Orographic Effects on Typhoons Ming-Jen Yang 楊明仁 Dept. of Atmospheric Sciences National Central University 2009 Typhoon Summer School at NUIST

Outline Track discontinuity Impacts on storm intensity Impacts on surface features Impacts on rainfall

Track discontinuity

Track discontinuity --- Idealized modeling study Chang (1982; MWR) An idealized PE model with 60-km horizontal grid size, 7 sigma layers, and parameterized physics is used to study the effects of an island mountain range on translating TCs. Note that the topography is introduced by a linear growth from 0-12 h, and a quasi-steady flow is established by 24 h. Mean hurricane sounding Easterly flow of 5 m/s Initial TC Rankine vortex

Track discontinuity Wang (1980): Observation data of 53 typhoons that came close to Taiwan in Continuous Track Discontinuous Track

Track discontinuity --- Idealized modeling study Chang (1982): The TC is weakened but tends to translate at about twice the speed of the basic flow near the mountain. The model TC makes a cyclonic curvature in its path around the northern end of the island mountain.

Track discontinuity --- Idealized modeling study Bender et al. (1987; MWR):  Using a triply-nested movable mesh of GFDL model with finest mesh of 1/6 degree

Track discontinuity --- Idealized modeling study Bender et al. (1987): In the case of Taiwan and 10 m/s easterly zonal flow, secondary surface lows develop behind the CMR.

Track discontinuity --- Idealized modeling study Bender et al. (1987): After landfall, the upper-level TC vortex became detached from the original surface low and eventually coupled with a secondary one. The intensity change of the TC near and over the island was strongly related to the latent hear supply and the vertical coherence of the TC.

Track discontinuity --- Idealized modeling study Yeh and Elsberry (1993a; MWR)  Using the NRL model with 45-km horizontal grid size and 16 vertical levels. For the westward-moving TCs approaching the southern end of Taiwan island, the mean track deflections are larger. For TCs approaching the northern end of Taiwan island, the mean track deflections are smaller. Easterly flow of 5 m/s

Track discontinuity --- Idealized modeling study Yeh and Elsberry (1993a): The track deflections are larger for weaker and slower-moving TCs.

Track discontinuity --- Idealized modeling study Yeh and Elsberry (1993b; MWR) For westward-moving TCs approaching northern Taiwan, TCs tend to track continuously around the northern end of Taiwan due to the deflection of the deep-layer mean flow, and the vortex circulation is modified less.

Track discontinuity --- Idealized modeling study Yeh and Elsberry (1993b) For westward-moving TCs approaching central and southern Taiwan, TC track tend to be discontinuous. More intense and fast-moving TCs are more likely to cross directly over the Taiwan mountain barrier and maintain a continuous track.

Track discontinuity --- Idealized modeling study Lin et al. (1999; JAS) An idealized mesoscale model with 20-km horizontal grid size and 500-m vertical grid side is used to study the orographic influence on a drifting cyclone over an idealized elliptical mountain barrier. For a cyclone propagating from the east and impinging on the central portion of the mountain, a northerly surface jet tends to form upstream of the mountain between the TC and the mountain due to blocking and channeling effects.

Track discontinuity --- Idealized modeling study Lin et al. (1999)

Track discontinuity --- Idealized modeling study Lin et al. (1999)

Track discontinuity --- Idealized modeling study Lin et al. (2002; MWR) : When Vmax/Nh, Vmax/U, and Vmax/Rf are simultaneously large, such as Vmax/Nh > 1.6, Vmax/U > 7.0, and Vmax/Rf > 4.0, then the TC track is continuous. Otherwise, the TC track is discontinuous. Note that Vmax is the TC’s maximum tangential wind, N the Brunt-Väisälä frequency, h the mountain height, U the basic flow speed perpendicular to the mountain, R the TC’s radius of maximum wind, and Ly the north-south scale of the mountain.

Track discontinuity --- Idealized modeling study Lin et al. (2002; MWR) :

Track discontinuity --- Idealized modeling study Lin et al. (2005; JAS) : When the nondimensional parameter of R/Ly are small (large), the TC’s track would be discontinuous (continuous), and the track deflection is large (small). Note that The parameter Vmax/Nh may be regarded as a vortex Froude number of the airstream associated with typhoon’s tangential circulation. The parameter Vmax/U measures the relative strength of the typhoon vortex compared to the basic flow.

Track discontinuity --- Idealized modeling study Lin et al. (2005): With weak orographic blocking, a TC crosses over the mountain range with some northward deflection. With moderate orographic blocking, northward deflection is greater upstream of the mountain range and a secondary leeside vortex forms to the southwest of the mountain range. With strong orographic blocking, a westward-moving TC is deflected southward and a secondary cyclone forms to the northwest of the mountain range. Weak Blocking Moderate Blocking Strong Blocking

Track discontinuity --- real-case modeling study Wu et al. (2002; WAF) “For the Supertyphoon Herb (1996), the existence of the Central Mountain Range only has a minor impact on storm track.”

