Melvyn E. Gelman

Detection of long-term trends in global stratospheric temperature from NMC analyses derived from NOAA satellite data

Abstract Since 24 September 1978 global daily fields of temperature and geopotential height at 8 stratospheric pressure levels 70 to 0.4 mb (18–55 km) have been produced at the U.S. National Meteorological Center. Temperature profiles derived from NOAA operational satellites constitute the sole data source for the upper stratospheric levels 5, 2, 1, and 0.4 mb (35, 42, 48 and 55 km). Significant changes in upper stratosphere reported temperatures have accompanied each of the eight changes in either operational satellite or method of data processing. Comparisons with rocketsonde data from 1978 to 1986 show bias changes of 1 to 5 Celsius degrees at various levels. For detecting long term trends of ambient stratospheric temperature, adjustments based on rocket comparisons must be applied to the NMC fields. Lack of data at north polar latitudes and in the southern hemisphere limits comprehensive characterization of temperature uncertainty. We discuss in detail our ability to characterize temperature uncertainty of the NMC stratospheric analyses. We specifically discuss our ability to detect a trend in the middle stratosphere temperature of about 1.5 celsius degrees per decade, the amount of change indicated likely by current theoretical models.

Comparison of U.K. Meteorological Office and U.S. National Meteorological Center stratospheric analyses during northern and southern winter

Meteorological data from the United Kingdom Meteorological Office (UKMO), produced using a data assimilation system, and the U.S. National Meteorological Center (NMC), produced using an objective analysis procedure, are compared for dynamically active periods during the Arctic and Antarctic winters of 1992. The differences seen during these periods are generally similar to those seen during other winter periods. Both UKMO and NMC analyses capture the large-scale evolution of the stratospheric circulation during northern hemisphere (NH) and southern hemisphere (SH) winters. Stronger vertical and horizontal temperature gradients develop in the UKMO than in the NMC data during stratospheric warmings; comparison with satellite measurements with better vertical resolution suggests that the stronger vertical temperature gradients are more realistic. The NH polar vortex is slightly stronger in the UKMO analyses than in the NMC in the middle and upper stratosphere, and midstratospheric temperatures are slightly lower. The SH polar vortex as represented in the UKMO analyses is stronger and colder in the midstratosphere than its representation in the NMC analyses. The UKMO analyses on occasion exhibit some difficulties in representing cross-polar flow or changes in curvature of the wind field at very high latitudes. In addition to the above study of two wintertime periods, a more detailed comparison of lower-stratospheric temperatures is done for all Arctic and Antarctic winter periods since the launch of the Upper Atmosphere Research Satellite. In the NH lower stratosphere during winter, NMC temperatures are consistently lower than UKMO temperatures and closer to radiosonde temperatures than are UKMO temperatures. Conversely, in the SH lower stratosphere during winter, UKMO temperatures are typically lower than NMC and are closer to radiosonde temperature observations.

Interannual variability of the North Polar Vortex in the lower stratosphere during the UARS Mission

Northern winters since the 1991 launch of UARS are compared to earlier years (1978–1991) with respect to the potential for formation of Polar Stratospheric Clouds and for isolation of the north polar vortex. Daily NMC temperature minima at 465 K late in the 1993–94 winter and again in December 1994 were the lowest values experienced at those times of year (since 1978). Northern PV gradients were unusually strong in 1991–92 prior to late January and throughout the winter in both 1992–93 and 1994–95. Of all northern winters since 1978, 1994–95 with its early extended cold spell and persistently strong PV gradients most resembled the Antarctic winter lower stratosphere. Even so, temperatures were never as low, nor was the polar vortex as large, as during a typical southern winter. Judged by daily temperature minima and PV gradients at 465 K, meteorological conditions in the Arctic winter lower stratosphere during the UARS period were more conducive to vortex ozone loss by heterogeneous chemistry than in most previous winters since 1978–79.

Evaluation of NMC Upper-Stratospheric Temperature Analyses Using Rocketsonde and Lidar Data

Daily NMC analyses, constructed from operational TOVS data since 1978, are used to monitor behavior of middle atmospheric temperature. Capability of the upper-stratospheric analyses (5,2,1, and 0.4 mb) to provide temporally consistent temperature fields depends on adjustments derived from ground-truth observations. These adjustments compensate for biases in the analyses caused by behavioral differences in data derived from successive operational satellite instruments and by changes in data and analysis procedures. This paper supports previous studies showing that observations from the datasonde rocket system provide ground-truth adjustments with a precision of 1°–3°C. The number of datasonde observations has diminished substantially in recent years, putting this adjustment system at risk. Falling-sphere rocket temperature data are shown to have variability in excess of that judged to be acceptable for use in the adjustment system. The capability for Rayleigh lidar to provide high-quality temperature data ...

Troposphere-Stratosphere (Surface-55 km) Monthly Winter General Circulation Statistics for the Northern Hemisphere-Four Year Averages

Abstract Monthly mean Northern Hemisphere general circulation statistics are presented for the four-year average December, January and February months of the winters 1978–79 through 1981–82. These calculations start with daily maps for eighteen pressure levels between 1000 and 0.4 mb of Northern Hemisphere temperature at 1200 GMT that are supplied by NOAA/NMC. Geopotential height and geostrophic wind are constructed using the hydrostatic and geostrophic relationships, respectively. Fields presented in this paper are zonally averaged temperature, mean zonal wind, and amplitude and phase of planetary waves with zonal wave-numbers 1-3. Diagnostic quantities, such as the northward fluxes of heat and eastward momentum by standing and transient eddies along with their wavenumber decomposition and Eliassen-Palm flux propagation vectors and divergences by the standing and transient eddies along with their wavenumber decomposition, are also given. The observations indicate that polar temperatures in the lower stra...

