Jean-Philippe Duvel

An Analysis of Tropical Ocean Diurnal Warm Layers

Abstract During periods of light surface wind, a warm stable layer forms at the ocean surface with a maximum sea surface temperature (SST) in the early afternoon. The diurnal SST amplitude (DSA) associated with these diurnal warm layers (DWLs) can reach several degrees and impact the tropical climate variability. This paper first presents an approach to building a daily time series of the DSA over the tropics between 1979 and 2002. The DSA is computed over 2.5° of latitude–longitude regions using a simple DWL model forced by hourly-interpolated surface radiative and turbulent fluxes given by the 40-yr ECMWF Re-Analysis (ERA-40). One advantage of this approach is the homogeneity of the results given by the relative homogeneity of ERA-40. The approach is validated at the global scale using empirical DWL models reported in the literature and the Surface Velocity Program (SVP) drifters of the Marine Environmental Data Service (MEDS). For the SVP dataset, a new technique is introduced to derive the diurnal var...

Role of non-linear oceanic processes in the response to Westerly Wind Events: new implications for the 1997 El Niño onset

In March 1997, a strong westerly wind event (WWE) occurred in the western equatorial Pacific prior to the 1997-1998 El Nino event. It produced downwelling Kelvin waves that interacted non linearly with the surface temperature, salinity and zonal current fronts located at the eastern edge of the warm-fresh pool (EEWP). This non-linear interaction locally increased zonal currents by a factor of three compared to a theoretical linear response, and advected the EEWP at an unexpected rate (? 1m/s) to which the ocean-atmosphere coupled system may have been responding rapidly to trigger El Nino conditions.

Influence of the vertical structure of the atmosphere on the seasonal variation of precipitable water and greenhouse effect

By using satellite observations and European Centre for Medium Range Weather Forecasts analyses, we study the seasonal variations of the precipitable water and the greenhouse effect, defined as the normalized difference between the longwave flux emitted at the surface and that emergent at the top of the atmosphere. Results show a strong systematic influence of the vertical structure of the atmosphere on geographical and seasonal variations of both precipitable water and greenhouse effect. Over ocean, in middle and high latitudes, the seasonal variation of the mean temperature lapse rate in the troposphere leads to large seasonal phase lags between greenhouse effect and precipitable water. By contrast, the seasonal variation of the clear-sky greenhouse effect over tropical oceans is mainly driven by the total atmospheric transmittance and thus by precipitable water variations. Over land, the seasonal variation of the tropospheric lapse rate acts to amplify the radiative impact of water vapor changes, giving a strong seasonal variation of the greenhouse effect. Over tropical land regions, monsoon activity generates a seasonal phase lag between surface temperature and relative humidity variations that gives a seasonal lag of about 2 months between the surface temperature and the clear-sky greenhouse effect. Generally, the cloudiness amplifies clear-sky tendencies. Finally, as an illustration, obtained results are used to evaluate the general circulation model of the Laboratoire de Meteorologie Dynamique.

Satellite validation of GCM-simulated annual cycle of the earth radiation budget and cloud forcing

Earth Radiation Budget Experiment (ERBE) data are used to validate radiative fluxes and cloud radiative forcing (CRF) simulated by the Laboratoire de Meteorologie Dynamique (LMD) general circulation model (GCM). The emphasis of the work is on the development of new tests to obtain more significant elements of comparison between model simulations and satellite observations. These tests are applied to the clear-sky fluxes and the cloud radiative forcing. The validation of the CRF described by a model requires to test the consistency between the solar or shortwave (SW: 0.2 to 5 μm) and longwave (LW: 5 to 50 μm) cloud forcing. For this purpose, we compute the mean cloud perturbation of the planetary albedo as a function of the LW cloud forcing for both model results and ERBE observations. In the SW spectral domain, the consideration of total fluxes does not provide very constraining elements of validation because most of the observed variations are prescribed (incoming solar radiation, solar zenith angle). We therefore distinguish the part of the SW seasonal variations related only to the variation of external parameters (mainly the insolation) from the part which arises from the combined variation of internal climate parameters (mainly cloud albedo and snow/ice cover) with the insolation. Fourier analysis is used to study the seasonal amplitude and phase of the CRF. The seasonal variation of the cloudiness is, respectively, out of phase (in phase) with the insolation in mid-latitudes (in low and high latitudes). We show that this acts to enhance (to reduce) the seasonal amplitude of the absorbed SW flux in mid-latitudes (in low and high latitudes). Finally, we show that the impact of the seasonal variation of the cloudiness on the variation of the net CRF is less than 10 W m−2.