Track discontinuity --- real-case modeling study Yang et al. (2008; JAS) “Changing the terrain heights produces nonlinear tracks with circular shapes and variable movements associated with different degrees of blocking effects.”

Track discontinuity --- real-case modeling study Jian and Wu (2008; MWR)  Looping track of Typhoon Haitang (2005) before landfall due to the channeling effect.

Track discontinuity --- real-case modeling study Jian and Wu (2008):  Enhancement of Haitang’s southward deflection before landfall due to channeling effect.

Track discontinuity --- real-case modeling study Jian and Wu (2008): Less southward deflection before landfall due to reduced channeling effect in lower terrain heights.

Track discontinuity --- real-case modeling study Jian and Wu (2008): Good correlations between the storm motion and the area-mean asymmetric flow within 100-km radius from the storm center at 700-hPa. Storm motion vector Area-mean asymmetric flow

Summary I The TC is weakened but tends to translate at about twice the speed of the basic flow near the mountain. The model TC makes a cyclonic curvature in its path around the northern end of the island mountain. After landfall, the upper-level TC vortex became detached from the original surface low and eventually coupled with a secondary one. For the westward-moving TCs approaching the southern end of Taiwan island, the mean track deflections are larger.

Summary II For TCs approaching the northern end of Taiwan island, the mean track deflections are smaller. For westward-moving TCs approaching northern Taiwan, TCs tend to track continuously around the northern end of Taiwan due to the deflection of the deep-layer mean flow, and the vortex circulation is modified less. For westward-moving TCs approaching central and southern Taiwan, TC track tend to be discontinuous. More intense and fast-moving TCs are more likely to cross directly over the Taiwan mountain barrier and maintain a continuous track.

Summary III When Vmax/Nh, Vmax/U, and Vmax/Rf are simultaneously large, such as Vmax/Nh > 1.6, Vmax/U > 7.0, and Vmax/Rf > 4.0, then the TC track is continuous. Otherwise, the TC track is discontinuous. When the nondimensional parameter of R/Ly are small (large), the TC’s track would be discontinuous (continuous), and the track deflection is large (small). With weak orographic blocking, a TC crosses over the mountain range with some northward deflection. With moderate orographic blocking, northward deflection is greater upstream of the mountain range and a secondary leeside vortex forms to the southwest of the mountain range. With strong orographic blocking, a westward-moving TC is deflected southward and a secondary cyclone forms to the northwest of the mountain range.

Summary IV Changing the terrain heights produces nonlinear tracks with circular shapes and variable movements associated with different degrees of blocking effects. A looping TC track before landfall may occur due to the blocking and channeling effects. Less TC southward deflection before landfall is found by the reduced channeling effect in lower terrain heights. Good correlations are obtained between the storm motion and the area-mean asymmetric flow within 100- km radius from the storm center at 700-hPa.

Impact on storm intensity

Impact on storm intensity --- Idealized modeling study Bender et al. (1985; MWR) A triply-nested movable-mesh GFDL model is used to study the effects of a mountain range on a landfalling TC embedded in an easterly flow of 10 m/s. Easterly flow of 10 m/s

Impact on storm intensity --- Idealized modeling study Bender et al. (1985; MWR)  Weakening of storm intensity after landfall

Impact on storm intensity --- real-case modeling study Yang et al. (2008): The impact of terrain height on TC’s intensity is nearly linear, with stronger storm intensity in lower-terrain runs Landfall

Summary When a TC makes landfall, the storm’s intensity becomes weaker because of the increased surface friction and reduced moisture supply. The impact of terrain height on TC’s intensity is nearly linear, with stronger storm intensity in lower-terrain runs.

Impact on surface features

Foehn Wu et al. (2002; WAF)  Extremely dry and warm air on the lee side due to adiabatic warming during Typhoon Herb (1996) F

Secondary low Wang (1980): Surface pressure and flow pattern associated with typhoons (continuous and discontinuous tracks) Continuous TC tracks Discontinuous TC tracks

Secondary low Yang et al. (2008): Weaker secondary low on the lee side in a lower-terrain run Full-terrain 50%-terrain 9/16/12Z9/16/18Z Secondary Low Secondary Low Trough Weaker Trough

Summary After TC’s landfall on a mountainous island, Foehn phenomenon with extremely dry and warm air may occur on the leeside due to adiabatic warming. A secondary low may also occur on the leeside, and a weaker secondary low can be found if the terrain height is lower.

Impact on rainfall

Terrain resolution vs. Model resolution Wu et al. (2002):  Central Mountain Range plays a key role in increasing total rainfalls of Typhoon Herb (1996) on Taiwan.  The model and terrain resolutions play an equally important role in the heavy precipitation over Mount A-Li. 60km 20km 6.7km 2.2km

Rainfall enhancement by terrain Yang et al. (2008):  More rainfalls are produced in higher-terrain runs

Summary For topographically-enhanced rainfall, The model and terrain resolutions play an equally important role in production of heavy precipitation. More rainfalls are generated in higher-terrain runs.