Tropopause Altitude Research Articles

Analysis of global Terrestrial Gamma Ray Flashes distribution and special focus on AGILE detections over South America

Global distribution of the Terrestrial Gamma-ray Flashes (TGFs) detected by AGILE and RHESSI for the period from March 2009 to July 2012 has been analysed. A fourth TGF production region has been distinguished over the Pacific. It is confirmed that TGF occurrence follows the Intertropical Convergence Zone (ITCZ) seasonal migration and prefers afternoons. TGF/lightning ratio appears to be lower over America than other regions suggesting that meteorological regional differences are important for the TGF production. Diurnal cycle of TGFs peaks in the afternoon suggesting that Convective Available Potential Energy (CAPE) and convection are important for TGF production. Moreover all AGILE passages over South America in the same period have been analysed to find meteorological preferences for TGF occurrence. In each passage the analysis of Cloud Top Altitude (CTA), CAPE, number of strokes, number of storms and coverage area of clouds with temperatures below −70°C (Cloud Top Coverage area, CTC) are computed. On average, AGILE has been exposed to 19,100 strokes between each TGF representing ∼68h of exposure over active storms. High CAPE values, high cloud tops and high stroke occurrence suggest that meteorological conditions conducive to vigorous and electrically active storms are important for TGF production. It is shown that a high number of thunderstorms is preferable for TGF production which may be explained by a greater chance of the presence of a thunderstorm in the best development stage for TGF production. High tropopause altitude seems to be important but not primordial for TGF production.

Characteristics of tropopause parameters as observed with GPS radio occultation

Abstract. Characteristics of the lapse rate tropopause are analyzed globally for tropopause altitude and temperature using global positioning system (GPS) radio occultation (RO) data from late 2001 to the end of 2013. RO profiles feature high vertical resolution and excellent quality in the upper troposphere and lower stratosphere, which are key factors for tropopause determination, including multiple ones. RO data provide measurements globally and allow examination of both temporal and spatial tropopause characteristics based entirely on observational measurements. To investigate latitudinal and longitudinal tropopause characteristics, the mean annual cycle, and inter-annual variability, we use tropopauses from individual profiles as well as their statistical measures for zonal bands and 5° × 10° bins. The latitudinal structure of first tropopauses shows the well-known distribution with high (cold) tropical tropopauses and low (warm) extra-tropical tropopauses. In the transition zones (20 to 40° N/S), individual profiles reveal varying tropopause altitudes from less than 7 km to more than 17 km due to variability in the subtropical tropopause break. In this region, we also find multiple tropopauses throughout the year. Longitudinal variability is strongest at northern hemispheric mid latitudes and in the Asian monsoon region. The mean annual cycle features changes in amplitude and phase, depending on latitude. This is caused by different underlying physical processes (such as the Brewer–Dobson circulation – BDC) and atmospheric dynamics (such as the strong polar vortex in the southern hemispheric winter). Inter-annual anomalies of tropopause parameters show signatures of El Niño–Southern Oscillation (ENSO), the quasi–biennial oscillation (QBO), and the varying strength of the polar vortex, including sudden stratospheric warming (SSW) events. These results are in good agreement with previous studies and underpin the high utility of the entire RO record for investigating latitudinal, longitudinal, and temporal tropopause characteristics globally.

Thunderstorms enhance tropospheric ozone by wrapping and shedding stratospheric air

AbstractA significant source of ozone in the troposphere is transport from the stratosphere. The stratospheric contribution has been estimated mainly using global models that attribute the transport process largely to the global‐scale Brewer‐Dobson circulation and synoptic‐scale dynamics associated with upper tropospheric jet streams. We report observations from research aircraft that reveal additional transport of ozone‐rich stratospheric air downward into the upper troposphere by a leading‐line‐trailing‐stratiform mesoscale convective system with convection overshooting the tropopause altitude. The fine‐scale transport demonstrated by these observations poses a significant challenge to global models that currently do not resolve storm‐scale dynamics. Thus, the upper tropospheric ozone budget simulated by global chemistry‐climate models where large‐scale dynamics and photochemical production from lightning‐produced NO are the controlling factors may require modification.

