Mark A. Olsen

Role of stratospheric air in a severe weather event: Analysis of potential vorticity and total ozone

The role of dry stratospheric air descending to low and middle tropospheric levels in a severe weather outbreak in the midwestern United States is examined using NCEP Eta model output, Rapid Update Cycle (RUC) analyses, and Earth probe Total Ozone Mapping Spectrometer (EP/TOMS) total ozone data. While stratospheric air was not found to play a direct role in the convection, backward trajectories show stratospheric air descended to 800 hPa just west of the convection. Damaging surface winds not associated with thunderstorms also occurred in the region of greatest stratospheric descent. Small-scale features in the high-resolution total ozone data compare favorably with geopotential heights and potential vorticity fields, supporting the notion that stratospheric air descended to near the surface. A detailed vertical structure in the potential vorticity appears to be captured by small-scale total ozone variations. The capability of the total ozone to identify mesoscale features assists model verification. The total ozone data suggest biases in the RUC analysis and Eta forecast of this event. The total ozone is also useful in determining whether potential vorticity is of stratospheric origin or is diabatically generated in the troposphere.

Fine-scale comparison of TOMS total ozone data with model analysis of an intense Midwestern cyclone

High-resolution (-40 km) along-track total column ozone data from the Total Ozone Mapping Spectrometer (TOMS) instrument are compared with a high-resolution mesoscale numerical model analysis of an intense cyclone in the Midwestern United States. Total ozone increased by -100 DU (nearly 38%) as the TOMS instrument passed over the associated tropopause fold region. Complex structure is seen in the meteorological fields and compares well with the total ozone observations. Ozone data support the meteorological analysis showing that stratospheric descent was confined to levels above -600 hPa; significant positive potential vorticity at lower levels is attributable to diabatic processes. Likewise, meteorological fields show that two pronounced ozone streamers extending north and northeastward into Canada at high levels are not bands of stratospheric air feeding into the cyclone; one is a channel of exhaust downstream from the system, and the other apparently previously connected the main cyclonic circulation to a southward intrusion of polar stratospheric air and advected eastward as the cut-off cyclone evolved. Good agreement between small-scale features in the model output and total ozone data underscores the latters potential usefulness in diagnosing upper- tropospheric/lower-stratospheric dynamics and kinematics. This paper compares high-resolution satellite-derived total column ozone observations with a high-resolution mesoscale model analysis near the time of lowest central pressure of an intense storm system over the central United States. Significant consistency of the analysis and ozone fields at fine-scales will be demonstrated. Before the comparisons are made, we review several concepts central to the investigation, potential vorticity (PV) and total column ozone. In the 1940s and 1950s, PV was shown to be a promising diagnostic tool in understanding midlatitude cyclogenesis and tropospheric dynamics (e.g., Rossby, 1940; Kleinschmidt, 1951; Reed, 1955). Ertel (1942) showed that PV evaluated on isen- tropic surfaces is conserved in the absence of diabatic or fric- tional effects. More recently, Haynes and Mcintyre (1987) showed that changes in the magnitude of PV within a layer bounded by isentropic surfaces can only come about through flow across isentropes. Ertels (1942) potential vorticity in isentropic coordinates is 00

Evidence of stratosphere‐to‐troposphere transport within a mesoscale model and Total Ozone Mapping Spectrometer total ozone

We present evidence for stratospheric mass transport into, and remaining in, the troposphere in an intense midlatitude cyclone. Mesoscale forecast model analysis fields from the Mesoscale Analysis and Prediction System were compared with total ozone observations from the Total Ozone Measurement Spectrometer. Coupled with parcel back trajectory calculations, the analyses suggest that two mechanisms contributed to the mass exchange: (1) A region of dynamically induced exchange occurred on the cyclones southern edge. Parcels originally in the stratosphere crossed the jet core and experienced dilution by turbulent mixing with tropospheric air. (2) Diabatic effects reduced parcel potential vorticity (PV) for trajectories traversing precipitation regions, resulting in a “PV hole” signature in the cyclone center. Air with values of ozone and water vapor typical of the lower stratosphere was left in the troposphere. The strength of the latter process may be atypical. These results, combined with other research, suggest that precipitation-induced diabatic effects can significantly modify (either decreasing or increasing) parcel potential vorticity, depending on parcel trajectory configuration with respect to jet core and maximum heating regions. The diabatic heating effect on stratosphere-troposphere exchange (STE) is more important to tropopause erosion than to altering parcel trajectories. In addition, these results underscore the importance of using not only PV but also chemical constituents for diagnoses of STE.