The anomalous Arctic lower stratospheric polar vortex of 1992-1993

Potential vorticity (PV) gradients defining the lower stratospheric vortex during the 1992–1993 winter were anomalously strong and persistent compared to those during the last 16 Arctic winters. For ≈3 months PV gradients were closer to typical Antarctic values than to most Arctic values. Air motion diagnostics computed for 3-dimensional air parcel trajectories confirm that the 1992–1993 Arctic lower stratospheric vortex was substantially more isolated than is typical. Such isolation will delay and reduce the export of the higher ozone typical of the winter lower stratospheric vortex to mid-latitudes. This may have contributed to the record-low total ozone amounts observed in northern mid-latitudes in 1993.

An inter-hemisphere comparison of the persistent stratospheric polar vortex

Based on 19 years (1979–1998) of NCEP/NCAR reanalyses data and potential vorticity (PV) area diagnostics, we found that in the southern hemisphere (SH) the polar vortex has lasted about two weeks longer in the 1990s than in the early 1980s and the northern hemisphere (NH) polar vortex has lasted four weeks longer. The SH vortex persisted within the layer (12–22 km) with almost complete ozone loss, but did not persist at higher altitudes where ozone was not depleted. However, the NH vortex persisted in a broader vertical range not limited to the ozone-depletion layer. We show that wave activity has weakened in recent years in the NH, but not in the SH. The springtime Antarctic ozone hole seems to be the main cause for the SH polar vortex persistence, while the cause for the NH vortex persistence involves changes in polar ozone as well as changes in dynamics.

Evaluation of NMC upper-stratospheric temperature analyses using rocketsonde and lidar data. [NMC (National Meteorological Center)]

Daily NMC analyses, constructed from operational TOVS data since 1978, are used to monitor behavior of middle atmospheric temperature. Capability of the upper-stratospheric analyses (5,2,1, and 0.4 mb) to provide temporally consistent temperature fields depends on adjustments derived from ground-truth observations. These adjustments compensate for biases in the analyses caused by behavioral differences in data derived from successive operational satellite instruments and by changes in data and analysis procedures. This paper supports previous studies showing that observations from the datasonde rocket system provide ground-truth adjustments with a precision of 1°–3°C. The number of datasonde observations has diminished substantially in recent years, putting this adjustment system at risk. Falling-sphere rocket temperature data are shown to have variability in excess of that judged to be acceptable for use in the adjustment system. The capability for Rayleigh lidar to provide high-quality temperature data ...

Semidiurnal and diurnal temperature tides (30–55 km): Climatology and effect on UARS-LIDAR data comparisons

Very good agreement is shown for diurnal and semidiurnal temperature variations calculated from lidar measurements in southern France and from data of the microwave limb sounder of the Upper Atmosphere Research Satellite (UARS). Tides induce temperature deviations observed in southern France to be as large as +3 K, with a maximum at the stratopause. The amplitudes and phases of the semidiurnal variation change significantly with season and location. Seasonal changes up to 2 K have been clearly identified for the diurnal component. An analytic model of the diurnal component, based on sinusoidal functions, fits the data well, but is less successful for the semidiurnal component. Substantial agreement is also reported for the diurnal component between the results of our analytical model and the published results of a two- dimensional global-scale wave model. In contrast, the semidiurnal component is in total disagreement with numerical simulations that report very small amplitudes, as compared with the observations reported here. The confidence in detecting bias in data comparisons is improved if data used are limited to periods from April to September and if time-of-day adjustments are applied. Comparison between lidar and nearly coincident UARS temperature measurements have revealed, systematically, for the 4 experiments aboard UARS, a significant residual mean difference of up to 3 K around 35-43 km. A comparison using simultaneous measurements suggests that the bias is associated with the variability of migrating tides and/or the presence of nonmigrating tides rather than instrumental characteristics.

Investigations on long‐term temperature changes in the upper stratosphere using lidar data and NCEP analyses

OHP lidar data and National Centers for Environmental Prediction (NCEP) stratospheric temperature analyses provide long and continuous databases for the middle and upper stratosphere that are highly valuable for long-term studies. However, each data set has limitations. Comparisons between lidar data from 1979 to 1993 and NCEP data interpolated from the global analyses to the lidar location reveal significant mean temperature differences. Insight into the origin of the differences offers an opportunity to improve the overall quality of temperature monitoring in the stratosphere. Some of the differences can be explained by instrumental effects in the lidar system. In the stratosphere most of the limitations in lidar temperatures appear below 35-40 km, due to events of lidar misalignment (as large as 10 K) or to the effects on lidar data of volcanic aerosols (as large as 15 K). Changing biases between lidar and NCEP temperatures above 5 hPa coincide with replacement of satellites used in the NCEP analyses. However, some bias differences in upper stratospheric temperatures remain even after NCEP adjustments are made, based on rocketsonde comparisons. While these biases have been already suspected, they had never been explained. Here we suggest that the remaining bias (2-4 K) is caused by tidal influences, heretofore not accounted for by the NCEP adjustment procedure. Lidar profiles have been filtered in their lower part for misalignment and aerosol contamination. Long-term changes have been compared, and a factor of 2 in trend differences have been reported. No significant trends (at 95% confidence) have been detected except with lidar around the stratopause and with NCEP analyses at 5 and 10 hPa. According to instrumental limitations of both data sets the temperature trend may vary from 1 to 3 K with altitude (10-0.4 hPa). Because only satellite data can provide global trend estimates and because lidar data have been chosen for ground-based stratospheric monitoring programs, we suggest some plans to overcome these difficulties for past and future measurements. This should allow a more confident use for future trend estimates from both data sets.