Differences in deep convective transport characteristics between quasi‐isolated strong convection and mesoscale convective systems using seasonal WRF simulations

AbstractWe utilize the Weather Research and Forecasting (WRF) model with chemistry to simulate mass transport during the 2007 convective season in the U.S. Southern Great Plains at convection‐allowing scale. Resolved storms are classified using an object‐based classification scheme. This scheme uses model‐derived radar reflectivity to classify storm type as quasi‐isolated strong convection (QISC) or mesoscale convective system (MCS). Differences between QISCs and MCSs are investigated by analysis of two transport parameters for each convective object: the level of maximum detrainment (LMD) and the detrainment mass flux. Analysis of the mean LMD showed differences between the two regimes is statistically significantly different in May, as the mean QISC LMD is 440 m higher than the mean MCS LMD in May, and statistically insignificant in July where the mean QISC LMD is only 350 m higher. The detrainment flux per deeply convective object showed statistically significant differences between the two regimes in both May (MCS 4.8 times greater than QISC) and July (MCS 6.8 times greater than QISC). Over the entire study period, MCS storms accounted for 72% of the total mass detrainment, even though QISCs were twice as common as MCSs. However, differences in the detrainment flux per unit area of deep convection showed that QISCs exhibited stronger flux (1.1 times greater) than MCSs in both months. Analysis of tropopause‐relative LMDs showed that QISCs detrained the maximum amount of mass closer to the tropopause altitude than MCSs for both months. However, only in May is the difference statistically significant (430 m closer).

Detection of tropopause altitude using Indian MST radar data and comparison with simultaneous radiosonde observations

The structure and variability of tropical tropopause over Gadanki (13.5°N, 79.2°E) are delineated using data obtained from Indian MST radar operated in the vertical mode as a part of intense Tropical Tropopause Dynamics (TTD) campaigns conducted under the CAWSES India Phase II (Theme 3) program. Radar measurements for 72 h in each month from December 2010 to September 2013 have been considered. The identified tropopause altitude with radar (RTH) is compared with the cold point (CPH) and lapse rate tropopause altitudes (LRH) obtained from simultaneous radiosonde data at three hourly intervals during these campaigns. Most of the time, a very good agreement between the RTH and CPH and/or LRH from radiosonde measurements is observed. The mean difference between RTH and CPH and RTH and LRH is found to be 0.1±1 km and 0.5±1 km, respectively. The smaller differences between RTH and CPH noticed in the present work when compared to other mid- and polar latitudes might be due to the well defined tropopause structure in the tropical latitudes. As the radar provides reliable data on the tropopause, its long-term variability is investigated using the data from 2007 to 2012 available from the MST radar. • Tropopause altitude detected by MST radar agrees well with radiosonde. • Long-term in the tropopause altitude detected by radar is noticed. • This study expected to provide stronger insights on STE processes.

A volcanogenic intensification factor of stratosphere-troposphere exchange

A mechanism of volcanic aerosol transport to the stratosphere after Plinian-type eruptions, when the maximal height of the emission does not exceed the tropopause altitude, is suggested. The NOAA HYSPLIT trajectory model and open global temperature data are used in order to show the role of the volcanic gas-ash clouds in the temperature change of the upper troposphere and lower stratosphere, tropopause destruction, and, as a consequence, intensification of the stratosphere-troposphere exchange. An abnormal increase in surface ozone concentrations is recorded during the passage of volcanic clouds.

Trajectory matching of ozonesondes and MOZAIC measurements in the UTLS – Part 2: Application to the global ozonesonde network

