During the last two decades (1979–2002), there has been an ever-increasing frequency of summer severe-rainfall events over Ontario, Canada. This observed upward trend is robust as demonstrated through the Mann–Kendall test with consideration of removing a lag-1 autoregressive process. It is shown through composite analyses using the NCEP reanalysis data that in the presence of warming conditions the summer severe-rainfall events occur more frequently over Ontario, especially under atmospheric conditions with stronger low-level cyclonic circulations and more precipitable water. Further analyses indicate that over north and central Ontario the summer severe-rainfall frequency is linked with a positive trend of precipitable water whereas over central and south Ontario there is a strong interannual response of summer severe-rainfall frequency to the changes in precipitable water through the variations of air temperature. Abstract During the last two decades (1979–2002), there has been an ever-increasing frequency of summer severe-rainfall events over Ontario, Canada. This observed upward trend is robust as demonstrated through the Mann–Kendall test with consideration of removing a lag-1 autoregressive process. It is shown through composite analyses using the NCEP reanalysis data that in the presence of warming conditions the summer severe-rainfall events occur more frequently over Ontario, especially under atmospheric conditions with stronger low-level cyclonic circulations and more precipitable water. Further analyses indicate that over north and central Ontario the summer severe-rainfall frequency is linked with a positive trend of precipitable water whereas over central and south Ontario there is a strong interannual response of summer severe-rainfall frequency to the changes in precipitable water through the variations of air temperature.

The effects of different moisture distributions on the generation of moist potential vorticity (MPV) in extratropical cyclones are examined by numerical simulations. These sensitivity experiments show that low-level negative MPV generation depends on the initial distribution of potential temperature and moisture gradients. The generation of negative MPV depends on the baroclinic and moisture gradient vectors. It can also depend on the locations of condensation. Although condensation has no direct effect on MPV production, MPV can be generated in the neighborhood of condensation where the gradients of potential temperature and moisture have significantly changed. It is found in the sensitivity experiments that the bent-back warm front is the most favored regions for negative MPV generation because strong deformation of moisture and potential temperature fields occur. After the cyclones mature, negative MPV moves into the warm core. It can be transported upward through the enhanced vertical motion ... Abstract The effects of different moisture distributions on the generation of moist potential vorticity (MPV) in extratropical cyclones are examined by numerical simulations. These sensitivity experiments show that low-level negative MPV generation depends on the initial distribution of potential temperature and moisture gradients. The generation of negative MPV depends on the baroclinic and moisture gradient vectors. It can also depend on the locations of condensation. Although condensation has no direct effect on MPV production, MPV can be generated in the neighborhood of condensation where the gradients of potential temperature and moisture have significantly changed. It is found in the sensitivity experiments that the bent-back warm front is the most favored regions for negative MPV generation because strong deformation of moisture and potential temperature fields occur. After the cyclones mature, negative MPV moves into the warm core. It can be transported upward through the enhanced vertical motion ...

Even with ever-increasing societal interest in tornado activities engendering catastrophes of loss of life and property damage, the long-term change in the geographic location and environment of tornado activity centers over the last six decades (1954–2018), and its relationship with climate warming in the U.S., is still unknown or not robustly proved scientifically. Utilizing discriminant analysis, we show a statistically significant geographic shift of U.S. tornado activity center (i.e., Tornado Alley) under warming conditions, and we identify five major areas of tornado activity in the new Tornado Alley that were not identified previously. By contrasting warm versus cold years, we demonstrate that the shift of relative warm centers is coupled with the shifts in low pressure and tornado activity centers. The warm and moist air carried by low-level flow from the Gulf of Mexico combined with upward motion acts to fuel convection over the tornado activity centers. Employing composite analyses using high resolution reanalysis data, we further demonstrate that high tornado activities in the U.S. are associated with stronger cyclonic circulation and baroclinicity than low tornado activities, and the high tornado activities are coupled with stronger low-level wind shear, stronger upward motion, and higher convective available potential energy (CAPE) than low tornado activities. The composite differences between high-event and low-event years of tornado activity are identified for the first time in terms of wind shear, upward motion, CAPE, cyclonic circulation and baroclinicity, although some of these environmental variables favorable for tornado development have been discussed in previous studies.

A variational method is developed to retrieve the von Kármán constant κ from the CASES-97 dataset, collected near Wichita, Kansas, the United States from 6 April to 24 May 1997. In the variational method, a cost function is defined to measure the difference between observed and computed gradients of wind speed, air temperature and specific humidity. An optimal estimated von Kármán constant is obtained by minimizing the cost function through adjusting values of the von Kármán constant. Under neutral stratification, the variational analysis confirms the conventional value of κ (=0.40). For non-neutral stratification, however, κ varies with stability. The computational results show that the κ decreases monotonously from stable to unstable stratification. The variational calculated mean value of the von Kármán constant is 0.383~0.390 when the atmospheric stratification is taken into consideration. Relations between κ and surface momentum and heat flux are also examined.

An upward trend in Ontario tornado frequency (about 1.6 tornadoes/decade with the statistically significant level at least at 95%) is identified using three independent approaches. The first method is the conventional linear regression method that had no disturbance to the original tornado time series. The second approach is to employ the Mann-Kendall test with consideration of removing a lag one autoregressive process. The trend is further firmed up using the Monte Carlo simulation. The robustness of the detected tornado frequency trend presented in this work offers an example for tornado frequency trend analysis over other regions of the world. It is shown in this study that the Ontario tornado frequency is linked to ENSO (El Niño/Southern Oscillation) signals with a statistically significant level at 99%.