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INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 33 : 109–120 (2013)Published online 21 December 2011 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/joc.3412

A multi-sensor study of water vapour from radiosonde,MODIS and AERONET: a case study of Hong Kong

Zhizhao Liu, a Man Sing Wong, a* Janet Nichol a and P. W. Chan ba Department of Land Surveying and Geo-Informatics, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong

b Hong Kong Observatory, Hong Kong

ABSTRACT: Water vapour is one of green house gases (GHG) and a key parameter affecting weather forecasting. Situatedon the edge of the South China Sea and in the path of the wet Asian monsoon, Hong Kong experiences more rainstorms thanmost other cities. An accurate observation of water vapour has a special signicance in severe weather prediction for HongKong, one of the cities with the highest density of population in the world. In this paper, water vapour observations overthe last 6 years from an AErosol RObotic NETwork (AERONET) sunphotometer, 9 years from the MODerate resolution

Imaging Spectroradiometer (MODIS) TERRA satellite images and 38 years from radiosonde were analysed and cross-validated. The operational MODIS water vapour products, namely MOD05 and MOD07, with a spatial resolution of 1and 5 km, respectively, were compared with both radiosonde and AERONET data. The correlation coefcients betweenMODIS water vapour products and radiosonde data are r = 0.878 and 0.876 for MOD05 and MOD07 products, and thecorrelations of those with AERONET data are r = 0.822, r = 0.976, respectively. The results also indicate that radiosondeand AERONET water vapour observations have a good agreement, with a correlation of r = 0.988 and a small meanabsolute difference (MAD), and root mean square error (RMS) of 0.197 and 0.289 cm, respectively. Although the satellitedata (with a frequency of once per day) represent water vapour coverage of all the territories of Hong Kong, they do notmeet short-term weather prediction demand due to their low temporal resolution. Radiosonde observations with a frequencyof twice per day are also temporally inadequate. This study demonstrates that the AERONET sunphotometer can provideaccurate and high temporal resolution water vapour data which can be used for short-term weather prediction and long-termclimate change research. Copyright © 2011 Royal Meteorological Society

KEY WORDS radiosonde; water vapour; AERONET; MODIS

Received 16 July 2011; Revised 4 November 2011; Accepted 24 November 2011

1. Introduction

Water vapour is one of the green house gases (GHGs)and is an essential parameter in the prediction of thun-derstorms, weather, and visibility. It is also a key factorin the study of many parameters such as the Earth’s radia-tion budget, global warming, cloud formation, the hydro-logical cycle, and atmospheric chemistry and dynamics(Fix et al ., 2002; Rama Varma Raja et al ., 2008). The

amount of water vapour inuences cloud formation, bothof which are essential components in atmospheric cool-ing and latent heating (Zveryaev and Allan, 2005); andthis atmospheric cooling and heating feedback, in turn,controls the amount of water vapour condensation presentin the atmosphere. Thus, precisely measuring the watervapour content of the atmosphere is critical for under-standing many feedback processes (Zveryaev and Allan,2005).

Despite its importance in understanding atmosphericprocesses and in weather forecasting, water vapour is one

∗Correspondence to: M. S. Wong, Department of Land Surveying andGeo-Informatics, The Hong Kong Polytechnic University, Hunghom,Kowloon, Hong Kong. E-mail: [email protected]

of the most poorly described components in the atmo-sphere for several reasons (Fix et al ., 2002). Firstly, watervapour is highly variable in nature both spatially and tem-porally. The upper troposphere shows more variabilitywhen compared with the lower layers (Van Baelen andPenide, 2009). Secondly, the temperature increase asso-ciated with global warming has the potential to acceleratethe global hydrological cycle due to increased evapora-tion (Semenov and Bengtsson, 2002; Labat et al ., 2004;Xu et al ., 2006). The inuence of global warming onweather is profound, with extreme weather events suchas extended periods of precipitation, drought, and rain-storms likely to occur more frequently (Shouraseni andBalling, 2004; Dore, 2005; Zhang et al ., 2008). Thirdly,in addition to global warming effects at global scale,water vapour variability is also greatly affected by anthro-pogenic activities that have signicantly altered distri-bution of water resources at local and regional scales,primarily in the form of virtual water import and export.Virtual water resources refer to water that is used in theproduction chain and is embedded in products (Allan,

1996; Guan and Klaus, 2007). Studies show that HongKong’s neighboring region, the Pearl River Delta (PRD),actually imports more virtual water than it exports (Guan

Copyright © 2011 Royal Meteorological Society



110 Z. LIU et al .

and Hubacek, 2007). All these factors affect the spatialand temporal distribution of water vapour in Hong Kong.