Abstract. Both balloon-borne electrochemical ozonesondes and MOZAIC (measurements of ozone, water vapour, carbon monoxide and nitrogen oxides by in-service Airbus aircraft) provide very valuable data sets for ozone studies in the upper troposphere/lower stratosphere (UTLS). Although MOZAIC's highly accurate UV-photometers are regularly inspected and recalibrated annually, recent analyses cast some doubt on the long-term stability of their ozone analysers. To investigate this further, we perform a 16 yr comparison (1994–2009) of UTLS ozone measurements from balloon-borne ozonesondes and MOZAIC. The analysis uses fully three-dimensional trajectories computed from ERA-Interim (European Centre for Medium-Range Weather Forecasts Re-analysis) wind fields to find matches between the two measurement platforms. Although different sensor types (Brewer-Mast and Electrochemical Concentration Cell ozonesondes) were used, most of the 28 launch sites considered show considerable differences of up to 25% compared to MOZAIC in the mid-1990s, followed by a systematic tendency to smaller differences of around 5–10% in subsequent years. The reason for the difference before 1998 remains unclear, but observations from both sondes and MOZAIC require further examination to be reliable enough for use in robust long-term trend analyses starting before 1998. According to our analysis, ozonesonde measurements at tropopause altitudes appear to be rather insensitive to changing the type of the Electrochemical Concentration Cell ozonesonde, provided the cathode sensing solution strength remains unchanged. Scoresbysund (Greenland) showed systematically 5% higher readings after changing from Science Pump Corporation sondes to ENSCI Corporation sondes, while a 1.0% KI cathode electrolyte was retained.

Complexity signatures for short time scales in the atmosphere above Adventdalen, Svalbard

AbstractAtmospheric parameters from the troposphere above Adventdalen, Svalbard, 78°N, 16°E, are examined for signatures of complexity in their respective stochastic components over time scales from ~1 h to 1 year. Several approaches are used, all of which can estimate values of the generalized Hurst exponent, α, which can in turn be compared with each other and with similar independent characterizations, usually via the classic Hurst exponent, H, obtained from location‐specific and globally averaged time series. For tropopause altitude, the stochastic component exhibits the signature of a persistent fractional Gaussian noise (fGn) with α ≈ 0.75. For surface air temperature, the indications are for fractional Brownian motion (fBm) with α ≈ 1.4. Using recent high time‐resolution data from a single high‐latitude location, this identification of fBm is relevant for short‐term memory as opposed to findings from many other studies addressing possible long‐term memory, which demonstrate fGn with α = H ≈ 0.7. Furthermore, the lack of similarity between the results for surface air temperature and tropopause altitude suggests that different underlying processes are responsible for stochastic variability.

Latitudinal and altitudinal variability of lower atmospheric inertial gravity waves revealed by U.S. radiosonde data

We adopt a broad spectral data analyzing method to derive the continuous altitude variability of inertial gravity wave (GW) parameter properties in the altitude range of 2–25 km from 11 year (1998–2008) radiosonde observations over 92 United States stations locating in the latitude range from 5°N to 75°N. To our knowledge, this is the first time presenting latitudinal and continuous altitudinal variability of lower atmospheric GW parameters. The presented latitudinal distribution of GW parameters indicates that the wave energy in the troposphere and lower stratosphere peaks, respectively, at the middle and lower latitudes; and at lower latitudes, GWs usually have larger ratios of wave intrinsic frequency to Coriolis parameter, smaller intrinsic frequencies, shorter vertical wavelengths, and longer horizontal wavelengths. Our analyses also revealed continuous altitudinal variations of GW parameters, most of which are closely related to those of the background temperature and wind fields, indicating the important role of background atmosphere in excitation and propagation of GWs. Moreover, our results suggested the profound climatological impacts of GWs on background atmosphere. The GW‐induced force tends to decelerate the zonal jet at middle latitudes and produces a negative vertical shear in the northward wind closely above the tropopause altitude. The GW heat flux tends to cool the atmosphere around the tropospheric jet altitude and contributes significantly to the forming of tropospheric inversions at middle latitudes. Additionally, we demonstrated that GW energy densities, momentum, and heat fluxes have evident seasonal variations, especially at middle latitudes.

