Numerical Study on Spatio-Temporal Distribution of Cold Front-Induced Waves along the Southeastern Coast of China

Cold fronts, as one of the most frequent extreme weather events, can induce significant waves on the sea. This work analyzes the spatial and temporal variations in cold front events, especially the characteristics of wind directions during cold fronts in the East China Sea (ECS). The SWAN (Simulating Waves Nearshore) model is applied to simulating the waves induced by cold fronts. To calibrate the model, two typical cold front events were selected to simulate the corresponding waves in the ECS. The results indicate that the data misfit between the observed and modeled significant wave heights (SWH) is within a reasonable range. Idealized sensitivity experiments were then designed in order to analyze and discuss the responses of ocean waves to wind direction, swell distribution, maximum of significant wave heights (MSWH), and time lag during the cold fronts. The results show that the average MSWH in the ECS decreases monotonically with the deflection of wind direction from north-east to north-west, while specific nearshore sites do not conform to this pattern due to topography. The time series of SWH indicate that the action of the swells leads to a prolongation of the duration of catastrophic waves. This work investigates the temporal and spatial distribution characteristics of cold front-induced wind wave fields in offshore Zhejiang, which has important value for the study of the impact of cold fronts on the ocean as well as disaster prevention and mitigation efforts.

Comparing Frontal Structures of Extratropical Cyclones in the Northwestern Pacific and Northwestern Atlantic Storm Tracks

The frontal structures of extratropical cyclones developing in the Northwestern Pacific storm track are relatively poorly understood compared with those in Europe and the Atlantic Ocean, for which representative conceptual models have been developed. In this paper, the structures of cyclones and their associated fronts in the Northwestern Pacific (NP), as well as in the Okhotsk Sea and Sea of Japan (OJ), are examined at their developing and mature stages using Japanese 55-year reanalysis dataset. Furthermore, the frontal structures in the NP are compared with those in the Northwestern Atlantic (NA). At the time of maximum deepening rate, cyclones in the NP are accompanied by strong warm and cold fronts, whereas cyclones in the OJ are more frequently accompanied by cold fronts than by warm fronts and tend to have stronger cold fronts than warm fronts. The weaker warm fronts than cold fronts to the east and northeast of cyclones in the OJ is likely due to the cyclones developing to the north and away from the region where the horizontal gradient of environmental potential temperature is strong. A comparison between mature cyclones in the NP and NA shows that the warm fronts in the NA tend to extend northeastward, whereas those in the NP extend more southeastward. These differences in warm fronts between NP and NA are suggested to be due to the difference in the horizontal structures of the warm currents between NP and NA.

On the relationship between CYGNSS surface heat fluxes and the lifecycle of low-latitude ocean extratropical cyclones

Surface latent and sensible heat fluxes are important for extratropical cyclone evolution and intensification. Because extratropical cyclone genesis often occurs at low-latitude, CYGNSS surface latent and sensible heat flux retrievals are composited to provide a mean picture of their spatial distribution in low-latitude oceanic extratropical cyclones. CYGNSS heat fluxes are not affected by heavy precipitation and offer observations of storms with frequent revisit times. Consistent with prior results obtained for cyclones in the Gulf Stream region, the fluxes are strongest in the wake of the cold fronts, and weakest to negative in the warm sector in advance of the cold fronts. As cyclone strength increases, or mean precipitable water decreases, the maximum in surface heat fluxes increases while the minimum decreases. This impacts the changes in fluxes during cyclone intensification: the post-cold frontal surface heat flux maximum increases due to the increase in near surface winds. During cyclone dissipation, the fluxes in this sector decrease, due to the decrease in winds and in temperature and humidity contrast. The warm sector minimum decreases throughout the entire cyclone lifetime and is mostly driven by sea-air temperature and humidity contrast changes. However, during cyclone dissipation, the surface heat fluxes increase along the cold front in a narrow band to the east, independently from changes in the cyclone characteristics. This suggests that, during cyclone dissipation, energy transfers from the ocean to the atmosphere are linked to frontal in addition to synoptic-scale processes.

