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

Abstract

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

A review of sources of systematic errors and uncertainties in observations and simulations at 183 GHz

“…For instance, ATMS offers a large suite of channels that can complement each other to separate clear and cloudy scenes (Bormann et al, 2013), while SAPHIR provides only channels in the 183.31 GHz line, giving fewer possibilities for cloud detection (Chambon et al, 2015). Several filtering techniques using only 183 GHz channels exist, relying on the strong reduction in BT induced by scattering and/or absorption, which can reach up to 20 K in regions of intense convection (Greenwald and Christopher, 2002;Hong et al, 2005;Buehler et al, 2007).…”

Investigating the potential benefit to a mesoscale NWP model of a microwave sounder on board a geostationary satellite

“…The six MWGEO channels are all rejected when the CIWP exceeds a given predefined threshold. Rejecting the six channels at once mimics the cloud screening of SAPHIR data within the ARPEGE 4D‐Var assimilation system (Chambon et al ., ) which rejects all channels depending on the ice scattering signature in SAPHIR channel 6. The 25 infrared channels from the IRS instrument are significantly affected by both liquid and solid hydrometeors so the selected proxy is an integrated cloud water + cloud ice water path (CIWP + CLWP thereafter CLCIP). This quantity is computed over three nested atmospheric layers: from model top to 300 hPa (CLCIP1), from model top to 600 hPa (CLCIP2) and from model top to the ground (CLCIP3).…”

Benefits of assimilating SAPHIR observations on analysis and forecasts of tropical fields in the Met Office global model

“…Examples of weighting functions for (a) MHS, (b) SAPHIR, and (c) ATMS for a tropical atmospheric profile at 29°N, 71°W on 1 July 1993 at 0000 UTC from the Chevalier () dataset, showing the increased vertical coverage provided by SAPHIR over the other instruments. (d) and (e) show, respectively, the temperature and specific humidity profile for the point used…”

“…Chambon et al . () investigated the impact of SAPHIR radiances in the global numerical weather prediction (NWP) model operational at Météo‐France, Action des Recherche Petit Echelle et Grand Echelle (ARPÈGE); they found that the combination of the MT low inclination orbit with SAPHIR's six channels increased the number of assimilated humidity‐sensitive microwave observations in the Tropics by a factor of 3.8, compared to the control experiment in which three AMSU‐B/MHS microwave sounders were assimilated. Chambon et al .…”

“…Chambon et al . () demonstrated benefit in assimilating SAPHIR data both on analyses and on forecasts, with relative humidity RMSE reduced by ∼10% between 400 and 150 hPa for 12 h forecasts.…”