Junhong Wang

An Overview of the International H2O Project (IHOP_2002) and Some Preliminary Highlights

The International H2O Project (IHOP_2002) is one of the largest North American meteorological field experiments in history. From 13 May to 25 June 2002, over 250 researchers and technical staff from the United States, Germany, France, and Canada converged on the Southern Great Plains to measure water vapor and other atmospheric variables. The principal objective of IHOP_2002 is to obtain an improved characterization of the time-varying three-dimensional water vapor field and evaluate its utility in improving the understanding and prediction of convective processes. The motivation for this objective is the combination of extremely low forecast skill for warm-season rainfall and the relatively large loss of life and property from flash floods and other warm-season weather hazards. Many prior studies on convective storm forecasting have shown that water vapor is a key atmospheric variable that is insufficiently measured. Toward this goal, IHOP_2002 brought together many of the existing operational and new st...

Corrections of Humidity Measurement Errors from the Vaisala RS80 Radiosonde—Application to TOGA COARE Data

A series of laboratory tests have been conducted on several different batches of Vaisala RS80 radiosondes to understand and develop methods to correct six humidity measurement errors, including chemical contamination, temperature dependence, basic calibration model, ground check, sensor aging, and sensor arm heating. The contamination and temperature-dependence (TD) errors dominate total errors. The chemical contamination error produces a dry bias, and is due to the occupation of binding sites in the sensor polymer by nonwater molecules emitted from the sonde packaging material. The magnitude of the dry bias depends on sensor polymer type (RS80-A and RS80H), age of the sonde, relative humidity (RH), and temperature, and it exists throughout the troposphere. The contamination error generally increases with age and RH, and is larger for the RS80-H than the RS80-A. It is ;2% and ;10% at saturation for 1-yr-old RS80-A and RS80-H sondes, respectively. The TD error for the RS80-A results from an approximation of a linear function of temperature to the actual nonlinear temperature dependence of the sensor, and also introduces a dry bias. The TD error mainly exists at temperatures below 2208C, increases substantially with decreasing temperatures below 2308C, and is much larger for the RS80-A than the RS80-H. The RS80-A’s TD correction (CTA) dominates the total correction at temperatures below 2408C and has a correction factor [CTA 5 (RH) (CTApfactor)] of 0.15, 0.75, and 2.3 at 2408, 2608, and 2808C, respectively. The correction methods are applied to 8129 Vaisala RS80 soundings collected during the Tropical Ocean and Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE) and are applicable to RS80 radiosonde data from other field experiments and historical and operational radiosonde datasets. The methods are validated by examining various summary plots of the TOGA COARE data and comparing them with other independent data. The corrections greatly improve the accuracy of the TOGA COARE radiosonde dataset. These correction methods have their own uncertainties and may not correct all errors in Vaisala RS80 humidity data. Analyses of these uncertainties are presented in the paper.

Diurnal and Semidiurnal Tides in Global Surface Pressure Fields

Global surface pressure data from 1976 to 1997 from over 7500 land stations and the Comprehensive Ocean‐ Atmosphere Data Set have been analyzed using harmonic and zonal harmonic methods. It is found that the diurnal pressure oscillation (S1) is comparable to the semidiurnal pressure oscillation ( S2) in magnitude over much of the globe except for the low-latitude open oceans, where S2 is about twice as strong as S1. Over many land areas, such as the western United States, the Tibetan Plateau, and eastern Africa, S1 is even stronger than S2. This is in contrast to the conventional notion that S2 predominates over much of the globe. The highest amplitudes (;1.3 mb) of S1 are found over northern South America and eastern Africa close to the equator. Here S1 is also strong (;1.1 mb) over high terrain such as the Rockies and the Tibetan Plateau. The largest amplitudes of S2 (;1.0‐1.3 mb) are in the Tropics over South America, the eastern and western Pacific, and the Indian Ocean. Here S1 peaks around 0600‐0800 LST at low latitudes and around 1000‐1200 LST over most of midlatitudes, while S2 peaks around 1000 and 2200 LST over low- and midlatitudes. Here S1 is much stronger over the land than over the ocean and its amplitude distribution is strongly influenced by landmasses, while the land‐sea differences of S2 are small. The spatial variations of S1 correlate significantly with spatial variations in the diurnal temperature range at the surface, suggesting that sensible heating from the ground is a major forcing for S1. Although S2 is much more homogeneous zonally than S1, there are considerable zonal variations in the amplitude of S2, which cannot be explained by zonal variations in ozone and water vapor. Other forcings such as those through clouds’ reflection and absorption of solar radiation and latent heating in convective precipitation are needed to explain the observed regional and zonal variations in S2. The migrating tides and 1 S 1 predominate over other zonal wave components. However, the nonmigrating tides are substantially stronger 2 S 2 than previously reported. The amplitudes of both the migrating and nonmigrating tides decrease rapidly poleward with a slower pace at middle and high latitudes.