Modeling of the Number of Cold Fronts in Cuba Using the Objective Regressive Regression (ROR) Methodology; Impact of Sunspots

The objective of this work is to model the variable number of cold fronts that affect the Cuban territory in a winter season for a long series of data, to establish if the trend is significant and to see which are the main statistics of the model, to observe the impact of prediction using the number of sunspots with the help of Objective Regressive ROR modeling. In this work, the series of cold fronts per season that affect the Cuban territory was modeled in the years from the 1916-1917 seasons to the 2006-2007 seasons. There are more moderate cold fronts than any other front, on average there are more classic fronts than any other type, on average 19 fronts can be presented per season with a standard deviation of 4.8 Sunspots and they only have a significant linear correlation with sunspots. In moderate fronts, as the stains increase, the number of fronts decreases. The ROR model explains 98% of the variance with an error of 4.2 cases and depends on the fronts returned in 5 seasons, which could coincide with the ENSO event, and also depends on the number of sunspots returned in 12 years. From 1916-1917 approximately the 1952-1953 season, moderate fronts predominated, later from 1953-1954 to the end of the data, weak fronts predominate over the rest with some exceptions throughout history. No significant trend was observed in the model. It is concluded that forecasts of the number of cold fronts can be made with the variable number of sunspots.

The interaction between cold fronts and convection in the mid-latitudes

<p>Cold fronts provide an environment favourable for convective initiation in the mid-latitudes. Some studies also show the presence of a cold front can increase the chance of certain convective hazards, such as hail and heavy rain. Convection initiates in three locations in respect to cold fronts: <em>ahead</em> of the cold front in the warm sector of the cyclone, directly <em>at</em> the cold frontal boundary and also <em>behind</em> the cold front. Previous literature has typically focused on each initiation location independently, thus a comprehensive study investigating the link between cold fronts and convection is currently lacking from literature. This study seeks to better understand the climatology, scale interactions and forcing mechanisms of convection at each initiation location relative to the front (i.e., behind, at, ahead).</p><p>Automatic front detection methods are applied to reanalysis data and a convective cell-tracking dataset from the German Weather Service is used to build a climatology of cold fronts and convection between April–September. Convective cells are found to initiate most commonly 200–300km ahead of the cold front during late afternoon. Cells behind the front primarily initiate in north-western Germany and exhibit a strong diurnal cycle. On the contrary, cells at and ahead of the front initiate most frequently in southern Germany and exhibit a less prominent diurnal cycle, especially for cells at the frontal boundary. Lightning probability decreases with closing proximity to the cold front and the average number of cell initiations per day is significantly higher on days with cold fronts opposed to days without. The next stages of research will investigate the relative importance of various forcing mechanisms on the development of convective cells at different cell-front positions.</p>

A 20-year analysis of disturbance-driven rainfall on O’ahu, Hawai’i

Undisturbed trade-wind conditions comprise the most prevalent synoptic weather pattern in Hawai’i and produces a distinct pattern of orographic rainfall. Significant total rainfall contributions and extreme events are linked to four types of atmospheric disturbances: cold fronts, Kona lows, upper-tropospheric disturbances, and tropical cyclones. In this study, a 20- year (1990-2010) categorical disturbance time series is compiled and analyzed in relation to daily rainfall over the same period. The primary objective of this research is to determine how disturbances contribute to total wet season rainfall on the Island of O’ahu, Hawai’i. On average, 41% of wet seasonal rainfall occurs on disturbance days. Seventeen percent of seasonal rainfall can be directly attributed to disturbances (after a background signal is removed) and as much as 48% in a single season. The intensity of disturbance rainfall (mm/day) is a stronger predictor (r2 = 0.49; p < 0.001) of the total seasonal rainfall than the frequency of occurrence (r2 = 0.11; p = 0.153). Cold fronts are the most common disturbance type; however, the rainfall associated with fronts that cross the island is significantly higher than rainfall produced from non-crossing fronts. In fact, non-crossing fronts produce significantly less rainfall than under mean non-disturbance conditions 76% of the time. While the combined influence of atmospheric disturbances can account for almost half of the rainfall received during the wet season, the primary factor in determining a relatively wet or dry season/year on O’ahu are the frequency and rainfall intensity of Kona Low events.