Abstract

In a recent publication (), the authors introduced a data source—Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC)—for monitoring and studying upper-tropospheric water vapor (UTWV) and analyzed 10 yr (1994–2004) of MOZAIC measurements of tropical UTWV in its climatology, variability, transport, and relation to deep convection. In this study (I), MOZAIC is used to assess the ECMWF humidity analysis over the tropics, taking advantage of the unique nature of the MOZAIC data, namely, the long data record, near-global coverage, and high accuracy.

In parallel to , the ECMWF UTWV analysis is assessed against MOZAIC in the following five aspects: 1) annual cycle, 2) vertical structure, 3) probability density functions (PDFs), 4) moisture flux divergence, and 5) interannual variability. The annual cycle of the ECMWF UTWV shows a similar pattern as MOZAIC but has an overall dry bias of about 10%–30% relative humidity with respect to ice (RHi). The dry biases are larger in the deep tropics than the subtropics and larger over the Asian monsoon region than the tropical Atlantic region. The increase in RH with height (from about 300 to 200 hPa) as observed by MOZAIC is largely missing in the ECMWF analysis, which has a roughly constant RH profile. The bimodal distribution of tropical UTWV is well established in MOZAIC, but for ECMWF, the moist mode is abruptly cut off at 100% RHi due to the lack of ice supersaturation (ISS) in the forecast model. Lack of ISS capability is, however, not the only cause for the dry bias in the ECMWF; it also has more occurrences of lower humidity compared to MOZAIC. There is also evidence that ECMWF underestimates the range of upper-tropospheric humidity (UTH) variation. A comparison of moisture flux divergence is conducted to assess the ability of ECMWF to capture the divergent transport of water vapor. It is shown that the ECMWF can represent the distribution of this quantity fairly well, although the dry bias leads to some underestimate of the magnitude. Finally, the authors show a comparison of the ECMWF and MOZAIC depictions of the interannual variation of UTWV during the 1997/98 ENSO event as an illustration that UTWV variations are more difficult to capture than those of the UT temperature.

Abstract

Ten years (1994–2004) of measurements of tropical upper-tropospheric water vapor (UTWV) by the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) are investigated over three regions—the tropical Atlantic, tropical Africa, and the Asian monsoon region—to determine the UTWV climatology and variability on multiple scales and to understand them in relation to moisture transport and deep convection.

The seasonal migration of upper-tropospheric humidity (UTH) keeps pace with that of the ITCZ, indicating the convective influence on UTH distribution. Some significant regional differences are identified with the tropical Africa and the Asian monsoon regions being moister than the tropical Atlantic. UTH generally increases with height by 10%–20% relative humidity with respect to ice (RHi) from about 300 to 200 hPa, and the differences are larger in the deep Tropics than in the subtropics. The probability density functions of tropical UTH are often bimodal. The two modes stay rather constant; differences in the mean value are largely due to the variations in the proportion of the two modes as opposed to changes in the modes themselves. In the deep Tropics, the moisture level frequently reaches ice supersaturation, the most notable case being the near-equatorial Asian monsoon region during the wet season when ice supersaturation is observed 46% of the time.

Interannual variations are observed in association with the 1997–98 ENSO event. A warming of about 1–2 K is observed for all three regions equatorward of roughly 15°. Specific humidity also increases somewhat for the tropical Atlantic and tropical Africa, but the increase in temperature outweighs the increase in specific humidity such that RH decreases by 5%–15% RHi. In addition to the ENSO-related variation, MOZAIC also sees increases in both RH and specific humidity over tropical Africa from 2000 onward.

Moisture fluxes are computed from MOZAIC data and decomposed into contributions from the mean circulation and from eddies. The flux divergence, which represents the moisture source/sink from horizontal transport, is also estimated. Finally, the MOZAIC climatology and variability are revisited in relation to deep convection obtained from the International Satellite Cloud Climatology Project (ISCCP).

