On the role of air temperature in humidity variations in the Northern hemisphere in the second part of XX century Ashkhen A. Karakhanyan*, Geliy А. Zherebtsov, Vladimir А. Kovalenko, Sergey I. Molodykh Institute of Solar-Terrestrial Physics SB RAS, 664033 P.O. Box 291, Irkutsk, Russia ABSTRACT A water vapor is the most important greenhouse gas in the atmosphere. Therefore, variations of the water vapor content can be one of the important factors determining observable changes of a climate. Based on NCEP/NCAR Reanalysis data (http://www.cdc.noaa.gov/), a complex analysis of humidity and air temperature is carried out at standard isobaric levels at middle and high latitudes in the Northern hemisphere. It is shown that atmospheric water vapor variations are produced, for the most part, by air temperature variations caused by radiation balance variations. Keywords: water vapor content, specific humidity, relative humidity, temperature, vertical profile

1. INTRODUCTION A water evaporation from the surface is practically the only process providing for a water vapor inflow into the atmosphere. A water vapor is the most important greenhouse gas in the atmosphere because of its rather high concentration and bands of strong absorption in the near, middle and far IR spectral ranges. Therefore, variations of the water vapor content can be one of the important factors influencing observable changes of a climate. Quantitatively the water vapor content in the atmosphere can be defined with some humidity characteristics the basic of which are the following: partial pressure of a water vapor, absolute and relative humidity, mass fraction of a water vapor (specific humidity) [1, 2]. The water vapor content in air above the surface has a diurnal and annual course. Two types of the diurnal variation of the partial pressure of a water vapor, absolute and specific humidity can be determined. The first type is an ordinary one that is similar to the course of the air temperature: humidity is higher during the daytime when temperature is higher. Such diurnal variation usually takes place above water surfaces (seas) and continents in winter time. The second type of a diurnal variation of the water vapor pressure, absolute and specific humidity is a double wave characterized by two maxima (near 9–10 and 20–21 LT) and two minima (early in the morning and during the most developed air turbulence at afternoon time). Such type of diurnal variation is usually observed on continents in summertime. The double diurnal variation of the air humidity is caused by a convection development above the surface in summer at the afternoon time, with the following decrease of the water vapor content in a surface layer because of the insufficient velocity of its inflow from soil. Annual courses of the water vapor partial pressure, absolute and specific humidity correlate with a temperature annual course: amplitudes of these parameters are higher in summer and lower in winter. The diurnal relative air humidity near the surface anticorrelates with the diurnal air temperature. An amplitude of diurnal fluctuations of the relative humidity is high on continents especially in summer, and much lower above water surfaces. In mountains and in a free atmosphere a diurnal variation of the relative humidity correlates with a temperature diurnal variation.

. * [email protected]

Fourteenth International Symposium on Atmospheric and Ocean Optics/Atmospheric Physics, edited by Gennadii G. Matvienko, Victor A. Banakh, Proc. of SPIE Vol. 6936, 69361L, (2008) 0277-786X/08/$18 · doi: 10.1117/12.783574 Proc. of SPIE Vol. 6936 69361L-1 2008 SPIE Digital Library -- Subscriber Archive Copy

The relative humidity anticorrelates with the air temperature in their annual courses. In general, annual amplitudes of humidity agree with the temperature. An annual variations of humidity decreases with altitude. In the upper troposphere these variations are insignificant. The humidity decreases with altitude: the specific humidity decreases slower than partial pressure of a water vapor. The absolute humidity decreases with altitude in a same way. In general, the relative humidity also decreases with altitude but this decrease is much less natural [3,4]. The purpose of the study is to establish the basic factor determining observable variations of the water vapor content in the atmosphere of the Northern hemisphere. Based on NCEP/NCAR Reanalysis data (http://www.cdc.noaa.gov/), a complex analysis of humidity and air temperature is carried out at standard isobaric levels at middle (50 – 65° N) and high (> 65° N) latitudes in the Northern hemisphere.

