Hélène Brogniez

Evaluation of the distribution of subtropical free tropospheric humidity in AMIP‐2 simulations using METEOSAT water vapor channel data

In the framework of the Atmospheric Model nIntercomparison Project (AMIP) phase 2, we have nestablished a diagnostic of the free tropospheric humidity n(FTH) distribution using METEOSAT data over the 1984– n1995 period for 14 climate models. The methodology of nevaluation follows a two step ‘‘model-to-satellite’’ approach. nFirst the raw METEOSAT ‘‘Water Vapor’’ radiances are nsimulated from the model profiles of temperature and nhumidity using the RTTOV-7 radiative transfer model. nSecond, the radiances are converted into FTH using the nsame coefficients as in the satellite product offering a direct ncomparison. The analysis is focused on the dry subtropical nareas observed by METEOSAT: the Eastern Mediterranean nand the tropical South Atlantic Ocean. Most of the models nreproduce the observed seasonal cycle both in terms of nphasing and magnitude, despite an overall moist bias. A few nmodels are in close agreement with the satellite data. The nmagnitude of the satellite estimated inter-annual variability is nalso generally captured by models. Again, a small subset nof models shows close agreement with the observations. nThis comparison suggests general improvements of the nmodels with respect to the AMIP-1 simulations.

The Megha-Tropiques mission: a review after three years in orbit

The Megha-Tropiques mission is operating a suite of payloads dedicated to the documentation of the water and energy cycles in the intertropical region in a low inclination orbit. The satellite was launched in October, 2011 and we here review the scientific activity after the first three years of the mission. The microwave sounder (SAPHIR) and the broad band radiometer (SCARAB) are functioning nominally and exhibit instrumental performances well within the original specifications. The microwave imager, MADRAS, stopped acquisition of scientific data on January 26th, 2013 due to a mechanical failure. During its 16 months of operation, this radiometer experienced electrical issues making its usage difficult and delayed its validation. A suite of geophysical products has been retrieved from the Megha-Tropiques payloads, ranging from TOA radiative flux to water vapor profiles and instantaneous rain rates. Some of these geophysical products have been merged with geostationary data to provide, for instance, daily accumulation of rainfall all over the intertropical region. These products compare favorably with references from ground based or space-borne observation systems. The contribution of the mission unique orbit to its scientific objectives is investigated. Preliminary studies indicate a positive impact on both, humidity Numerical Weather Prediction forecasts thanks to the assimilation of SAPHIR Level 1 data, and on the rainfall estimation derived from the Global Precipitation Mission constellation. After a long commissioning phase, most of the data and the geophysical products suite are validated and readily available for further scientific investigation by the international community.

Isentropic modeling of a cirrus cloud event observed in the midlatitude upper troposphere and lower stratosphere

[1] This publication provides a detailed study of one cirrus cloud observed by lidar at the Observatory of Haute-Provence (~44°N) in January 2006 in the vicinity of the tropopause (12-14 km/~136-190 hPa/328-355 K). The higher part of the air mass observed comes from the wet subtropics while the lower part comes from the midlatitudes. Both are advected by the Azores anticyclone, encounter cold temperatures (~205 K) above the North Atlantic Ocean, and flow eastward along the anticyclonic flank of the polar jet stream. A simulation of this cloud by an isentropic model is tested to see if synoptic-scale atmospheric structures could explain by itself the presence of such clouds. The developments made in the Modelisation Isentrope du transport Meso-echelle de lOzone Stratospherique par Advection (MIMOSA) model to take into account the three phases of water and their interactions allow reproduction of the occurrence of the cirrus and its temporal evolution. MIMOSA-H 2 O reproduces the atmospheric water vapor structures observed with Atmospheric Infrared Sounder (AIRS) with, however, an apparent wet bias of around 50%. Reliable water vapor fields appear to be the main condition to correctly simulate such cirrus clouds. The model reproduces the cirrus cloud altitude for fall speeds around 1 cm/s and gives ice water content around 3-4 mg/m 3 . Fall speed is also a critical parameter, and a better parameterization with altitude or other atmospheric conditions in the modeling of such cirrus clouds is required. This study also shows that supersaturation threshold impacts strongly the vertical and horizontal extension of the cirrus cloud but more slightly the ice water path.

Is There Really a Closure Gap Between 183.31-GHz Satellite Passive Microwave and In Situ Radiosonde Water Vapor Measurements?

We present a new closure study between radiosonde and microwave satellite humidity measurements. The radiosonde data are from the Global Climate Observing System Reference Upper-Air Network. The satellite data are from the radiometers: MHS, Advanced Technology Microwave Sounder, and Sondeur Atmosphérique du Profil d’Humidité Intertropicale par Radiométrie. Like previous studies, we find the satellite data to be “colder” than simulated radiosonde data. But the mean bias value (0.4 K) is smaller than previously reported and, according to our analysis, not significant. The error budget suggests an uncertainty of 0.52–1.06 K. We also show that the improvement in closure can be attributed to improvements in the intercomparison methodology.