• Introduction
• Conclusions
• References

FIGURES

• Figure 1
• Figure 2
• Figure 3
• Figure 4
• Figure 5
• Figure 6

More than 90 high resolution (~30 m), United States radiosonde stations have been analyzed for gravity wave activity for the year 1998. The analysis techniques closely follow methods previously used by Allen and Vincent (1995) and Vincent et al. (1997). In agreement with those earlier Australian studies, it is found that lower stratosphere gravity wave energies decrease poleward in the subtropics and extropics; and are stronger in winter than in summer in the subtropics.

New results from our study are that gravity wave energies in the lower stratosphere are stronger in winter than in summer even at low latitudes; are greater in the vicinity of the Rocky Mountains in all seasons, especially in fall and winter, with a smaller enhancement over the Appalachian Mountains; and the seasonal variation of stratospheric gravity wave energies correlates very well with that of the tropospheric jet wind speeds. Some preliminary analysis results are suggestive about source mechanisms. For instance, approximately 50% of the tropospheric wave energy shows upward energy propagation, whereas there is about 75% upward propagation in the lower stratosphere. Preliminary results also show that in the lower stratosphere, gravity wave intrinsic frequencies (scaled by the Coriolis parameter f), vertical wavelengths, and horizontal wavelengths all decrease poleward. It is also interesting that on the average gravity waves propagate eastward for locations south of about 30°N and westward north of about 30°N.

Introduction

Important information on gravity waves and their effects in the troposphere and lower stratosphere can be derived from high resolution radiosonde data (Allen and Vincent, 1995 and Vincent et al., 1997, among others). In this study, more than 90 high resolution (~30 m), United States radiosonde stations, which cover a large latitude range from the tropics to the Arctic (Figure 1), have been analyzed for gravity wave activity for the year 1998. A time-latitude cross-section of zonally averaged gravity wave energy density in the lower stratosphere is shown in Figure 2, followed by the maps of gravity wave energy density over the contiguous United States for four different seasons (Figure 3). Figure 4 shows the fraction of upward gravity wave energy propagation for both the lower stratosphere and troposphere, and Figure 5 shows the correlation between the monthly gravity wave energy density and the zonal wind at 200 hpa. Finally, preliminary results show the latitudinal dependence of several important wave parameters (Figure 6).

Figure 1: Location map of U.S. stations with High Resolution Radiosonde data in 1998. Radiosonde observations were made twice daily at 0000 and 1200 UT at 93 stations over the contiguous United States (green dots), Alaska (blue), Hawaii (pink), Caribbean islands (red), and western tropical Pacific islands (pink). The altitude resolution of the data is about 30m.

Figure 2: Contoured time-latitude cross-section of zonally averaged gravity wave energy density in the lower stratosphere (18-24.9 km) for the year 1998. Contour interval is 1.0 J/kg.

It is evident that gravity wave energy density decreases with latitude from the tropics to the Arctic. The seasonal variation of gravity wave energy density exhibits winter maximum and summer minimum throughout the whole latitude band. It is also worth noting that the seasonal variation of gravity wave energy density is larger in absolute terms in lower latitudes than in higher latitudes, but similar on a percentage basis.

Figure 3: Contoured longitude-latitude maps of gravity wave energy density over the contiguous United States in four different seasons: winter (DJF), spring (MAM), summer (JJA), and fall (SON). Contour interval is 1.4 J/kg.

Note that the gravity wave energies are greater in the vicinity of the Rocky Mountains in all seasons, especially in winter and fall. There is also a suggestion of larger gravity wave energies over the Appalachian Mountains. This indicates that orographic forcing is an important source for gravity waves over these regions.

Figure 4: Upward energy propagation fraction for both lower stratosphere and troposphere. The upper two panels show the one-year average of the fraction for all the stations available while the lower two panels show the histogram of upward energy propagation fraction for Bismarck, North Dakota (46.77°N, 100.75°W) for the year 1998.

It is clear that approximately 50% of the tropospheric wave energy show upward energy propagation, whereas there is about 75% upward propagation in the lower stratosphere, indicating that some waves are generated in the upper troposphere.

Figure 5: Seasonal variation of gravity wave energy density (pink lines) and zonal wind at 200 hpa (blue lines) over U.S. radiosonde stations which cover a large range in latitude and longitude (25.8-46.9°N, 68?101.7°W), together with their linear correlation.

At many sites, the seasonal variation of gravity wave energy in the lower stratosphere correlates very well with the zonal wind variation at 200 hpa.

Figure 6: Latitudinal dependence of one-year averaged gravity wave intrinsic frequency (scaled by the Coriolis parameter f), horizontal propagation direction (in degrees), vertical wavelength (in km), and horizontal wavelength (in km). These preliminary results show that, in the lower stratosphere, gravity wave intrinsic frequencies (scaled by f), vertical wavelengths and horizontal wavelengths all decrease poleward. It is also interesting that in general gravity waves propagate eastward south of about 30°N, and westward north of about 30°N.

Conclusions:

From preliminary analysis of the high resolution U.S. radiosonde data for the year 1998 we find the following:

• In the lower stratosphere, gravity wave energies decrease poleward at all latitudes from the tropics to the Arctic regions and are stronger in winter than in summer.

• Gravity wave energies in the lower stratosphere are greater in the vicinity of the Rocky Mountains in all seasons, especially in fall and winter, with a smaller enhancement over the Appalachian Mountains.

• In the troposphere, approximately 50% of the gravity wave energy shows upward energy propagation while there is about 75% upward propagation in the lower stratosphere.

• The seasonal variation of stratospheric gravity energies correlates very well with tropospheric jet wind speeds.

• In the lower stratosphere, on the average, gravity wave intrinsic frequencies (scaled by the Coriolis parameter f), vertical wavelengths, and horizontal wavelengths all decrease poleward, and gravity waves propagate eastward south of about 30°N, and westward north of 30°N.

References

Allen, S. J., and R. A. Vincent, Gravity wave activity in the lower atmosphere: Seasonal and latitudinal variations, J. Geophys. Res., 100, 1327-1350, 1995.

Vincent, R. A., S. J. Allen, and S. D. Eckermann, Gravity-wave parameters in the lower stratosphere, in Gravity Wave Processes: Their Parameterization in Global Climate Models, edited by K. Hamilton, NATO ASI Ser. I, 50, 7-25, 1997