Radiosonde is the most traditional method of watervapour observation and appears to be the most accuratetechnique of observation (Idrac and Bureau, 1927; Brettleand Galvin, 2003). Radiosonde measures the pressure and

temperature at different altitudes from a balloon-borneplatform. The water vapour (total precipitable water)from the Earth’s surface to the top of the atmospherecan then be calculated. Radiosonde is a one-off andexpensive observation technique because the observationballoon is difcult to control and cannot be reused. Theaccuracy of estimation of water vapour from radiosondebased on numerous studies is likely to be 0.1 g cm − 2

(Wolfe and Gutman, 2000; Niell et al ., 2001; M atzleret al ., 2002; Bokoye et al ., 2003). Owing to their highaccuracy and high vertical resolution, radiosonde data areoften used as a reference to evaluate retrievals of watervapour from other methods of estimating water vapour,e.g. ground-based and satellite remote sensors (e.g. Sodenand Lanzante, 1996; Soden et al ., 2004). Owing to thehigh costs associated with launching radiosonde, there areapproximately only 850 radiosonde stations worldwide(Kuo et al ., 2005). Radiosonde data are single-pointmeasurements that lack regional coverage. In addition,their temporal resolution is very low, usually with onlytwo radiosonde observations each day at a station. Thus,for monitoring purposes, the amount of water vapour dataobtained from radiosondes is rather limited.

Satellite images provide another means of water vapourobservation over a large region on a regular basis. Water

vapour retrieval from satellite remote sensing has beenstudied for two decades (King et al ., 1992, 2003; Parkin-son, 2003; Prasad and Singh, 2009). For instance, awidely used remote sensing sensor, the MODerate res-olution Imaging Spectroradiometer (MODIS) sensor onboard the NASA TERRA and AQUA Spacecraft plat-forms provides water vapour images twice per daycovering the entire world using three near-infrared chan-nels located in the 940-nm region, and three additionalinfrared channels in the water vapour absorption regions(Gao and Kaufman, 2003). The MODIS water vapouroperational products (MOD05 and MOD07) have spatialresolutions of 1 and 5 km, respectively (Kaufman andGao, 1992). Besides MODIS, there are many other satel-lites that can perform water vapour retrieval such as thegeostationary MTSAT (Sohn et al ., 2008).

Another technique for retrieving high temporal-resolution water vapour data is to estimate this from solartransmittance with sunphotometers (Volz, 1974). Numer-ous studies using sunphotometers to retrieve water vapourhave been conducted in the last two decades (Brueggeet al ., 1992; Schmid et al ., 1996; Halthore et al ., 1997;Bokoye et al ., 2003, 2007). The water vapour dataretrieved from sunphotometers show consistent and reli-able accuracy when compared with other methods of

measuring water vapour. For example, Bokoye et al .(2007) obtained an RMS error of 0.14–0.48 g cm − 2 ,although this error is greater than the 0.1 g cm − 2

estimated for radiosonde (Wolfe and Gutman, 2000;Niell et al ., 2001; M atzler et al ., 2002; Bokoye et al .,2003). A network of about 300 sunphotometers hasbeen deployed worldwide within the AErosol ROboticNETwork (AERONET) (Holben et al ., 1998). At eachAERONET station, one sunphotometer is deployed and

is calibrated annually by NASA. In addition to watervapour, AERONET stations also measure other parame-ters such as aerosol optical thickness, and other micro-physical properties of aerosols (Holben et al ., 1998).

