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1. The Response of the Near-Surface Tropical Cyclone Wind Field to Inland Surface Roughness Length and Soil Moisture Content during and after Landfall

   https://doi.org/10.1175/JAS-D-20-0211.1  

   Hlywiak, James; Nolan, David S. (March 2021, Journal of the Atmospheric Sciences)

   null (Ed.)

   Abstract The sensitivity of the inland wind decay to realistic inland surface roughness lengths and soil moisture contents is evaluated for strong, idealized tropical cyclones (TCs) of category 4 strength making landfall. Results show that the relative sensitivities to roughness and moisture differ throughout the decay process, and are dependent on the strength and size of the vortex. First, within 12 h of landfall, intense winds at the surface decay rapidly in reaction to the sudden change in surface roughness and decreasing enthalpy fluxes. Wind speeds above the boundary layer decay at a slower rate. Differences in soil moisture contents minimally affect intensity during the first 12 h, as the enhancement of latent heat fluxes from high moisture contents is countered by enhanced surface cooling. After TCs decay to tropical storm intensities, weakening slows and the sensitivity of the intensity decay to soil moisture increases. Increased latent heating becomes significant enough to combat surface temperature cooling, resulting in enhanced convection outside of the expanding radius of maximum winds. This supports a slower decay. Additionally, the decay of the radial wind profile by quadrant is highly asymmetric, as the rear and left-of-motion quadrants decay the fastest. Increasing surface roughness accelerates the decay of the strongest winds, while increasing soil moisture slows the decay of the larger TC wind field. Results have implications for inland forecasting of TC winds and understanding the potential for damage. 

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2. Impacts of Coastal Terrain on Warm-Sector Heavy-Rain-Producing MCSs in Southern China

   https://doi.org/10.1175/MWR-D-21-0190.1  

   Zhang, Murong; Rasmussen, Kristen L.; Meng, Zhiyong; Huang, Yipeng (March 2022, Monthly Weather Review)

   Abstract Warm-sector heavy rainfall in southern China refers to the heavy rainfall that occurs within a weakly forced synoptic environment under the influence of monsoonal airflows. It is usually located near the southern coast and is characterized by poor predictability and a close relationship with coastal terrain. This study investigates the impacts of coastal terrain on the initiation, organization, and heavy rainfall potential of MCSs in warm-sector heavy rainfall over southern China using quasi-idealized WRF simulations and terrain-modification experiments. Typical warm-sector heavy rainfall events were selected to produce composite environments that forced the simulations. MCSs in these events all initiated in the early morning and developed into quasi-linear convective systems along the coast with a prominent back-building process. When the small coastal terrain is removed, the maximum 12-h rainfall accumulation decreases by ∼46%. The convection initiation is advanced ∼2 h with the help of orographic lifting associated with flow interaction with the coastal hills in the control experiment. Moreover, the coastal terrain weakens near-surface winds and thus decreases the deep-layer vertical wind shear component perpendicular to the coast and increases the component parallel to the coast; the coastal terrain also concentrates the moisture and instability over the coastal region by weakening the boundary layer jet. These modifications lead to faster upscale growth of convection and eventually a well-organized MCS. The coastal terrain is beneficial for back-building convection and thus persistent rainfall by providing orographic lifting for new cells on the western end of the MCS, and by facilitating a stronger and more stagnant cold pool, which stimulates new cells near its rear edge. 

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3. The Evolution of Asymmetries in the Tropical Cyclone Boundary Layer Wind Field during Landfall

   https://doi.org/10.1175/MWR-D-21-0191.1  

   Hlywiak, James; Nolan, David S. (March 2022, Monthly Weather Review)

