Ž .
Atmospheric Research 54 2000 59–85 www.elsevier.comrlocateratmos
On the influence of surface heterogeneity on latent heat fluxes and stratus properties
Katja Friedrich
1
, Nicole Molders
¨
LIM-Institut fur Meteorologie, UniÕersitat Leipzig, Stephanstraße 3, D-04103 Leipzig, Germany
¨ ¨
Received 26 August 1999; received in revised form 16 November 1999; accepted 6 December 1999
Abstract
A mesoscale atmospheric model is used to examine the three-dimensional structure and evolution of low extended stratus over various synthetic landscapes of different heterogeneity in
mid-latitudes in spring. The simulation results substantiate that surface heterogeneity nonlinearly influences the distributions of latent heat fluxes, vertical motions, and cloud-water presupposed
the length of the patches of equal surface type is about 10 km or larger than that. For low degrees
Ž .
of heterogeneity large patch sizes , a great coverage by lowly evapotranspiring, but strongly heating patches may enhance vertical motion. Moreover, this constellation may increase the
cloud-water amount of low extended stratus as compared to that of the other heterogeneous landscapes or that with the highest domain-averaged daily sum of latent heat fluxes. Although
there exists a relationship between the degree of heterogeneity and the modulation of latent heat fluxes as well as cloud-water amount, the kind of surface characteristics is also important for the
modulation of the properties of low extended stratus. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Degree of heterogeneity; Latent heat fluxes; Low extended stratus; Mesoscale modeling; Surface atmosphere interaction
1. Introduction
Flying over a landscape in mid-latitudes presents a fantastic view of patchy fields with various surface properties and different sizes. Obviously, this surface heterogeneity
Corresponding author. Tel.: q49-341-9732-872; fax: q49-341-9732-899. Ž
. E-mail address: moelderscurie.meteo.uni-leipzig.de N. Molders .
¨
1
Present affiliation: DLR, Institut fur Physik der Atmosphare, Oberpfaffenhofen, Postfach 1116, 82230
¨ ¨
Wessling, Germany. 0169-8095r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved.
Ž .
PII: S 0 1 6 9 - 8 0 9 5 9 9 0 0 0 5 0 - 2
can be significant at the mesoscale or global scale. The varying nature and structure of land-surface result in different fluxes of momentum, water vapor, matter, and heat due to
differences in water availability, surface temperature, plant and soil characteristics as Ž
. well as hill slopes e.g., Li and Avissar, 1994 . Thus, the meteorological processes
Ž .
taking place in the atmospheric boundary layer ABL and at the interface earth–atmo- sphere are, among others, governed by surface characteristics and surface discontinu-
ities. This impact is exacerbated, for instance, by moistening of the ABL through Ž
. evapotranspiration, the rising of lighter, moist air as compared with dry air , and the
additional ascending motion induced by surface thermal heterogeneity. Therefore, it has to be expected that the degree of heterogeneity may affect the water and energy fluxes
as well as cloud formation.
The impact of surface characteristics and discontinuities on the ABL was investigated Ž
in many theoretical and numerical studies as well as field experiments e.g., Anthes, 1984; Pinty et al., 1989; Mahrt et al., 1994; Zhong and Doran, 1995; Molders and
¨
. Raabe, 1996 . Investigating interactions between land cover and cloud cover by means
Ž .
of GOES satellite data on a 18 = 18 grid, O’Neal 1996 hypothesized that it could be possible to define a measure denoted as ‘‘degree of surface heterogeneity’’ within larger
areas to test whether areas with greater land-surface heterogeneity have significantly less or larger cloud cover. The intensity of thermally induced mesoscale circulations between
vegetated and bare soil areas — so-called vegetation breezes — was found to be
Ž directly related to the characteristics of the bare soil and surface fluxes Mahrt et al.,
. 1994; Hong et al., 1995 . In general, upward motion in such mesoscale circulations is
Ž .
stronger than thermal cells induced by turbulence e.g., Seth and Giorgi, 1996 . Their ability to transport moist, warm air upward increases the amount of water that can be
condensed and precipitated. In a relatively dry atmosphere, clouds and precipitation Ž
appear to be randomly distributed when the domain is homogeneous e.g., Avissar and .
Liu, 1996; Seth and Giorgi, 1996 . However, when the landscape structure triggers the formation of mesoscale circulations, they concentrate on the originally dry parts of the
domain. A negative feedback is created, which tends to eliminate the effect of the Ž
landscape discontinuities and spatially homogenize soil moisture content e.g., Avissar .
and Liu, 1996 . Most studies on the atmospheric impact of land-surface heterogeneity were carried
Ž out for arid or semiarid regions and convective precipitating clouds e.g., Anthes, 1984;
. Mahrt et al., 1994; Zhong and Doran, 1995 . In these regions, it is of interest, for
instance, for water management, irrigation purposes or limitations of grazing, whether there exist land-use pattern distributions that favor cloud formation. Even in high
mid-latitudes, where, however, the atmosphere is usually relatively humid, the analyses of aircraft data show that moisture variability is likely to have an impact on relative
Ž .
humidity variations e.g., Frech et al., 1998 . Therefore, it has to be expected that, here, cloudiness may be modulated by the underlying surface too. As a consequence of the
relatively humid atmosphere in mid-latitudes, however, low extended stratus frequently occurs in the boundary layer under anticyclonic conditions, especially in spring, autumn,
and winter. Thus, the aim of this paper is to examine how surface heterogeneity may affect latent heat fluxes, vertical motions as well as the properties of low extended
stratus. These investigations are of main interest for environmental questions. Namely,
Ž in mid-latitudes, land-surface conditions are anthropogenically altered e.g., through
urbanization, deforestation and afforestation, subsidy politics, open-pit mining and .
recultivation of open-pit mines, etc. . These land-use changes go along with modifica- tions of surface heterogeneity. Since low extended stratus may significantly alter
Ž .
photolysis rates Molders et al., 1995 as well as evapotranspiration, and, thus, ground-
¨
Ž water recharge, land-use changes may not only affect the water and energy cycle e.g.,
. Molders, 1998 , but also the trace gas concentrations. Moreover, since in mid-latitudes,
¨
extended low stratus is often supercooled, here, land-use changes that contribute to enhance stratus should be avoided in areas of airports due to the danger of icing.
2. Model description and initialization