Other water vapour measuring techniques includeGlobal Positioning System (GPS) (Bevis et al ., 1994;Wolfe and Gutman, 2000), radar, and numerical models.The US NOAA GPS-Met network provides nation-wideGPS integrated water vapour data for weather forecast-ing and climate monitoring (Bevis et al ., 1992; RamaVarma Raja et al ., 2008). The potential of GPS watervapour measurement was demonstrated in the GPS/METexperiment (Rocken et al ., 1997). It was assessed thatthe water vapour derived from GPS data can reach anaccuracy of better than 1 mm (Rama Varma Raja et al .,2008). Compared to point measurement by GPS, weatherradars can efciently provide water vapour measurementsover large areas. However, the observation from weatherradar is not a direct measurement. It requires a calibra-tion by gauge data and the uncertainty of the water vapourestimate increases with range (Battan, 1973; Zawadzki,1984; Pedersena et al ., 2010). In addition, the quality of weather radar measurements of near-surface phenomenaat S-band and C-band, degrades with range due to theincreasing gap between the beam and the Earth’s sur-

face resulting from the curvature of the Earth (Pedersenaet al ., 2010). Test data showed that X-band weather radarcan produce a water vapour measurement accuracy of about 2 mm according to gauge data quality. NumericalWeather Prediction (NWP) models are primarily based onthe simulation of water vapour from radiosonde observa-tions, satellite-based infrared, and microwave radiometerdata (Kanamitsu et al ., 1991; Kistler et al ., 2001; Ander-sson et al ., 2007; Vey et al ., 2010). NWP models haveregional and global coverage, and some have high tem-poral and spatial resolutions. For example, the spatialresolution of the NWP model from National Centers forEnvironmental Prediction (NCEP) is 2 .5° × 2.5° and thetemporal resolution is 6 h (Vey et al ., 2010). When watervapour from the NWP model at NCEP was assessed withGPS-derived data, the accuracy was found to be within1.5 ∼ 3.0 mm (Vey et al ., 2010).

Despite the special importance of water vapour datain weather forecasting concerning a highly populatedand extremely humid coastal city like Hong Kong,only limited information from radiosonde and satelliteobservations is currently available for daily weatherprediction by the Hong Kong Observatory (HKO). Atpresent, only two sources of water vapour data areused by the HKO, namely the traditional radiosonde and

satellite remote sensing. The water vapour data from theradiosonde have a low resolution in time (12 h), and themethod is expensive. Moreover, it is not environmentally

Copyright © 2011 Royal Meteorological Society Int. J. Climatol. 33 : 109–120 (2013)



A MULTI-SENSOR STUDY OF WATER VAPOUR IN HONG KONG 111

friendly since the balloon and the meteorological sensorscannot be reused. In the space domain, there is onlyone radiosonde station in Hong Kong to represent theterritory’s total area of 1104 km 2 . In spite of the largeareal coverage by satellite remote sensing, satellite-retrieved water vapour has low temporal resolution (e.g.

only twice per day) as well as relatively low accuracybecause the retrieval algorithm uses a global modeldevised to encompass all environments and conditions.

It is desirable to have accurate and more water vapourobservations to improve weather forecasting in HongKong. The Hong Kong Polytechnic University operatestwo AERONET stations, one of which is deployed inthe city centre for measuring local and anthropogenicaerosols and water vapour, and the other situated at aremote peninsula for measuring long-distance aerosolsand coastal water vapour. Each station has a sunphotome-ter that can measure water vapour in the atmosphere con-tinuously when the sun is visible. However, AERONETwater vapour data have never contributed to Hong Kong’sweather forecasting. Furthermore, a systematic evaluationof the accuracies of local observations by the three tech-niques (radiosonde, MODIS, and AERONET) has neverbeen reported. The main objectives of this study are:(1) to evaluate the accuracies of water vapour observa-tions of MODIS and AERONET data using in situ data(radiosonde) as the reference; (2) to characterize localwater vapour variations in Hong Kong at different timescales, such as monthly, seasonally, and annually usinghistorical water vapour data; and (3) to suggest the appro-priate data and describe its potential benets in applica-

tion in day-to-day weather forecasting services.

2. Study area and data description

Hong Kong, located at 22 °N latitude 114 °E longitudeand surrounded by the South China Sea (SCS) on theeast, west, and south, has a climate typical of SouthChina as it is affected by the Asian Summer Monsoon(ASM) and weather systems from both tropical and mid-latitudes (Zhao et al ., 2007). The monsoonal stream fromthe SCS is identied as one of the three main low-level monsoonal streams that transports a large amount of water vapour towards China during the ASM (Zhao et al .,2007; Chow et al ., 2008). This monsoonal rainfall is themost important climatic factor affecting China and India(Chow et al ., 2008). Over the last decade, Hong Konghas suffered serious air pollution coupled with the inter-actions among atmospheric aerosols, clouds, and highconcentrations of water vapour introducing instabilitiesand unpredictabilities in local weather forecasting. Thesub-tropical zone tends to be cloudy, with high precipita-tion especially in the summer. The HKO is equipped with29 weather stations (27 automatic and 2 manned) and 16rainfall stations, but only one radiosonde station to take

water vapour measurements. Like many other worldwideweather stations, the HKO launches a radiosonde balloononly twice daily.