Determination of Cloud Vertical Structure from Upper-Air Observations

Abstract A method is described to use rawinsonde data to estimate cloud vertical structure, including cloud-top and cloud-base heights, cloud-layer thickness, and the characteristics of multilayered clouds. Cloud-layer base and top locations are identified based on three criteria: maximum relative humidity in a cloud of at least 87%, minimum relative humidity of at least 84%, and relative humidity jumps exceeding 3% at cloud-layer top and base, where relative humidity is with respect to liquid water at temperatures greater than or equal to 0°C and with respect to ice at temperatures less than 0°C. The analysis method is tested at 30 ocean sites by comparing with cloud properties derived from other independent data sources. Comparison of layer-cloud frequencies of occurrence with surface observations shows that rawinsonde observations (RAOBS) usually detect the same number of cloud layers for low and middle clouds as the surface observers, but disagree more for high-level clouds. There is good agreement be...

Cloud Vertical Structure and Its Variations from a 20-Yr Global Rawinsonde Dataset

Abstract A global cloud vertical structure (CVS) climatic dataset is created by applying an analysis method to a 20-yr collection of twice-daily rawinsonde humidity profiles to estimate the height of cloud layers. The CVS dataset gives the vertical distribution of cloud layers for single and multilayered clouds, as well as the top and base heights and layer thicknesses of each layer, together with the original rawinsonde profiles of temperature, humidity, and winds. The average values are cloud-top height = 4.0 km above mean sea level (MSL), cloud-base height = 2.4 km MSL, cloud-layer thickness = 1.6 km, and separation distance between consecutive layers = 2.2 km. Multilayered clouds occur 42% of the time and are predominately two-layered. The lowest layer of multilayered cloud systems is usually located in the atmospheric boundary layer (below 2-km height MSL). Clouds over the ocean occur more frequently at lower levels and are more often formed in multiple layers than over land. Latitudinal variations o...

Systematic Errors in Global Radiosonde Precipitable Water Data from Comparisons with Ground-Based GPS Measurements

Abstract A global, 10-yr (February 1997–April 2006), 2-hourly dataset of atmospheric precipitable water (PW) was produced from ground-based global positioning system (GPS) measurements of zenith tropospheric delay (ZTD) at approximately 350 International Global Navigation Satellite Systems (GNSS) Service (IGS) ground stations. A total of 130 pairs of radiosonde and GPS stations are found within a 50-km distance and 100-m elevation of each other. At these stations, 14 types of radiosondes are launched and the following 3 types of humidity sensors are used: capacitive polymer, carbon hygristor, and goldbeater’s skin. The PW comparison between radiosonde and GPS data reveals three types of systematic errors in the global radiosonde PW data: measurement biases of the 14 radiosonde types along with their characteristics, long-term temporal inhomogeneity, and diurnal sampling errors of once- and twice-daily radiosonde data. The capacitive polymer generally shows mean dry bias of −1.19 mm (−6.8%). However, the c...

Effects of Cloud Vertical Structure on Atmospheric Circulation in the GISS GCM

Abstract Thirteen experiments have been performed using the Goddard Institute for Space Studies General Circulation Model (GISS GCM) to investigate the response of the large-scale circulation to different macroscale cloud vertical structures (CVS). The overall effect of clouds, the role of their geographic variations, and difference between the transient and equilibrium responses of the atmospheric circulation are also studied. Clouds act to suppress the Hadley circulation in the transient response, but intensify it in the equilibrium state. Changing CVS affects the atmospheric circulation directly by modifying the radiative cooling profile and atmospheric static stability, but the effect is opposed, on average, by an indirect effect on the latent heating profile produced by deep (moist) convection. Different interactions of radiation and convection with land and ocean surfaces mean that this cancellation of CVS effects on radiative and latent heating is not the same at all locations. All three parameters...