2. ANALYSIS RESULTS As evident from the observation data analysis performed, any increase of the temperature causes the increase of the water vapor amount, as pressure of the saturated vapors (Clausius–Clapeyron equation) rises with increasing temperature; and evaporation and following raise usually provide necessary vapor income. As an example, Fig. 1 presents variations of the specific humidity and air temperature on standard isobaric surfaces in the Polar region of the Northern hemisphere in winter and summer seasons. T, K 256 R = 0.78 254

1000 gPa

winter

q*10-3, kg/kg T, K 1.6 281 R = 0.89 1 1.4 280

252

2

250 248 246 1950 1960 T, K 252 R = 0.55 250

1970

1980

1990

1.2

2

q*10-3, kg/kg 6 1 5.8

279

1

278

0.8

277

0.6

276

5.6 2

5.4 5.2 5

2000 Year 700 gPa 1950 1960 q*10-3, kg/kg T, K 0.8 268 R = 0.80 1 0.7 267

248

summer

1970

1980

1990

2000 Year q*10-3, kg/kg 2.8 1 2.6

0.6

266

246

0.5

265

2.2

244

0.4

264

2

242

0.3

263

1.8

1950 1960 T, K 238 R = 0.54 236

1970

1980

1990

234

2000 Year 500 gPa 1950 1960 q*10-3, kg/kg T, K 0.32 253 R = 0.64 1 0.28 252 2

0.24 0.2

250

230

0.16

249

0.12

248

228 1960

1970

1980

1990

2000

Year

1970

1980

1990

2.4

2000 Year q*10-3, kg/kg 1.1 1

251

232

1950

2

1 0.9

2

0.8 0.7 0.6

1950

1960

1970

1980

1990

2000

Year

Fig. 1. The variations of the air temperature (1) and specific humidity (2) on standard isobaric surfaces in the Polar region in winter and summer seasons, R – correlation coefficient between the air temperature and specific humidity.

Proc. of SPIE Vol. 6936 69361L-2

As the water vapor is a greenhouse gas, the long-wave radiation absorption and therefore the temperature increase with increasing content in this volume. Thus, the atmosphere with a water vapor possesses a positive feedback, and any temperature increase leads to the increase of the water vapor content and the subsequent increase of the temperature. This process stops only when either the content of the vapor reaches saturation, or radiation energy losses from this volume compensate the energy influx into the volume that results in temperature not to increase further. Any temperature decrease results in the vapor saturation, whereas the latent heat release under condensation stops next cooling. The above peculiarity of the atmosphere with a water vapor and asymmetry of the processes that stop instability result in water vapor content to aspire to saturation; but in a greater part of the atmosphere radiation losses limit the vapor content below the saturation level. To illustrate atmosphere peculiarities mentioned above, Fig. 2 displays variations of the relative humidity and air temperature on standard isobaric surfaces in the Siberian sector (50–65° N, 60–119° E) of the Northern hemisphere in winter and summer seasons. The analysis of Figure 2 allows us to conclude that just requirements of the radiation balance determine the water vapor content in the atmosphere. T, K R1 = 0.95 268 R2 = 0.87 264

1000 gPa

winter 3

260 256 252 248

f, % q, kg/kg 95 0.0024 1 90 0.002 85 2 0.0016 80 0.0012 75 0.0008 70

0 10 20 30 40 50 60 70 80 90 100 Days

244 240 236 232

0 10 20 30 40 50 60 70 80 90 100 Days

T, K 218 216 214 212 210 208

R1 = 0.83 R2 = 0.39

1

80

5e-005

2

70

4e-005

60

3e-005

50

2e-005

40

1e-005

0 10 20 30 40 50 60 70 80 90 100 Days

292 290 288 286

summer

f, % q, kg/kg 95 0.012 90 0.01 85 2 0.008 3 80 0.006 75 1 0.004 70

R1 = 0.96 R2 = 0.19

0 10 20 30 40 50 60 70 80 90 100 Days

T, K 258 256 254 252 250

R1 = 0.86 f, % q, kg/kg R2 = -0.18 70 0.0014 65

0.0012

1

60

0.001

2 3

55

0.0008

50

0.0006

248

300 gPa

f, % q, kg/kg

3

294

500 gPa

f, % q, kg/kg 70 0.0005 60 0.0004 3 50 0.0003 2 40 1 0.0002 30 0.0001 20

T, K 252 R1 = 0.87 R2 = -0.43 248

T, K 296

0 10 20 30 40 50 60 70 80 90 100 Days

T, K 230 228 226

R1 = 0.93 f, % q, kg/kg R2 = 0.36 80 0.00024 1 0.0002 75 2

224 222

3

220

70

0.00016

65

0.00012

60

8e-005

55

4e-005

0 10 20 30 40 50 60 70 80 90 100 Days

Fig. 2. The variations of the air temperature (1), specific humidity (2) and relative humidity (3) on standard isobaric surfaces in the Siberian sector (50–65° N, 60–119° E) in winter and summer seasons, 1976 year, R1 – correlation coefficient between the air temperature and specific humidity and R2 – correlation coefficient between the air temperature and relative humidity.