2.1. Radiosonde data

In this study, radiosonde data over the past 38 years (from1973 to 2010) were acquired from the HKO. The dailyradiosondes are launched twice at the Hong Kong King’sPark station (latitude 22.30 ° and longitude 114.16 °), at8 : 00 a.m. and 8 : 00 p.m. local time, respectively. The

integrated water vapour column was derived using thefollowing formula:

W =1g

p 2

p 1

xdp (1)

where W is the precipitable water vapour (unit: mm)between a layer bounded by pressures p 1 and p 2 (unit:Pascal) that are measured by radiosonde sensors at twoaltitudes; g is the acceleration of gravity (i.e. 9.806 ms − 2

used in this study); x is the mixing ratio (unit: g kg− 1)that is dened as shown below:

x = 0.622 · ep − e

(2)

p is the pressure (unit: Pascal); e is the vapour pressure(unit: Pascal) that can be derived from e = RH · es (T), es

is the saturation vapour pressure (unit: Pascal) that is afunction of temperature (unit: degree Celsius) and RH isthe relative humidity (unit: %) of the atmosphere.

2.2. MODIS data

The TERRA satellite passes the zenith of Hong Kongfrom north to south at about 10: 30 am local time(UTC + 8 h) each day. The water vapour products arederived from near-infrared and infrared measurements.In this study, the MODIS level 2 operational prod-ucts, namely MOD05 and MOD07 at 1 and 5 kmwere acquired. The MOD05 and MOD07 TERRA datarecorded from 2002 to 2010 for the Hong Kong regionwere acquired from NASA Goddard Earth Science Dis-tributed Active Archive Center (DAAC).

The MOD07 operating in the longwave infrared spec-tral region allows daytime and nighttime measurements,while MOD05 working in the near-infrared permits day-time measurements only. The ‘differential absorption

technique’ is employed to retrieve water vapour from theMOD05 and MOD07 images (e.g. Gao and Goetz, 1990;Kaufman and Gao, 1992; Gao et al ., 1993; Gao and Kauf-man, 2003; King et al ., 2003). The rationale behind thismethod is to rst estimate the water vapour attenuationof near-infrared solar radiations at several wavelengths.The radiative transfer model with numerous atmosphericproles is used for estimating the radiance ratio. On thebasis of further inversion procedures such as a lookuptable, the water vapour content can be derived using alookup table.

2.3. AERONET data

The AERONET is a federated network of over 300ground sunphotometers around the world (Holben et al .,




112 Z. LIU et al .

Figure 1. Left: MODIS MOD07 water vapour image for entire Hong Kong (annual average of 2009); Right: monitoring stations in Hong

Kong overlaid with IKONOS image. Red triangle symbol represents Hong Kong PolyU AERONET station; yellow star represents radiosondemeasurement in King’s Park. This gure is available in colour online at wileyonlinelibrary.com/journal/joc

1998). All the instruments in the AERONET are annu-ally calibrated with reference to the world standard:the Mauna Loa Observatory. Thus, the measuring accu-racies among different AERONET stations are accu-rate and consistent. An AERONET station comprisesa sunphotometer that measures the aerosol extinctionevery 15 min using multiple wavelengths. The derivedproducts include near-real-time aerosol optical thickness,precipitable water, aerosol size distribution, single scat-tering albedo, and refractive index, based on the solu-tions of radiative transfer equations (Dubovik and King,2000; Holben et al ., 2001). The AERONET providesthree levels of data: 1, 1.5, and 2 which represent theraw data, cloud-screened data, and cloud-screened, andquality-assured data, respectively (Holben et al ., 1998).The AERONET water vapour data used in this studywere retrieved from an urban AERONET station estab-lished by the Hong Kong Polytechnic University in 2005.This AERONET station is only approximately 1 kmaway from the Hong Kong radiosonde station, thus, theAERONET and radiosonde data can be considered ascollocated observations (Figure 1). The AERONET data

collected over the past 6 years (2005–2010) are used inthis study and compared with water vapour data from theradiosonde and MODIS images.