The radar tropopause at 78°N, 16°E: Characteristics of diurnal variation

A continuous 5 year time series of tropopause altitude made by the SOUSY VHF radar on Svalbard (78°N, 16°E) is examined for diurnal variation. The data have a nominal time resolution of 1 h, allowing for investigation of intraday timescales not normally possible using radiosonde observations. Periodicities identified in tropopause altitude are found to vary with season. Surface air temperature measurements naturally exhibit diurnal variations which depend on season: however, it is demonstrated that, using a simple scenario of constant adiabatic lapse rate in the troposphere, the tropopause altitude variation that might be anticipated from that of the surface air temperature is not in agreement with the radar observation. Of several mechanisms that could be postulated for this discrepancy, it is demonstrated that a likely candidate is intraday top‐down forcing from the stratosphere.

The radar tropopause above Svalbard 2008–2012: Characteristics at various timescales

AbstractDeterminations of the tropopause altitude over Adventdalen, Svalbard (78°N, 16°E) have been assembled for the period 2008–2012. These reveal characteristics at various timescales: interannual, seasonal, and stochastic. First, it is established that the inclusion of considerably more data does not alter earlier findings of good temporal agreement with radiosonde measurements, that the radar tropopause is located at a fairly constant altitude above the meteorological tropopause, and that the seasonal variation lags the surface temperature variation by approximately 1 month, indicating a degree of top‐down control from the stratosphere. Over the longest timescale available, i.e., 5 years, we find an increase in tropopause altitude and identify this with increasing solar flux during the growth phase of solar cycle 24, and contrary to the seasonal variation, there is evidence that this is controlled by troposphere warming. At stochastic timescales, the scaling characteristics of the signal are examined, and in an exploratory investigation, the Hurst exponent is found to be ~0.82, which is in excellent agreement with independent findings for high latitude surface temperature data and therefore supportive of a complexity matching approach to identification of causal relationships between atmospheric metrics.

High resolution VHF radar measurements of tropopause structure and variability at Davis, Antarctica (69° S, 78° E)

Abstract. Two years of Very High Frequency (VHF) radar echo power observations are used to examine the structure and variability of the tropopause at Davis, Antarctica. Co-located radiosonde and ozonesonde launches provide data with which to calculate the lapse-rate and chemical tropopauses. The radar tropopause, defined as the maximum vertical gradient of echo return power, can be used as a definition of the Antarctic tropopause throughout the year under all meteorological conditions. During the extended summer period of December–April (DJFMA) inclusive, radar tropopauses are (0.2 ± 0.4) km lower than radiosonde lapse-rate (i.e. the World Meteorological Organisation – WMO) tropopauses and during the extended winter period of June–October (JJASO) inclusive, the radar tropopauses are (0.8 ± 1.0) km lower. A potential vorticity tropopause is defined as the altitude of the −2 PVU surface (where 1 PVU = 106 m2 s−1 K kg−1). This is (0.3 ± 0.5) km lower than the radar tropopause during DJFMA and (0.5 ± 1.0) km lower during JJASO. The radar, potential vorticity and ozone tropopauses decrease in altitude during increasingly strong cyclonic conditions, in contrast to the radiosonde WMO tropopause which remains nearly constant. During strong JJASO cyclonic conditions, there are large (several km) differences between WMO tropopause altitudes and radar tropopause altitudes. A seasonal cycle in tropopause fold occurrence is observed, with approximately a three-fold increase during JJASO.

Mean structure of the tropical tropopause and its variability over the Indian longitude sector

The mean spatiotemporal variations in tropopause parameters over the tropics (±35°, in latitude) in the Indian monsoon region are examined using the upper air data for an extended period obtained from radiosonde and Radio Occultation measurements. In general, the altitude of cold point tropopause (CPT) is a minimum near the equator and increases with latitude on either side. While CPT over the entire southern tropical latitudes and northern equatorial region is cooler (higher) during boreal winter and warmer (lower) during boreal summer, the annual pattern of CPT-temperature reverses in the northern hemispheric off-equatorial region. The temperature of lapse rate tropopause (LRT) is always negatively correlated with its altitude. While the annual variation of LRT-temperature in tropics is always positively correlated with CPT-temperature, the annual variation of LRT-altitude differs mainly in the off-equatorial regions. While the altitude of the convective tropopause is positively correlated with CPT-altitude over the latitude region 20°S–5°N, they are negatively correlated at the north of 10°N. In general, the tropical tropopause layer (TTL) is very thin (~3 km) near the equator and its thickness increases with latitude on either side of the equator to reach a peak value (of ~6 km) around ±30°. A pronounced decrease in TTL-thickness observed over the northern off-equatorial region during the ASM period can be attributed to the manifestation of very deep convection over the land near the Head Bay-of-Bengal region. The TTL-lapse-rate (γTTL) is large in the equatorial region and decreases with increase in latitude. While γTTL in the northern hemispheric off-equatorial region is low during winter, it increases and becomes comparable to that over equatorial region during the ASM period. The annual variations in CPT parameters as well as the TTL- thickness are significantly modulated by quasi-biennial oscillation and the El Nino Southern Oscillation.