   Abstract The evolution of the tropical cyclone boundary layer (TCBL) wind field before landfall is examined in this study. As noted in previous studies, a typical TCBL wind structure over the ocean features a supergradient boundary layer jet to the left of motion and Earth-relative maximum winds to the right. However, the detailed response of the wind field to frictional convergence at the coastline is less well known. Here, idealized numerical simulations reveal an increase in the offshore radial and vertical velocities beginning once the TC is roughly 200 km offshore. This increase in the radial velocity is attributed to the sudden decrease in frictional stress once the highly agradient flow crosses the offshore coastline. Enhanced advection of angular momentum by the secondary circulation forces a strengthening of the supergradient jet near the top of the TCBL. Sensitivity experiments reveal that the coastal roughness discontinuity dominates the friction asymmetry due to motion. Additionally, increasing the inland roughness through increasing the aerodynamic roughness length enhances the observed asymmetries. Last, a brief analysis of in situ surface wind data collected during the landfall of three Gulf of Mexico hurricanes is provided and compared to the idealized simulations. Despite the limited in situ data, the observations generally support the simulations. The results here imply that assumptions about the TCBL wind field based on observations from over horizontally homogeneous surface types—which have been well documented by previous studies—are inappropriate for use near strong frictional heterogeneity. 

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4. Influence of convective processes on weather research and forecasting model precipitation biases over East Asia

   https://doi.org/10.1007/s00382-022-06587-5  

   Sugimoto, Shiori; Xue, Yongkang; Sato, Tomonori; Takahashi, Hiroshi G (April 2024, Climate Dynamics)

   Abstract Dynamical downscaling with a 20 km horizontal resolution was undertaken over East Asia for the period May–August in 1991–2015 using the Weather Research and Forecasting (WRF) model with Grell-3D ensemble cumulus parameterization as a product of the Impact of Initialized Land Temperature and Snowpack on Sub-Seasonal to Seasonal Prediction (LS4P) program. Simulated climatological precipitation biases were investigated over land during June when heavy precipitation occurred. Simulations underestimated precipitation along the Meiyu/Baiu rainband, while overestimating it farther north. Dry and wet biases expanded to south and north of the Yangtze River in China, respectively, marking years with poor precipitation simulations. Model biases in synoptic-scale circulation patterns indicate a weakened clockwise circulation over the western North Pacific in the model due to active convection there, and suppressed northward moisture transport to the Meiyu/Baiu rainband. Moisture convergence was slightly enhanced over central China due to an apparent anticyclonic circulation bias over northern China. In years with large biases, positive feedback between reduced moisture inflow and inactive convection occurred over southern China, while moisture transport to central China intensified on regional scales, with amplification of dry and wet biases over China. The Kain–Fritch scheme was used to test the influence of cumulus parameterization, improving the dry bias over southern China due to the modification of synoptic-scale circulation patterns in the lower troposphere. However, precipitation was further overestimated over central China, with the accuracy of precipitation distribution deteriorating. 

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5. The influence of sea- and land-breeze circulations on the diurnal variability in precipitation over a tropical island

   Lei Zhu1, 2 (November 2017, Atmospheric chemistry and physics)

   Abstract. This study examines the diurnal variation in precipitation over Hainan Island in the South China Sea using gauge observations from 1951 to 2012 and Climate Prediction Center MORPHing technique (CMORPH) satellite estimates from 2006 to 2015, as well as numerical simulations. The simulations are the first to use climatological mean initial and lateral boundary conditions to study the dynamic and thermodynamic processes (and the impacts of land–sea breeze circulations) that control the rainfall distribution and climatology. Precipitation is most significant from April to October and exhibits a strong diurnal cycle resulting from land–sea breeze circulations. More than 60% of the total annual precipitation over the island is attributable to the diurnal cycle with a significant monthly variability. The CMORPH and gauge datasets agree well, except that the CMORPH data underestimate precipitation and have a 1 h peak delay. The diurnal cycle of the rainfall and the related land– sea breeze circulations during May and June were well captured by convection-permitting numerical simulations with the Weather Research and Forecasting (WRF) model, which were initiated from a 10-year average ERA-Interim reanalysis. The simulations have a slight overestimation of rainfall amounts and a 1 h delay in peak rainfall time. The diurnal cycle of precipitation is driven by the occurrence of moist convection around noontime owing to low-level convergence associated with the sea-breeze circulations. The precipitation intensifies rapidly thereafter and peaks in the afternoon with the collisions of sea-breeze fronts from different sides of the island. Cold pools of the convective storms contribute to the inland propagation of the sea breeze. Generally, precipitation dissipates quickly in the evening due to the cooling and stabilization of the lower troposphere and decrease of boundary layer moisture. Interestingly, the rather high island orography is not a dominant factor in the diurnal variation in precipitation over the island. 

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