Tropical and Extra-Tropical Influences on the Distribution of Free Tropospheric Humidity Over the Intertropical Belt

Roca, Rémy; Guzman, Rodrigo; Lemond, Julien; Meijer, Joke; Picon, Laurence; Brogniez, Hélène

doi:10.1007/s10712-011-9169-4

Cited by 17 publications

(25 citation statements)

References 50 publications

(67 reference statements)

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“…Clear-sky OLR from CERES-EBAF data is derived only from measurements over clear-sky atmospheric columns which are generally drier than the clear part of a cloudy atmospheric CERES column. Because a drier atmospheric column leads to a stronger OLR (e.g., Spencer and Braswell, 1997;Dessler et al, 2008;Roca et al, 2012), OLR clear from CERES-EBAF should overestimate OLR clear from C3M on average. The diurnal cycle, which is taken into account in OLR clear from CERES-EBAF but not in OLR clear from C3M (since we only used nighttime observations) could also play a role in the difference.…”

Section: Gridded Olrmentioning

confidence: 99%

The link between outgoing longwave radiation and the altitude at which a spaceborne lidar beam is fully attenuated

et al. 2017

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Abstract. According to climate model simulations, the changing altitude of middle and high clouds is the dominant contributor to the positive global mean longwave cloud feedback. Nevertheless, the mechanisms of this longwave cloud altitude feedback and its magnitude have not yet been verified by observations. Accurate, stable, and long-term observations of a metric-characterizing cloud vertical distribution that are related to the longwave cloud radiative effect are needed to achieve a better understanding of the mechanism of longwave cloud altitude feedback. This study shows that the direct measurement of the altitude of atmospheric lidar opacity is a good candidate for the necessary observational metric. The opacity altitude is the level at which a spaceborne lidar beam is fully attenuated when probing an opaque cloud. By combining this altitude with the direct lidar measurement of the cloud-top altitude, we derive the effective radiative temperature of opaque clouds which linearly drives (as we will show) the outgoing longwave radiation. We find that, for an opaque cloud, a cloud temperature change of 1 K modifies its cloud radiative effect by 2 W m −2 . Similarly, the longwave cloud radiative effect of optically thin clouds can be derived from their top and base altitudes and an estimate of their emissivity. We show with radiative transfer simulations that these relationships hold true at single atmospheric column scale, on the scale of the Clouds and the Earth's Radiant Energy System (CERES) instantaneous footprint, and at monthly mean 2 • × 2 • scale. Opaque clouds cover 35 % of the ice-free ocean and contribute to 73 % of the global mean cloud radiative effect. Thin-cloud coverage is 36 % and contributes 27 % of the global mean cloud radiative effect. The link between outgoing longwave radiation and the altitude at which a spaceborne lidar beam is fully attenuated provides a simple formulation of the cloud radiative effect in the longwave domain and so helps us to understand the longwave cloud altitude feedback mechanism.

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Section: Gridded Olrmentioning

confidence: 99%

The link between outgoing longwave radiation and the altitude at which a spaceborne lidar beam is fully attenuated

et al. 2017

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“…Following Roca et al (2011) large-scale dynamics have a strong impact on the FTH distribution and its variability. Brogniez et al (2009) analyzed the FTH from MVIRI over northeastern Africa over the period 1983-2004 for the months of July/August and separated the analysis into dry and wet years.…”

Section: Variabilitymentioning

confidence: 99%

“…However, the outgoing longwave radiation (OLR) is much more sensitive to perturbations at the dry end than at the moist end of the distribution (Spencer and Braswell, 1997;Roca et al, 2011). The magnitude of the differential impact of a given change at the dry end of the RH distribution compared to at the moist end of it can exceed a factor of 3 (Roca et al, 2011).…”

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confidence: 99%

“…Most of the knowledge on the impact of climate change on RH in the free troposphere arises from theoretical and global average considerations (Pierrehumbert et al, 2007;Sherwood et al, 2010b;Shi and Bates, 2011;Roca et al, 2011, and references therein). On a global scale, the assumption of a constant upper-tropospheric RH under climate change conditions is supported by the work of Soden et al (2002) and Soden and Held (2006), among others.…”

mentioning

confidence: 99%

“…On the zonal mean scale, a common pattern emerges for a doubling of CO 2 : subtropical dry regions shift poleward (Sherwood et al, 2010a), dry minima become drier (Hurley and Galewsky, 2010) and the width of these dry regions increases. Such impacts on the frequency of occurrence of the dry end of the FTH range (below 10 %, noted FTHp10 in the following) have been simulated with the Institut Pierre Simon Laplace climate model based on the analysis of the FTH distribution under a climate change scenario (Roca et al, 2011). On a regional scale, it is challenging to assess whether any of these large-scale features are reproduced and what is the expected change in their pattern if they are not reproduced.…”