Abstract

A new in-flight calibration (IFC) method is described for the humidity sensor flown routinely since 1994 on the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) program’s aircraft. The IFC method corrects the potential drift of the sensor offset at zero relative humidity, which is the critical parameter in determining the uncertainty of the measurements. The sensor offset is determined from the measurements themselves as obtained during periods when the aircraft is flying in the lower stratosphere at or above the hygropause, where the H₂O mixing ratio reaches well-defined minimum values of about 5 ppmv and the contribution of atmospheric H₂O to the sensor signal is minimal. The selection of stratospheric data is achieved with the help of potential temperature, which can be calculated in situ from measured temperature and pressure. The IFC method is capable of providing humidity measurements in near–real time with an uncertainty of ±8% RH at the surface and ±7% RH in the upper troposphere.

For validation, the IFC method was applied to 5 yr of archived raw signals from the MOZAIC aircraft. The resulting humidity data are in good agreement (within 2% RH) with the original MOZAIC data that used monthly pre- and postflight calibrations of the sensor. The standard deviation of the differences varies with altitude between ±4% and ±6% RH, which is comparable to the accuracy of the MOZAIC laboratory calibrations. Compared to MOZAIC operation based on monthly calibrations in the laboratory, the use of IFC will substantially reduce the efforts for maintenance and thus will enable operation of the sensor on a large fleet of in-service aircraft for near-real-time measurements of humidity in the troposphere. Because the IFC method will not work on aircraft that never enter the lower stratosphere, for example, aircraft that fly exclusively regional routes or in the tropics, regular offline calibrations will remain important for such aircraft.

Abstract

Multiple years of measurements of tropical upper-tropospheric temperature and humidity by the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) project are analyzed in the vicinity of deep convective outflow to study the variations of temperature and humidity and to investigate the influence of the sea surface temperature (SST) on the outflow air properties. The principal findings are the following. 1) The distribution of relative humidity with respect to ice (RH _i ) depends on where a convective system is sampled by the MOZAIC aircraft: deep inside the system, RH _i is unimodal with the mode at ~114%; near the outskirts of the system, bimodal distribution of RH _i starts to emerge with a dry mode at around 40% and a moist mode at 100%. The results are compared with previous studies using in situ measurements and model simulations. It is suggested that the difference in the RH _i distribution can be explained by the variation of vertical motions associated with a convective system. 2) Analysis of MOZAIC data shows that a fractional increase of specific humidity with SST, q ⁻¹ dq/dSTT, near the convective outflow is about 0.16–0.18 K⁻¹. These values agree well with previous studies using satellite data. Because MOZAIC measurements of temperature and humidity are independent, the authors further analyze the SST dependence of RH _i and temperature individually. Temperature increases with SST for both prevalent flight levels (238 and 262 hPa); RH _i stays close to constant with respect to SST for 238 hPa but shows an increasing trend for the 262-hPa level. Analysis conducted in this study represents a unique observational basis against which model simulations of upper-tropospheric humidity and its connection to deep convection and SST can be evaluated.

Abstract

The results of four balloon flights of the NOAA ultraviolet fluorescence stratospheric water vapor instrument are presented. A series of improvements in the instrument has brought results which are credibly free from contamination by outgassing. The results are in essential agreement with the extensive soundings by H.J. Mastenbrook. The minimum water vapor mixing ratio occurs 2–3 km above the tropopause in both tropical and temperature latitudes. Our measured minimum values were 2.6 ppmv over Brazil (5°S) and 3.6 ppmv over Wyoming (41°N), with an estimated total error of 20%. This degree of dryness permits the conclusion that the global circulation originally proposed by Brewer is correct; i.e., that air enters the stratosphere from the troposphere in substantial quantities only through the tropical tropopause. This general circulation must apply to all other trace gases of tropospheric origin as well. The carbon monoxide measurements of Seiler support the conclusion.