Corrected TOGA COARE Sounding Humidity Data: Impact on Diagnosed Properties of Convection and Climate over the Warm Pool

This study reports on the humidity corrections in the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE) upper-air sounding dataset and their impact on diagnosed properties of convection and climate over the warm pool. During COARE, sounding data were collected from 29 sites with Vaisala-manufactured systems and 13 sites with VIZ-manufactured systems. A recent publication has documented the characteristics of the humidity errors at the Vaisala sites and a procedure to correct them. This study extends that work by describing the nature of the VIZ humidity errors and their correction scheme. The corrections, which are largest in lower-tropospheric levels, generally increase the moisture in the Vaisala sondes and decrease it in the VIZ sondes. Use of the corrected humidity data gives a much different perspective on the characteristics of convection during COARE. For example, application of a simple cloud model shows that the peak in convective mass flux shifts from about 88N with the uncorrected data to just south of the equator with corrected data, which agrees better with the diagnosed vertical motion and observed rainfall. Also, with uncorrected data the difference in mean convective available potential energy (CAPE) between Vaisala and VIZ sites is over 700 J kg21; with the correction, both CAPEs are around ;1300 J kg21, which is consistent with a generally uniform warm pool SST field. These results suggest that the intensity and location of convection would differ significantly in model simulations with humidity-corrected data, and that the difficulties which the reanalysis products had in reproducing the observed rainfall during COARE may be due to the sonde humidity biases. The humidity-corrected data appear to have a beneficial impact on budget-derived estimates of rainfall and radiative heating rate, such that revised estimates show better agreement with those from independent sources.

Reference upper-air observations for climate: rationale, progress, and plans.

While the global upper-air observing network has provided useful observations for operational weather forecasting for decades, its measurements lack the accuracy and long-term continuity needed for understanding climate change. Consequently, the scientific community faces uncertainty on key climate issues, such as the nature of temperature trends in the troposphere and stratosphere; the climatology, radiative effects, and hydrological role of water vapor in the upper troposphere and stratosphere; and the vertical profile of changes in atmospheric ozone, aerosols, and other trace constituents. Radiosonde data provide adequate vertical resolution to address these issues, but they have questionable accuracy and time-varying biases due to changing instrumentation and techniques. Although satellite systems provide global coverage, their vertical resolution is sometimes inadequate and they require independent reference observations for sensor and data product validation, and for merging observations from differ...

Variability of Cloud Vertical Structure during ASTEX Observed from a Combination of Rawinsonde, Radar, Ceilometer, and Satellite

The macroscale cloud vertical structure (CVS), including cloud-base and -top heights and layer thickness, and characteristics of multilayered clouds, is studied at Porto Santo Island during the Atlantic Stratocumulus Transition Experiment (ASTEX) by using rawinsonde, radar, ceilometer, and satellite data. The comparisons of CVS parameters obtained from four different approaches show that 1) by using the method developed by Wang and Rossow rawinsonde observations (raob’s) can sample all low clouds and determine their boundaries accurately, but oversample low clouds by about 10%, mistaking clear moist layers for clouds; 2) cloud-base heights less than 200 m in the radar data are ambiguous, but can be replaced by the values measured by ceilometer ; and 3) the practical limit on the accuracy of marine boundary layer cloud-top heights retrieved from satellites appears to be about 150‐300 m mainly due to errors in specifying the atmospheric temperature and humidity in the inversion layer above the cloud. The vertical distribution of clouds at Porto Santo during ASTEX is dominated by low clouds below 3 km, a cloud-free layer between 3 and 4 km, and ;20% high clouds with a peak occurrence around 7‐8 km. Low clouds have mean base and top heights of 1.0 km and 1.4 km, respectively, and occur as single layers 90% of the time. For double-layered low clouds, the tops of the uppermost layers and the bases of the lowermost layers have similar distributions as those of single-layered clouds. The temporal variations of low clouds during ASTEX are apparently dominated by advecting mesoscale (20‐200 km) horizontal variations. Coherent time variations are predominately synoptic (timescale 4.5‐6.8 days) and diurnal variability. On the diurnal timescale, all cloud properties show maxima in the early morning (around 0530 LST) decreasing to minima in the late afternoon. Diurnal variations appear to be altered when high clouds are present above low clouds. The general characteristics of CVS in three ASTEX and the First ISCCP Regional Experiment (FIRE87) regions derived from a 20-yr rawinsonde dataset are also presented. The results suggest that CVS characteristics obtained from data collected at Porto Santo during ASTEX (June 1992) are not representative of other marine stratiform cloud regions.