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Climatology of free-tropospheric humidity: extension into the SEVIRI era, evaluation and exemplary analysis

et al. 2014

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Abstract. A new free-tropospheric humidity (FTH) data record is presented. It is based on observations from the Meteosat Visible and Infrared Imager (MVIRI) onboardMeteosat-2-Meteosat-5, as well as Meteosat-7, and the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard Meteosat-8 and Meteosat-9 at the water absorption band near 6.3 µm. The data set is available under clear-sky and low-level cloud conditions. With the extension to SE-VIRI observations, the data record covers the period 1983-2009 with a spatial resolution of 0.625 • × 0.625 • and a temporal resolution of 3 h.The FTH is the mean relative humidity (RH) in a broad layer in the free troposphere. The relation between the observed brightness temperature (BT) and the FTH is well established. Previous retrievals are refined by taking into account the relative humidity Jacobians in the training process of the statistical retrieval. The temporal coverage is extended into the SEVIRI period, the homogenization of the BT record is improved, and the full archive is reprocessed using updated regression coefficients.The FTH estimated from the Meteosat observations is compared to the FTH computed from the RH profiles of the Analyzed RadioSoundings Archive (ARSA). An average relative bias of −3.2 % and a relative root-mean-square difference (RMSD) of 16.8 % are observed. This relative RMSD agrees with the outcome of an analysis of the total uncertainty of the FTH product. The decadal stability of the FTH data record is 0.5 ± 0.45 % per decade.As exemplary applications, the interannual standard deviation, the differences on decadal scales, and the linear trend in the FTH data record and in the frequency of occurrence of FTH < 10 % (FTHp10) are analyzed per season. Interannual standard deviation maxima and maxima in absolute decadal differences are featured in gradient areas between dry and wet regions, as well as in areas where FTH reaches minima and FTHp10 reaches maxima. An analysis of the FTH linear trends and of the associated uncertainty estimates is achieved to identify possible problems with the data record. Positive trends in FTHp10 are featured in gradient areas between wet and dry regions, in regions where the FTH is minimum, in regions where FTHp10 is maximum, and in regions where differences between FTHp10 averaged over the 2000s and 1990s are negative. However, these positive trends in FTHp10 are associated with maximum standard deviation and are thus hardly significant. This analysis and intercomparisons with other humidity data records are part of the Global Energy and Water Cycle Experiment (GEWEX) Water Vapor Assessment (G-VAP).

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Investigating the impact of the water‐vapour sounding observations from SAPHIR on board Megha‐Tropiques for the ARPEGE global model

Chambon

Meunier

Guillaume

et al. 2014

Quart J Royal Meteoro Soc

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Data assimilation experiments of radiances from the microwave sounder SAPHIR on board the Megha-Tropiques (MT) satellite were conducted using the global numerical weather prediction (NWP) model ARPEGE (Action de Recherche Petite Echelle Grande Echelle) operational at Météo-France for the period June to August 2012. The combination of the MT's low-inclined orbit with the SAPHIR's six channels was found to increase the number of assimilated humidity-sensitive microwave observations in the Tropics by a factor of 3.8, compared with a control experiment using three existing Advanced Microwave Sounding Unit B and Microwave Humidity Sounder instruments. Impacts of SAPHIR on both analyses through the 4D-Var system and forecasts are found to be positive with respect to radiosonde data and analyses by the European Centre for Medium-range Weather Forecasts up to 12 h and neutral to positive up to 72 h. In particular, the improvement of innovation statistics for other humidity-sensitive satellite observations when SAPHIR is assimilated indicates a positive synergy of the MT satellite with the present observing system. In addition to providing new humidity observations in the Tropics, the SAPHIR data can also be used to define new diagnostics for NWP models. In particular: (i) with the MT's orbit being non-heliosynchronous, all local times are observed by the same sensor for a given area, which makes diurnal cycle diagnostics straightforward to set up; (ii) with SAPHIR providing more information on the vertical (six channels) than similar sensors, it can be easily used to study water-vapour vertical structures in NWP models. The first diagnostic applied to the ARPEGE model reveals a good consistency between observed and modelled diurnal cycles in analyses over western Africa, but also reveals a 2-4 h time lag between mean observed and equivalent model brightness temperatures. The second diagnostic highlights a well-represented vertical correlation of simulated brightness temperatures from the ARPEGE model with respect to SAPHIR over western Africa, but a too-low vertical correlation over drier regions such as the southern Pacific Ocean during the

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Tropical and Extra-Tropical Influences on the Distribution of Free Tropospheric Humidity Over the Intertropical Belt

Cited by 17 publications

References 50 publications

The link between outgoing longwave radiation and the altitude at which a spaceborne lidar beam is fully attenuated

The link between outgoing longwave radiation and the altitude at which a spaceborne lidar beam is fully attenuated

Climatology of free-tropospheric humidity: extension into the SEVIRI era, evaluation and exemplary analysis

Investigating the impact of the water‐vapour sounding observations from SAPHIR on board Megha‐Tropiques for the ARPEGE global model

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