Retrieval of water vapour from AERONET is basedon the modied Langley algorithm by determining thespectral transmission of the solar irradiance at 940 nmwavelength (Reagan et al ., 1987).

W =1m

1a

lnV 0(λ) · R− 2

V(λ)− m r

· τ r (λ) − m · τ a (λ)

1b

(3)

where W is the derived water vapour content; m is airmass; a and b are constants that can be observed from

a curve-tting procedure (Schmid et al ., 1996); V 0(λ) isa constant that can be determined from sunphotometerdata using a modied Langley-plot technique; V (λ) isthe output from the sunphotometer; R is the Earth–Sundistance (in astronomical units) at the time of observation;m r is pressure-corrected air mass; τ r (λ) is Rayleigh-corrected aerosol optical thickness; and τ a (λ) is aerosoloptical thickness at 940 nm.

3. Intercomparison among AERONET, MODIS,and radiosonde

As stated earlier, one of the main objectives of this studyis to evaluate the accuracies of AERONET and MODISwater vapour data using radiosonde data as the reference.If their accuracies are consistent and good, there is apotential for the HKO to assimilate these water vapourmeasurements into the Hong Kong weather forecastingsystem. In this section, the water vapour data measuredby AERONET, MODIS, and radiosonde, respectively, arecompared and evaluated.

3.1. Comparison of AERONET with radiosonde

The radiosonde operates at approximately 8 : 00 a.m.(local time). Therefore, the water vapour data fromAERONET measurements for the period 7 : 30–8 : 30 a.m. (i.e. 30 min prior to and after 8 : 00 a.m.)were averaged for comparison with radiosonde. Over theapproximately 6-year period, there are a total of 336 datapairs from AERONET and radiosonde. Figure 2 showsthat the water vapour data from AERONET agree wellwith radiosonde data, with a high correlation coefcientr = 0.988. The mean absolute difference (MAD) and

RMS error are 0.197 and 0.289 cm, respectively. Themaximum and minimum differences between the twoobservation techniques are 1.411 and 0 cm, respectively





Figure 2. Comparison of water vapour between radiosonde andAERONET measurements.

(Table I). Since the AERONET operates only duringcloudless and rain-free days, many data were screenedout and only 336 paired data over the last 6 years wereused.

3.2. Comparison of MODIS and radiosonde

To match the location of the radiosonde station, theMODIS data from a 3 × 3 pixel window (3 × 3 km 2 ,and 15 × 15 km 2 for MOD05 and MOD07, respectively)were extracted, and the averaged values were comparedwith the radiosonde water vapour data. Because of incon-sistency between the satellite and radiosonde observationtimes (radiosonde data are measured at 8 : 00 a.m. whilethe MODIS TERRA satellite passes over Hong Kong at10 : 30 a.m.), comparison of the two sets of data may tosome extent be affected by water vapour variations duringthis 2.5-h period.

However, as shown in Figure 3, the two sets of watervapour observations still show a strong correlation. Thecorrelation coefcients between MODIS water vapourproducts and radiosonde data are r = 0.878 and 0.876 forMOD05 and MOD07 products, respectively. For MOD05data, the MAD and RMS are 1.140 and 1.309 cm,respectively. Compared to the results from AERONETand radiosonde, the correlation coefcient is considerablylowerr, and the MAD and RMS values are higher.This may be partially attributed to the large observation

(a)

(b)

Figure 3. Comparison of water vapour between (a) MODIS MOD05and radiosonde measurements, (b) MODIS MOD07 and radiosonde

measurements.

time gap (2.5 h), as water vapour may experience somevariation between 8 : 00 a.m. and 10 : 30 a.m.

3.3. Comparison between MODIS and AERONETdata

The AERONET data show a very high and almost perfectcorrelation ( r = 0.988) with the radiosonde referencedataset. Comparison of MODIS with AERONET can,therefore, indicate whether or not the apparent mismatch(lower correlation) between MODIS and radiosonde isdue to the time difference or due to real error in

Table I. Statistical summary of cross-comparisons between radiosonde, MODIS, and AERONET water vapour data.

