[1] This study examines the seasonal variation of the double tropopause (DT) using data from the High Resolution Dynamics Limb Sounder. The combination of high data density and high vertical resolution available with this satellite instrument allows for the detailed analysis of fine-scale structures such as the DT. The spatial distribution of DT frequency is examined for all seasons from 2005 to 2007. Global analysis of this distribution reveals that DTs have a strong tendency to follow the zonal wind pattern and are present all year over the Andes. Moreover, during Northern Hemisphere winter/spring, there is a 15% decrease in the DT frequency over the eastern Pacific and western Atlantic, two regions of wave breaking produced from a weakening of the zonal flow. Significant DT thickness values are also present over these regions, but are found to extend further poleward than the corresponding frequency pattern. A time series of DT frequencies is analyzed and shows an increase in daily frequencies during 2006 that highlights the interannual variability of this thermal structure. For the first time, DT duration is investigated in the extratropics utilizing a Hovmöller diagram of DT frequency. This representation highlights two preferred regions of formation, the Pacific and Atlantic. The slope of the feature gives it a speed of 18 m/s in the Northern Hemisphere and a slightly faster speed for the Southern Hemisphere. These speeds and the corresponding structures highlight a potential connection between upper tropospheric waves and DTs.

Using High Resolution Dynamic Limb Sounder observations and ERA-Interim reanalysis this study demonstrates that the warm conveyor belt (WCB) is a mechanism responsible for the relationship between the double tropopause (DT) and the tropopause inversion layer (TIL), a relationship recently suggested in the literature based on idealized model simulations of baroclinic disturbances. Using these data sets, spatial and temporal characteristics of the DT-TIL relationship are examined over a 3 year period, [2005][2006][2007][2008]. In the extratropics, results from satellite data show that as the TIL increases in strength, so does the frequency of the DT, regardless of season or hemisphere. The inverse relationship is found in the tropics. Using only DT profiles, zonal composites of wind, relative vorticity, and temperature from reanalysis data show that as the TIL increases in strength, the upper tropospheric circulation switches from cyclonic to anticyclonic, and the upward vertical motion increases. This result suggests the WCB as a mechanism since it is on the anticyclonic side of the jet and is characterized by the movement of tropical air poleward and upward from the surface. To verify this relationship, the vertical and horizontal development of a synoptic-scale baroclinic system is analyzed over a 4 day period. Results show the equatorward extension of the polar tropopause, and thus the formation of the DT, due to the strengthening of the TIL in the region of vertical motion associated with the WCB. Moreover, this result suggests that air movement within the DT could originate from high latitudes when associated with a baroclinic disturbance.

Severe weather events can have a significant impact on local communities because of the loss of life and property. Forecast busts associated with high-impact weather events have been attributed to initial condition errors over data-sparse regions, such as the Pacific Ocean. Numerous flight campaigns have found that targeted observations over these areas can improve forecasts. To better understand the impacts of measurement type and sampling domains on forecast performance, observing system simulation experiments are performed using the National Centers for Environmental Prediction Global Forecast System (GFS) with hybrid 3DEnVar data assimilation and the ECMWF T511 nature run. First, three types of simulated perfect dropsonde observations (temperature, specific humidity, and wind) are assimilated into the GFS over a large idealized sampling domain covering the Pacific Ocean. For the three winter storms studied, forecast error was found to be significantly reduced with all three types of measurements providing the most benefit (%–15% reduction in error). Instances when forecasts are not improved are investigated and concluded to be due to challenging meteorological structures, such as cutoff lows and interactions with atmospheric structures outside the sampling domain. Second, simulated dropsondes are assimilated over sensitive areas and flight tracks established using the ensemble transform sensitivity (ETS) technique. For all three winter storms, forecast error is reduced up to 5%, which is less than that found using an idealized domain. These results suggest that targeted observations over the Pacific Ocean may provide a small improvement to winter storm forecasts over the United States.