An Evaluation Metric for Intraseasonal Variability and its Application to CMIP3 Twentieth-Century Simulations

The intraseasonal variability (ISV) is an intermittent phenomenon with variable perturbation patterns. To assess the robustness of the simulated ISV in climate models, it is thus interesting to consider the distribution of perturbation patterns rather than only one average pattern. To inspect this distribution, the authors first introduce a distance that measures the similarity between two patterns. The reproducibility (realism) of the simulated intraseasonal patterns is then defined as the distribution of distances between each pattern and the average simulated (observed) pattern. A good reproducibility is required to analyze the physical source of the simulated disturbances. The realism distribution is required to estimate the proportion of simulated events that have a perturbation pattern similar to observed patterns. The median value of this realism distribution is introduced as an ISV metric. The reproducibility and realism distributions are used to evaluate boreal summer ISV of precipitations over the Indian Ocean for 19 phase 3 of the Coupled Model Intercomparison Project (CMIP3) models. The 19 models are classified in increasing ISV metric order. In agreement with previous studies, the four best ISV metrics are obtained for models having a convective closure totally or partly based on the moisture convergence. Models with high metric values (poorly realistic) tend to give (i) poorly reproducible intraseasonal patterns, (ii) rainfall perturbations poorly organized at large scales, (iii) small day-to-day variability with overly red temporal spectra, and (iv) less accurate summer monsoon rainfall distribution. This confirms that the ISV is an important link in the seamless system that connects weather and climate.

THE AEROCLIPPER A New Device to Explore Convective Systems and Cyclones

Aeroclipper balloons, designed for taking measurements at the air-sea interface, can withstand extreme conditions encountered when they are drawn into tropical cyclones and then follow the eye trajectory.

Outgoing longwave radiation and its diurnal variations from combined Earth Radiation Budget Experiment and Meteosat observations :2. Using Meteosat data to determine the longwave diurnal cycle

For April and July 1985, applying the narrow to broadband conversion of part 1 to Meteosat observations obtained at 3-hour intervals (ISCCP B2 data), we determine the monthly mean radiant exitance as well as the mean diurnal variation, over 2.5°×2.5° latitude-longitude regions of tropical Africa and the neighboring Atlantic Ocean. We compare these determinations with those obtained directly from the ERBS and NOAA 9 Earth Radiation Budget Experiment (ERBE) data for this month, for which the time sampling is sparser and not so uniform. Excellent agreement is obtained in most cases, in particular for the overall monthly means. However, for the monthly mean diurnal variation there are situations in which the ERBE time sampling, the nature of the ERBE diurnal modeling scheme and the convolution of weather system changes with the diurnal cycle, combine to produce significant differences between the ERBE determination and the Meteosat result. These differences would mostly have been much smaller had the third ERBE instrument package been in operation (at 0730/1930 LT) as originally planned. We consider possible improvements in diurnal interpolation procedures, but note that there is no general way to remove bias resulting from inadequate time sampling.

Tropospheric Turbulence over the Tropical Open Ocean: Role of Gravity Waves

A large set of soundings obtained in the Indian Ocean during 3 field campaigns is used to provide statistical characteristics of tropospheric turbulence and its link with gravity wave (GW) activity. The Thorpe method is used to diagnose turbulent regions of a few hundred meters depth. Above the mixed layer, turbulence frequency varies from ~10% in the lower troposphere up to ~30% around 12km heights. GW are captured by their signature in horizontal wind, normalized temperature and balloon vertical ascent rate. These parameters emphasize different parts of the wave spectrum from longer to shorter vertical wavelengths respectively. Composites are constructed in order to reveal the vertical structure of the waves and their link with turbulence. The relatively longer wavelength GW described by their signature in temperature (GWT) are more active in the lower troposphere where they are associated with clear variations in moisture. Turbulence is then associated with minimum static stability and vertical shear, stressing the importance of the former and the possibility of convective instability. Conversely, the short waves described by their signature in balloon ascent rate (GWw) are detected primarily in the upper troposphere and their turbulence is associated with a vertical shear maximum suggesting the importance of dynamic instability. Furthermore, GWw appear to be linked with local convection whereas GWT are more active in suppressed and dry phases in particular of the Madden-Julian Oscillation. These waves maybe associated with remote sources such as organized convection or local fronts such as those associated with dry air intrusions.

Comparison of SCARAB FM1 and SCARAB FM2 calibration performances

ScaRaB is a radiometer built to observe the Earth Radiation budget at the top of the atmosphere. This instrument has been produced by means of a joint program that gathers France, Russia and Germany. SCARAB has been planned to fill the gap between the flights of NASAs instruments ERB and CERS. Thus SCARAB mission is similar to the one fulfilled by ERB and CERES. SCARAB FM1 has been launched in January 1994. At the very beginning of the flight, a critical problem occurred on-board: three calibration lamps over 6 failed and were unusable for the rest of the mission. An alternative way of calibration was found that saved the mission. This method, explained in a previous paper, and its accuracy are briefly recalled in the article. FM2 has been launched in July 1998. The first data show a perfect functioning of the on-board calibration devices. It is an opportunity to test the 3 in-flight calibration methods developed: a method using on-board lamps and black-bodies as previously planned on FM1, another one using the dependence of gains to temperature as used on FM1, and a last one using only black body-simulators. After describing more in detail these different methods, a comparison of their performances is established. These results are compared with FM1 performances. Future prospects of the calibration are given in conclusion.