Data comparison Correlation(r )

Root Mean Squareerror (RMS)

Mean AbsoluteDifference (MAD)

Standard Deviation(sd )

Maximumdifference

Minimumdifference

Radiosonde vs AERONET 0.988 0.289 0.197 0.214 1.411 0MOD05 vs Radiosonde 0.878 1.309 1.140 0.675 3.197 0.002MOD07 vs Radiosonde 0.876 1.353 1.201 0.641 3.326 0.004MOD05 vs AERONET 0.822 1.380 1.231 0.676 3.412 0MOD07 vs AERONET 0.976 0.214 0.166 0.289 0.898 0.001




114 Z. LIU et al .

MODIS water vapour retrieval. Therefore, the MOD05(1 km resolution) and MOD07 (5 km resolution) watervapour products for the years 2005–2010 for the HongKong region were compared with AERONET level 1.5data observations, with time criteria set at ± 30 min of MODIS satellite overpass time. To match the location

of the AERONET station (on the campus of HongKong Polytechnic University), the MODIS data froma 3 × 3 pixel window (3 × 3 km 2 , and 15 × 15 km 2

for MOD05 and MOD07, respectively) were extracted,and the averaged values were compared with AERONETwater vapour data.

The columnar water vapours from the two MODISwater vapour products (MOD05 and MOD07) havehigh correlations with AERONET data (Figure 4), withcorrelation coefcients of r = 0.822 and r = 0.976 forMOD05 and MOD07 data, respectively. The MAD andRMS errors are 1.231 and 1.380 cm for MOD05 data, andthe MAD and RMS of MOD07 products are 0.166 and0.214 cm, respectively. These high correlations betweenMODIS products (within a 3 × 3 pixel window) and

(a)

(b)

Figure 4. Comparison of water vapour between (a) MODIS MOD05and AERONET measurements, (b) MODIS MOD07 and AERONET

measurements.

AERONET observations conrm that MODIS products,especially MOD07, have high accuracy for water vapourretrieval in Hong Kong. Another test was conducted byincreasing the MODIS window size from 3 × 3 pixels to5 × 5 pixels. Only a small decrease in the correlation wasobserved. This can probably be explained by the fact that

the Kowloon peninsula, where the AERONET is located,is a highly urbanized area and that it has generallyhomogenous landcover within a radius of 3–5 km aroundthe AERONET station.

4. Characteristics of Hong Kong water vapourfrom radiosonde, MODIS, and AERONET

In order to demonstrate the strengths and weaknesses of each method of water vapour retrieval and how they arecomplementary, the annual and seasonal trends of watervapour measurement from MODIS and AERONET wereestimated. However, the data archives for AERONET andMODIS cover a much shorter period than for radiosonde.The AERONET station has been operating for only6 years while MOD05 and MOD07 have 9 years of data,compared with the 38 years from radiosonde. Figure 5shows the (a) annual, (b) seasonal, and (c) monthly vari-ations of water vapour from four types of data.

The radiosonde data provide a long archive of watervapour measurements in Hong Kong since 1973. Theannual average of water vapour for the past 38 yearswas calculated, and the annual average water vapourin Hong Kong was found to be 4.179 cm per day.Some inter-annual uctuation is observed over this period

(Figure 5(a)). The highest annual average was observedin 1998 with 4.482 cm per day and the lowest of 3.698 cm per day occurred in 2004. The radiosonde watervapour observations were also grouped into four seasons,and the seasonal averages were estimated. The summerseason (June, July and August) has an average watervapour of 5.597 cm per day, while the winter season(December, January, and February) has an average watervapour of only 2.553 cm per day. Figure 5(b) indicatesthat the summer atmosphere contains approximately twotimes more humidity than during the winter season. In1998, the seasonal water vapour has highest values of 5.915 cm for summer and 3.065 cm for winter. In thedry year 2004, both summer and winter seasons havemuch less water vapour with 5.167 cm and 2.188 cm,respectively. It can be observed that during the monsoonand pre-monsoon months (July– September), there isan apparent increase of water vapour in Hong Kong’satmosphere (increase by 30% compared with the annualaverage).

Figure 5 indicates that the values of water vapour fromAERONET are much smaller than those of the other threetypes of data, and this is attributed to the nature of oper-ation of AERONET sunphotometers. The AERONETsunphotometers operate only on sunny days and can-

not take any measurements during nighttime or rainyperiods. Since the water vapour from sunphotometersis derived by measuring the ratio of direct sunlight





(b)

(c)

(a)

Figure 5. (a) Annual, (b) seasonal, and (c) monthly variation, of allthree types of data (radiosonde, MODIS, and AERONET), barsrepresenting standard deviation of data. This gure is available in

colour online at wileyonlinelibrary.com/journal/joc

transmitted through the atmosphere in and near thewater vapour absorption bands (Brooks et al ., 2007),AERONET water vapour products are only availablefor times and days when the Sun is visible. Thus, the

atmosphere under such conditions of observation is nat-urally much drier than in rainy conditions, when it is notoperating.

