyle \overrightarrow{V_M}$ is the reported maximum sustained wind at 10 meter height assuming in the same direction as $\displaystyle \overrightarrow{V_T}$ , $\displaystyle \overrightarrow{V_T}$ is the storm translational speed calculated from successive storm center locations, $\displaystyle W_{rf} = 0.9$ is the wind reduction factor for reducing wind speed from the gradient wind level to the surface at 10 meter height (Powell et al., 2003), and $\displaystyle \gamma$ is the damp factor for $\displaystyle V_T$ . The following formula of $\displaystyle \gamma$ is employ in the ASWIP program:

$\displaystyle \gamma = \frac{V_g}{V_{max}} \quad$ (15)

which is the ratio of gradient wind speed to the maximum wind speed along a radial wind profile. Thus, $\displaystyle \gamma$ is zero at storm center, and increases with $\displaystyle r$ until reaches a maximum value of 1 at $\displaystyle R_{max}$ , then gradually decreases outward to zero.

The gradient wind speed at the radii to specified storm isotaches in different storm quadrants can be calculated from the observed isotaches similarly as

$\displaystyle V_r = \vert \overrightarrow{V_r}\vert = \frac{\vert\overrightarrow{V_{isot}} - \gamma\overrightarrow{V_T} \vert} \quad$ (16)

where $\displaystyle \overrightarrow{V_{isot}}$ is the observed isotach wind speed with an unknown angle $\displaystyle \varepsilon$ , and $\displaystyle \overrightarrow{V_r}$ is the gradient wind speed with an inward rotation angle $\displaystyle \beta$ , defined as Equation (19) according to the Queensland Government's Ocean Hazards Assessment (2001):

$\displaystyle \beta = \begin{cases} 10^{\circ}, & r

Rewriting (16) in x- and y-components yields:

$\displaystyle V_r\cos(quad(i)+90+\beta)=V_{isot}\cos(\varepsilon)-\gamma {\mu}_T \quad$ (18)
$\displaystyle V_r\sin(quad(i)+90+\beta)=V_{isot}\sin(\varepsilon)-\gamma {\nu}_T \quad$ (19)

where $\displaystyle quad(i)$ is the azimuth angle of the $\displaystyle i-th$ storm quadrant (NE, SE, SW, NW at $\displaystyle 45^\circ, 135^\circ, 225^\circ, 315^\circ$ , respectively), $\displaystyle V_{isot}\cos(\varepsilon)$ and $\displaystyle V_{isot}\sin(\varepsilon)$ are the zonal and meridional components of $\displaystyle \overrightarrow{V_{isot}}$ , $\displaystyle {\mu}_T$ and $\displaystyle {\nu}_T$ are the zonal and meridional components of $\displaystyle \overrightarrow{V_T}$ .

Given an initial guess of $\displaystyle R_{max}$ , values of $\displaystyle B_g$ and $\displaystyle \varphi$ can be solved iteratively from (9) and (11) until both converge, and $\displaystyle V_r$ can be estimated by combining (15), (17), (18), and (19). Plugging $\displaystyle V_{max}$ from (14), the above calculated $\displaystyle B_g, \varphi, V_{max}, V_r$ and the observed radius $\displaystyle R_r$ to $\displaystyle V_r$ back into (13), a new $\displaystyle R_{max}$ can be inversely solved by a root-finding algorithm. Since the above calculations are carried out based on an initial guess of $\displaystyle R_{max}$ , wWe need to repeat the entire process until $\displaystyle R_{max}$ converges.

In case where multiple isotaches are given in the forecast/best track advisories, the $\displaystyle R_{max}$ for the highest isotach will be calculated using the above procedure, and used as the pseudo $\displaystyle R_{max}$ for the entire storm (physically, there is only one $\displaystyle R_{max}$ found along a radial wind profile ). For each lower isotach, $\displaystyle R_{max}$ will be calculated with the pseudo $\displaystyle R_{max}$ set as its initial value to determine the inward rotation angle $\displaystyle \beta$ following the above process only once. The use of the pseudo $\displaystyle R_{max}$ across all storm isotaches ensures that the cross-isobar frictional inflow angle changes smoothly along the radius according to (17).

Occasionally, we have to deal with situations where $\displaystyle V_{max} < V_r$ , which violate (13) so $\displaystyle R_{max}$ couldn't be calculated. These situations mostly happen in the right hand quadrants (in the Northern Atmosphere) of a weak storm with a relatively high translational speed. For cases like this, we assign $\displaystyle V_{max} = V_r$ , which is equivalent to assigning $\displaystyle R_{max} = R_r$ .

After the ASWIP program finishes processing the input file, it can be readily used by the GAHM to construct spatial pressure and wind fields in ADCIRC for storm surge forecast.