Long-term validation of tropospheric column-averaged CH<sub>4</sub> mole fractions obtained by mid-infrared ground-based FTIR spectrometry

Abstract. At the Izaña Atmospheric Research Center, high-resolution mid-infrared solar absorption spectra have been recorded for more than 12 yr using Fourier Transform InfraRed (FTIR) spectrometers. We use the spectral fitting algorithm PROFFIT to retrieve long-term time series of methane (CH4) from the measured spectra. We investigate the total column-averaged dry air mole fractions of methane (totXCH4) obtained from a profile scaling and a profile retrieval, and apply two approaches for deriving the tropospheric column-averaged dry air mole fractions: firstly, we use the FTIR hydrogen fluoride (HF) total column amounts as an estimator for the stratospheric CH4 contribution and a posteriori correct the totXCH4 data of a profile scaling retrieval accordingly (troXCH4post); secondly, we directly determine the tropospheric column-averaged dry air mole fractions of methane (troXCH4retr) from retrieved CH4 profiles. Our theoretical estimation indicates that the scaling retrieval leads to totXCH4 amounts that are subject to a large smoothing error, which can be widely avoided by applying a profile retrieval (for the latter we estimate an overall precision of 0.41%). We compare the different FTIR CH4 data to Izaña's Global Atmospheric Watch (GAW) surface in-situ CH4 data (CH4GAW), which in the case of the Izaña Atmospheric Research Center high mountain observatory are very representative for the free tropospheric CH4 amounts. Concerning totXCH4, the agreement between the FTIR data product and the in-situ measurement is rather poor documenting that totXCH4 is not a valid free tropospheric CH4 proxy, as it is significantly affected by the varying stratospheric CH4 contribution and it rather follows the variation in the tropopause altitude. The a posteriori correction method as applied here only removes a part of this stratospheric CH4 contribution. In contrast the profile retrieval allows for a direct estimation of the tropospheric column-averaged CH4 amounts. Results of the profile retrieval analysis correlate well with the CH4GAW data (correlation coefficient of 0.60, FTIR-GAW scatter of 0.97%), and both data sets show very similar annual cycles and trend behaviour for the 2001–2010 time period. Furthermore, we find a very good absolute agreement between the troXCH4retr and CH4GAW (mid-infrared FTIR/GAW scaling factor of 0.9987) suggesting that mid-infrared FTIR data can be well combined with the surface in-situ GAW data. Our study strongly supports the value of mid-infrared ground-based FTIR CH4 profile retrievals as well as the robustness of the approach for achieving total and tropospheric column-averaged XCH4 data of high quality.

Tropopause height at 78° N 16° E: average seasonal variation 2007–2010

Abstract. We present a seasonal climatology of tropopause altitude for 78° N 16° E derived from observations 2007–2010 by the SOUSY VHF radar on Svalbard. The spring minimum occurs one month later than that of surface air temperature and instead coincides with the maximum in ozone column density. This confirms similar studies based on radiosonde measurements in the arctic and demonstrates downward control by the stratosphere. If one is to exploit the potential of tropopause height as a metric for climate change at high latitude and elsewhere, it is imperative to observe and understand the processes which establish the tropopause – an understanding to which this study contributes.

