Antecedent mid-tropospheric frontogenesis caused by the interaction between a tropical cyclone and midlatitude trough: a case study of Typhoon Rusa (2002)

Abstract

This study examines antecedent mid-tropospheric frontogenesis (AMF) resulting from the interaction between Typhoon Rusa (2002) and a midlatitude trough over the Korean Peninsula. In this event, the AMF contributed to the first peak in the time series of rainfall in Gangneung (37.75°N, 128.90°E), occurring about 12 h before the time of the extratropical transition (ET) process of the tropical cyclone (TC). Using observations and high-resolution model outputs, we showed that the AMF contributed to the antecedent rainfall in Gangneung during the first rainfall period when Gangneung was located outside of Rusa's sphere of direct influence. A Weather Research Forecasting (WRF) model experiment was conducted to diagnose the frontogenetical features and associated precipitation processes in detail. The experiment revealed that the AMF was mainly forced by the horizontal deformation forcing (HDF). The direction of the HDF was oriented from southwest to northeast in the middle part of the peninsula. The HDF increased positively due to the confluence of the southeasterlies from the TC and the northwesterlies emanating from the midlatitude trough. The experiment also suggested that the mid-tropospheric moisture originated from the subtropical ocean and deposited into the frontal region by the southerlies on the eastern periphery of the TC, which enhanced the convergence of moisture flux in the frontal region during the first rainfall period. The thermally direct circulation associated with the AMF lead to the mid-tropospheric saturation, which enhanced the precipitation of the first rainfall event together with the orographically forced convection at the low level above Gangneung.

Appendix 1

Sedimentation of hydrometeor contents (SHC)

The vertically integrated water vapor and total hydrometeor contents budgets can be expressed as:

$$ \frac{\partial \left[{q}_{\mathrm{v}}\right]}{\partial t}=\left[{\mathrm{ADV}}_{q\mathrm{v}}\right]-\left[{S}_{q\mathrm{v}}\right], $$

(A1)

$$ \frac{\partial \left[C\right]}{\partial t}=\left[{\mathrm{ADV}}_C\right]-\left[S\mathrm{H}C\right]+\left[{S}_C\right], $$

(A2)

where a term in square brackets signifies a mass-weighted vertical integration between two levels (p ₁ and p ₂) as defined by $\left[F\right]=\frac{1}{g}{\displaystyle \underset{p_1}{\overset{p_2}{\int }}}F\mathrm{d}p$ for any variable F. In Eqs. A1 and A2, q _v is the mixing ratio of water vapor and C is the sum of mixing ratios of hydrometeor contents; that is, cloud water, rain water, cloud ice, snow, and graupel, (ADV_qv), and (ADV_C) are three-dimensional advection of water vapor and hydrometeor contents, respectively. S _qv and S _c are the source and sink in the water vapor and hydrometeor contents budget due to conversions between water vapor and various hydrometeor species. SHC is total sedimentation of hydrometeor contents, i.e., $\mathrm{SHC}=\frac{1}{g}\frac{\partial }{\partial p}\left({q}_{\mathrm{C}}{\upomega}_{\mathrm{T}}\right)$, where q _C and ω_T are mixing ratio of hydrometeor contents and terminal velocity, due to saturation in specific column. Note that (S _qv) = (S _c), when averaged over the specific region. Then (SHC) is expressed as:

$$ \left[ SHC\right]=-\left(\frac{\mathrm{d}\left[{q}_{\mathrm{v}}\right]}{\mathrm{d}t}+\frac{\mathrm{d}\left[C\right]}{\mathrm{d}t}\right), $$

(A3)

which means that SHC indicates the fallout residual following an air parcel. Equation A3 presents the difference between precipitation and evaporation at the surface if the SHC is integrated from the surface to the top of the model:

$$ {P}_{\mathrm{s}}-{E}_{\mathrm{s}}=-\left\{\frac{\mathrm{d}}{\mathrm{d}\mathrm{t}}\left(\frac{1}{g}{\displaystyle \underset{p_{\mathrm{s}}}{\overset{p_{\mathrm{t}}}{\int }}}{q}_v\mathrm{d}p\right)+\frac{\mathrm{d}}{\mathrm{d}\mathrm{t}}\left(\frac{1}{g}{\displaystyle \underset{p_{\mathrm{s}}}{\overset{p_{\mathrm{t}}}{\int }}}C\mathrm{d}p\right)\right\}, $$

(A4)

where P _s, E _s, p _s, and p _t denote precipitation, evaporation, surface pressure, and the pressure at the top of the model, respectively.