(a)

(b)

(c)

Figure 6. Diurnal variation of (a) AERONET data, (b) air temperaturedata, (c) wind speed data.

The AERONET data provide high temporal resolutionwater vapour data (i.e. every 15 min) during the daytime.The annual averages of daily water vapour for the years2005, 2006, 2007, 2008, 2009, and 2010 from AERONET

are 2.94, 3.27, 3.15, 2.62, 3.51, and 3.988 cm, respec-tively. A considerable increasing trend (increasing slopeof 0.07 cm h − 1) is observed over the course of a day




116 Z. LIU et al .

between 8 : 00 a.m. and 5 : 00 p.m., presumably due toboth anthropogenic effects and increasing temperatures(Figure 6(a)). The maximum water vapour concentrationsobserved from AERONET lag the maximum of tempera-ture (25.07 °C at 2 : 00 p.m.) by 3 h (Figure 6(b)), and themaximum of wind speed (2.79 m s − 1 at 1 : 00 p.m.) by

4 h (Figure 6(c)). However, these relationships cannot beconrmed unless more nighttime water vapour data fromGPS retrieval are incorporated in the study. It is knownthat water vapour as a GHG is always underestimatedand is probably one of the causes of temperature changes.Further study of diurnal variations in water vapour, andits relationship with other climatic factors, will be con-ducted in the near future.

5. Case study: comparison between AERONET,radar, and weather-prediction-model data

In this study, the water vapour contents retrieved fromAERONET have similar accuracy to radiosonde obser-vations but with higher temporal resolution. This capa-bility enables its use in numerical weather prediction.Two cases from 2010 and 2011 are demonstrated. In theearly morning of 20 August 2011 (8 : 00 a.m. local time),there were no showers in Hong Kong as shown on theweather radar (Figure 7(b)). Later in the morning around11 : 00 a.m., showers began to fall near the southeasterncoast of Hong Kong (Figure 7(c)), and in the afternoon(2 : 00 p.m.) there was a shower in the western part of Hong Kong (Figure 7(d)). A numerical weather predic-tion model (Non-Hydrostatic Model (NHM) model) was

also utilized to forecast the water vapour contents. Itwas initialized at 5 : 00 a.m. local time, 20 August 2011.The results are similar to those observed from weatherradar observations (Figure 7(e–g)). A shower occurred at2 : 00 p.m. in the western part of Hong Kong. The gradualrise of water vapour content shown by AERONET witha maximum around 11 : 00 a.m.– 2 : 00 p.m. suggests thechance occurrence of showers due to the accumulationof water vapour in the troposphere (Figure 7(a)). Themissing data from 2 : 00 p.m. to 4 : 00 p.m. local time areprobably due to the ‘wet sensor’ in AERONET beingexposed to precipitation which shut down the sensor.Another case on 13 September 2010 is demonstrated. Aneast-to-southeasterly airstream prevailed over the south-ern coast of China at the surface, and a troughing owcould be detected in the middle troposphere. At dawn,there were no showers around Hong Kong, as shown onthe weather radar (Figure 8(b)). Later in the morning,showers began to fall at the western coast of Guang-dong (Figure 8(c)), and at 2 : 00 p.m. local time it wasshowery near Hong Kong (Figure 8(d)). Some rainfallwas reported at HKO headquarter, which is close to theAERONET station. The gradual rise of water vapourcontent from AERONET suggests the chance of show-ers increases as water vapour accumulates in the tropo-

sphere (Figure 8(a)). AERONET data are missing from1 : 00 p.m. to 3 : 00 p.m., local time probably due to pre-cipitation. This consistency with the radar observations

suggests that AERONET data can help to alert weatherforecasters about occurrence of showers around HongKong.

