Klemens Hocke

Geographical distribution and interseasonal variability of tropical deep convection: UARS MLS observations and analyses

[1]xa0Tropical deep convection and its dynamical effect on the tropopause and stratosphere are investigated using a suite of data from the Upper Atmospheric Research Satellite (UARS) Microwave Limb Sounder (MLS), including upper tropospheric humidity, cloud radiance, and gravity wave measurements. For this purpose, geographical distributions of temperature, water vapor, and cloudiness in the tropical tropopause layer (TTL) are compared with corresponding maps of gravity wave variance in the stratosphere. In addition, ECMWF global wind divergent and velocity potential fields as well as NOAA outgoing longwave radiation and CMAP rainfall data are analyzed to help pinpoint the locations of deep convection. We found that high-altitude clouds near the bottom of TTL (∼147 hPa) are usually surrounded by high-humidity air, and their spatial pattern and seasonal variability are closely associated with regions of vigorous summertime deep convection. Upward propagating gravity waves generated from these convection regions are shifted poleward by prevailing stratospheric winds. We estimate that tropical deep convection lifts ∼5% of the cloud tops to altitudes above 100 hPa and that most of the extreme deep convection events occur in the Western Pacific and Indian monsoon regions. Low-temperature regions in the TTL are associated with, but often drift away from, the center of deep convection. Regions of water vapor maxima near the bottom of TTL are located directly above the deep convection centers, but this moisture behavior is somewhat reversed at the top of the TTL. The integrated picture derived from this study implies that convective scale motions could be important in affecting short-term dehydration processes in the TTL. Our results also suggest that the spatial organization and temporal development of tropical convective systems will be better monitored with the follow-on Earth Observing System (EOS) Aura satellite instruments and lead to improved understanding of the complex interaction of tropical convection with large-scale dynamic and thermodynamic conditions.

The gravity wave-TID relationship: insight via theoretical model-EISCAT data comparison

Abstract Atmospheric Gravity Waves (AGWs) in the thermosphere are of particular interest because of their role in the equatorward redistribution of auroral momentum and energy input. However, their direct measurement is difficult and so they are normally traced by their ionospheric signatures, the Traveling Ionospheric Disturbances (TIDs). These can be routinely observed, especially with incoherent scatter radars like the EISCAT-facility, which measure all the fundamental ionospheric parameters. In order to reliably infer AGW parameters from TID data, however, one needs to know the physics of the AGW-TID relationship as comprehensively as possible. We investigated this relationship by means of one-to-one comparison of theoretical model results with EISCAT data for several TID events. The relevant physics, the modeling procedure and the results of the comparisons are discussed. As a representative example, one typical event is presented in some detail. We found that the AGW-TID relationship can be quantitatively understood by means of careful physical modeling. A particular simulated TID shows quantitative consistency with a particular TID in EISCAT data only for a quite specific model-AGW ; thus, comprehensive AGW infonnation can be deduced by our method. We conclude that our use of TID ‘polarization information’ along a single incoherent scatter beam is basically as valuable for the unique determination of a causative AGW as is traditional TID ‘propagation/dispersion information’. The latter, however, requires several distributed stations. Finally, we address the possibility that radars like EISCAT could be used in future WAGS (Worldwide AGW Study) campaigns to provide almost real-time information on AGW activity for the benefit of mid-latitude monitoring stations.

Phases and amplitudes of TIDs in the high latitude F-region observed by EISCAT

Abstract Travelling Ionospheric Disturbances (TIDs) are the ionospheric response to gravity waves passing through the neutral atmosphere. They can be measured simultaneously as wavelike perturbations of the ionospheric electron density ( N e ), field-aligned ion velocity ( V i ), and ion and electron temperature ( T i and T e ) by the EISCAT radar at Tromso. In order to derive the average amplitude and phase relationships between N e , V i T i and T e we have selected 45 TIDs with periods in the range 30–150 min from EISCAT data of a quiet daytime F-region from November 1987 to December 1991. It is shown that there exist characteristic phase differences between the TID parameters which can be used to identify gravity waves in incoherent scatter data. Furthermore, we present height profiles of the average phases and amplitudes of the various TID parameters. Finally, the spectral behaviour of the TID amplitudes is studied using the average power spectra of N e , V i , T i and T e . We conclude that the use of the incoherent scatter information of all TID parameters allows a sure identification of gravity waves in the thermosphere.