Radio occultation bending angle anomalies during tropical cyclones

Abstract. The tropical deep convection affects the radiation balance of the atmosphere changing the water vapor mixing ratio and the temperature of the upper troposphere lower stratosphere. The aim of this work is to better understand these processes and to investigate if severe storms leave a significant signature in radio occultation profiles in the tropical tropopause layer. Using tropical cyclone best track database and data from different GPS radio occultation missions (COSMIC, GRACE, CHAMP, SACC and GPSMET), we selected 1194 profiles in a time window of 3 h and a space window of 300 km from the eye of the cyclone. We show that the bending angle anomaly of a GPS radio occultation signal is typically larger than the climatology in the upper troposphere and lower stratosphere and that a double tropopause during deep convection can easily be detected using this technique. Comparisons with co-located radiosondes, climatology of tropopause altitudes and GOES analyses are also shown to support the hypothesis that the bending angle anomaly can be used as an indicator of convective towers. The results are discussed in connection to the GPS radio occultation receiver which will be part of the Atomic Clock Ensemble in Space (ACES) payload on the International Space Station.

Sub-daily variations observed in Tropical Easterly Jet (TEJ) streams

To investigate the sub-daily variation in Tropical Easterly Jet (TEJ), a few special experiments have been conducted with mesosphere–stratosphere–troposphere (MST) radar located at Gadanki(13.5°N, 79.2°E) under Study on Atmospheric Forcing and Responses (SAFAR) campaign during 2008–2009. Large sub-daily changes in the TEJ characteristics are observed within a day in addition to day-to-day changes. Based on this observational study, three possible mechanisms for the sub-daily variations are proposed i.e., (1) motion of TEJ core, (2) large updrafts and downdrafts, and (3) strong wave activity. Interestingly, TEJ peak altitude is seen above the Cold Point Tropopause altitude for about 42% of the days.

Terrestrial gamma ray flashes correlated to storm phase and tropopause height

We compare the seasonal and geographical occurrence of terrestrial gamma ray flashes (TGFs) with global lightning maps and find that only part of the difference can be explained by differences in tropopause altitude. The altitude hypothesis suggests either that TGFs are produced only in conjunction with the highest lightning or that only the highest events are seen from space because of the easier escape of gamma rays from the atmosphere. We find that the differences in atmospheric transmission due to seasonal and geographical differences in tropopause height play a major but not dominant role in reconciling lightning and TGF maps and that other factors are needed to explain the remaining local differences. In a second analysis, we use radio atmospherics data from the World Wide Lightning Location Network to study at what time in the evolution of a storm TGFs tend to be seen. We find that, on average, TGFs lag the peak flash rate of the associated storm by 38 min, although the range of lags is extremely wide, including some cases where the TGF leads the peak flash rate.

First results of the refurbished SOUSY radar: Tropopause altitude climatology at 78°N, 16°E, 2008

The second generation SOUSY MST radar at 78°N, 16°E on Svalbard has recently completed its inaugural year of combined troposphere and mesosphere observations. Here, troposphere observations have been processed using a robust detection algorithm designed for obtaining tropopause climatology by automatic data processing, and the resulting monthly statistics have been compared with corresponding surface air temperatures. As our main objective, we describe the new radar system, present the tropopause detection method, and validate the results using radiosonde and surface temperature data. The tropopause height depends on the temperature of the underlying atmosphere but is also influenced by downward control from the stratosphere. We find that the climatological tropopause height is correlated with the surface temperature but with the former lagging the latter by approximately 1 month.

A robust method for tropopause altitude identification using GPS radio occultation data

A robust method to determine the tropopause altitude directly from GPS Radio Occultation (RO) measurements of bending angle is presented. An objective covariance transform method is applied to identify transitions in a bending angle profile. The tropopause is identified by the maximum in the convolution of the natural logarithm of an observed bending angle profile with a gradient window function. Identification of the tropopause from bending angles is of particular value since they are directly derived from climate benchmark observations. This method avoids additional RO data processing and assumptions to derive parameters such as dry temperature, and use of subjective tropopause identification criteria. The RO tropopause altitude is compared with lapse rate and cold point criteria using dry temperatures and radiosonde temperature profiles. A longer‐term tropopause altitude analysis for May to November 2008 using the RO bending angle method shows good agreement with tropopause altitudes computed from dry temperature parameters.