It should be noted that violation of this mechanism can be observed at fast changes of the radiation balance near the surface when the rate of vapor income into the atmosphere appears to be insufficiently high (deserts and some midland regions in summer season, Fig. 3).

Proc. of SPIE Vol. 6936 69361L-3

1000 gPa

winter

f, % 80

q, kg/kg T, K 310 0.005

f, % 120

300

0.004

100

3

290

0.003

80

50

2

280

0.002

60

40

1

270

0.001

40

0

20

70

R1= 0.95 R2= -0.11

60

30

260 0 10 20 30 40 50 60 70 80 90 100 Days

f, % 80

R1= 0.92 R2= -0.02

70 60 50 40

T, K 300

3

280

2 1

270

30

260

0.004

60

R1= 0.77 R2= -0.54

T, K 3

40 20

2 1

0 -20

296

0.012

2

292

0.008

3

288

0.004

284

0

0 10 20 30 40 50 60 70 80 90 100 Days R1= 0.12 f, % T, K q, kg/kg R2= -0.48 296 120 0.012 1 292 100 0.01

0.003

80

0.002

60

0.001

40

0

20

0 10 20 30 40 50 60 70 80 90 100 Days

f, % 80

R1= 0.14 q, kg/kg T, K R2= -0.44 0.02 304 1 0.016 300

850 gPa

q, kg/kg 0.005

290

summer

300

0.002

290

0.0016

280

0.0012

270

0.0008

260

0.0004

3

0.008

284

0.006

280

0.004

276

0.002

0 10 20 30 40 50 60 70 80 90 100 Days

700 gPa q, kg/kg

2

288

f, % 80

q, kg/kg T, K 292 0.014

R1= 0.34 R2= -0.42

60

288

0.012

40

3

284

0.01

20

1

280

0.008

276

0.006

272

0.004

0

2

-20

0 10 20 30 40 50 60 70 80 90 100 Days 500 gPa 0 10 20 30 40 50 60 70 80 90 100 Days q, kg/kg f, % q, kg/kg f, % T, K T, K R1= 0.78 0.0012 60 272 0.0048 260 R = -0.30 3 2 60 3 0.001 40 268 0.004 255 R1= 0.64 40 0.0008 20 R2= 0.04 264 0.0032 250 20 1 0.0006 0 260 0.0024 1 245 0 0.0004 -20 256 0.0016 240 2 -20 2 0.0002 -40 252 0.0008 235 Days 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Days

300 gPa

f, %

T, K 3

20 0 -20

R1= 0.57 R2= 0.27

222

1

-40 -60

224 220 218

2

216

0 10 20 30 40 50 60 70 80 90 100 Days

q, kg/kg 0.0002

f, % 60

0.00016

40

0.00012

20

8e-005

0

4e-005

-20

0

-40

3 R1= 0.74 R2= 0.12 1 2

T, K q, kg/kg 248 0.002 244

0.0016

240

0.0012

236

0.0008

232

0.0004

228

0

0 10 20 30 40 50 60 70 80 90 100 Days

Fig. 3. The variations of the air temperature (1), specific humidity (2) and relative humidity (3) on standard isobaric surfaces in the desert Gobi in winter and summer seasons, 1976 year, R1 – correlation coefficient between the air temperature and specific humidity and R2 – correlation coefficient between the air temperature and relative humidity.

Proc. of SPIE Vol. 6936 69361L-4

3. CONCLUSIONS Thus, the water vapor content and temperature depend on energy influx into this volume if the velocity of the vapor inflow is sufficiently high. In this case variations of specific and relative humidity will correlate with temperature. At fast variations of the radiation balance near the surface, the water vapor content will be limited due to insufficiently high rate of the vapor inflow into the atmosphere (deserts and some midland regions in summer season). In this case the correlation with temperature will be observed for the specific humidity, while the anticorrelation for the relative humidity.

ACKNOWLEDGMENTS This work is carried out under the support of Program for Fundamental Researches of Presidium RAS № 16, the RFFR grant № 06-05-81011-Бел_а. and SB RAS integration project № 11.2.

REFERENCES 1. 2. 3. 4.

O.A. Drozdov and et., Climatology. Leningrad, Gidrometeoizdat, p.568, 1989. (in Russian). V.E. Zuev and G.A. Titov, Atmospheric Optics and Climate. Tomsk, Spectrum, p.272, 1996. (in Russian). P.N. Tverskoi, Meteorology. Leningrad, Gidrometeoizdat, p.700, 1962. (in Russian). S.P. Khromov and M.A. Petrosyunts, Meteorology and Climatology. Moscow, MSU, p.528, 2001. (in Russian).

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