Gravity waves determined by modeling of traveling ionospheric disturbances in incoherent‐scatter radar measurements

With the advent of the era of thermosphere/ionosphere weather modeling there is an increasing need to routinely acquire information on important regional-scale phenomena in the upper atmosphere like atmospheric gravity waves (AGWs) in the thermosphere and their ionospheric signatures, the traveling ionospheric disturbances (TIDs). AGW information is especially desirable in view of its role in the momentum and energy balance of the upper atmosphere. Measuring AGWs directly is difficult, however, so they are usually traced by the TIDs, which can be routinely observed by the powerful incoherent-scatter technique even in all fundamental ionospheric parameters. Thus an important problem, and the central concern of this study, is how to infer, as comprehensively as possible, AGW information from measured TIDs. This problem was tackled up to now by direct inversion of TID parameters into AGW parameters, but severe limitations, which we shall briefly discuss, are inherent in this approach. Contrasting with this, we introduce a novel approach to AGW retrieval: realistic modeling of AGW/TID physics, constrained by TIDs measured by the incoherent-scatter technique. This method is found to be capable of yielding comprehensive AGW information, that is, the full polarization and dispersion information. We applied the method to four TID events, which cover different seasons and solar activities during quiet magnetic conditions, from European EISCAT radar data. One-to-one comparisons of modeling results with experimental data are presented and discussed. These show the feasibility and scope of the method and its potential to discriminate between TIDs from AGWs and non-AGW sources. In view of the satisfactory results, we finally address the potential of the method for routine processing of incoherent-scatter data with respect to AGWs, which could provide valuable AGW information for other research.

Cross‐validation of MIPAS/ENVISAT and GPS‐RO/CHAMP temperature profiles

[1]xa0The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board the ENVISAT and the Global Positioning System (GPS) receiver on the Challenging Mini-Satellite Payload (CHAMP) provide temperature profiles by limb-viewing midinfrared emission and radio occultation (RO) measurements, respectively. The MIPAS temperatures retrieved at the Institut fur Meteorologie und Klimaforschung (IMK) are compared with the GPS-RO/CHAMP observations derived at Jet Propulsion Laboratory (JPL) and GeoForschungsZentrum (GFZ) Potsdam. The three data sets show generally good agreement. The global mean differences averaged between 8 and 30 km in 14 days of September/October 2002 are −0.44 ± 0.02 K and 0.07 ± 0.02 K for MIPAS/GPS-RO JPL and GFZ comparisons, respectively. The MIPAS global mean temperatures below 25 km are slightly lower than those of GPS-RO JPL and GFZ by less than 1 K and 0.2 K, respectively. Above 25 km, the MIPAS temperatures are higher than the JPL and GFZ data, in particular near both poles and the equator, with maxima of 1 K for JPL and 1.5 K for GFZ at 30 km. The standard deviations are ∼2–4 K. Possible explanations for the observed differences include (1) effect of spatial and temporal mismatch between the correlative measurements on the observed standard deviations, in particular in regions and episodes of enhanced wave activity; (2) a negative bias in GPS-RO/CHAMP temperatures in regions of increased humidity; (3) a mapping of initialization temperature profiles on GPS-RO/CHAMP retrievals at altitudes where low refraction contains no information on air density; and (4) measurement errors of both instruments, particularly the errors due to insufficient knowledge of the instrument line shape and spectroscopy in current MIPAS retrievals.