6. Discussion and conclusion

Three types of water vapour measuring technologies:radiosonde, MODIS satellite, and AERONET sunpho-tometer were evaluated in the study, and their accuracieswere statistically tested. The strong correlations betweenradiosonde (the reference dataset) and AERONET ( r =

0.988) suggest the robustness of AERONET data forretrieving water vapour even though AERONET esti-mates are consistently lower due to their dependency onweather. A similar high correlation between the MODISMOD07 product and AERONET data ( r = 0.976) isobserved, again with AERONET consistently lower. Thisstudy adopted AERONET sunphotometer level 1.5 data

which are cloud-screened, therefore, certain biases maybe expected in comparison with radiosonde data espe-cially during the wet season, as the radiosonde works inall-weather conditions.

The yearly, seasonal, and monthly variations of watervapour observations from the three techniques werealso analysed in the study. Although some inter-annualuctuations were observed using all three sensors, nogeneral increasing or decreasing trend is evident fromany of the three techniques used. Seasonal and monthlyvariations from all three datasets indicate that the wetmonsoon season (summer) experiences water vapour

amounts to approximately two times higher than thewinter season. The pre-monsoon season (spring) alsoreceives relatively high water vapour, probably due tothe effect of cloud cover and wet atmospheric conditionsfrom rainstorms.

It would be expected that the accuracy of the MOD05product with a 1 km pixel size would be higher thanthe MOD07 product with 5 km pixel size for daytimemeasurements. Indeed, Seemann et al . (2003) and Kinget al . (2003) showed that the RMS error and slope of regression between MOD07 and microwave radiome-ter measurements are 4 kg m − 2 and 0.77, respectively,while MOD05 performed better, with an RMS error of 1.16 kg m − 2 and a slope of 0.96 (Gao and Kaufman,2003). However, this study indicates the opposite, withMOD07 performing better than MOD05. This may be dueto higher signal-to-noise ratio of the larger 5 km MOD07pixels compared with the 1 km MOD05 pixels, since alarge pixel size increases the signal strength relative tothe system noise.

In conclusion, both the radiosonde and AERONET canbe used as references for daily and routine monitoring of atmospheric water vapour content. While radiosonde maybe the most accurate, the water vapour amounts retrievedfrom AERONET are very similar, and AERONET’s high

(15 min) temporal resolution will enable its use in theprediction of the rapid onset of heavy rain from a troughof low pressure or tropical cyclones, by assimilation into





the numerical weather prediction (e.g. NHM) model. Theother AERONET stations in mainland China and eastAsia will enable more extensive water vapour data tobe ingested into mesoscale and regional climatic modelsfor improving the short-term prediction of larger-scaleweather systems. The MODIS water vapour data will

soon be ingested into the NHM model because althoughthe data are not perfect, they can, due to their large spatialcoverage contribute positively to model forecasting aftercareful compensation of the biases described here. This

study illustrates for the rst time the feasibility of integrating the AERONET water vapour data into HongKong’s daily weather forecasting operation.

Acknowledgements

The authors would like to acknowledge the NASA God-dard Earth Science Distributed Active Archive Centerfor the MODIS MOD05 and MOD07 products, andBrent Holben for assistance with the Hong Kong PolyU

(b) (c)

(d) (e)

(a)

Figure 7. (a) AERONET water vapour data on 20 August 2011; radar rainfall observation (b) at 8 : 00 a.m. (local time), (c) at 11 am (localtime), (d) at 2 : 00 p.m. (local time); NHM model at (e) at 8 : 00 a.m. local time (00 : 00 UTC), (f) at 11 : 00 a.m. local time (03 : 00 UTC), (g) at

2 : 00 p.m. local time (06 : 00 UTC). This gure is available in colour online at wileyonlinelibrary.com/journal/joc




118 Z. LIU et al .

(f) (g)

Figure 7. ( Continued ).

(b)

(d)

(c)

(a)

Figure 8. (a) AERONET water vapour data on 13 September 2010; radar rainfall observation (b) at 8 : 00 a.m. (local time), (c) at 11 : 00 a.m.(local time), (d) at 2 : 00 p.m. (local time). This gure is available in colour online at wileyonlinelibrary.com/journal/joc





AERONET station. The Grants PolyU 1-ZV4V from theHong Kong Research Grants Council, and PolyU researchgrant B-Q23L supported this study. The rst author isgrateful for receiving the support from the Hong KongPolytechnic University projects 1-ZV6L, A-PJ63, and A-PJ78. The rst author also thanks the support extended

by the Programme of Introducing Talents of Discipline toUniversities (Wuhan University, GNSS Research Center),China.

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