Some observational and model evidence for dry soils supporting cumulus onset

Research output: Chapter in Book/Report/Conference proceedingConference paperAcademic

Abstract

The tendency of relative humidity at the top of a clear convective boundary layer (RHtop) is studied as indicator of cumulus cloud formation over a semi-arid region. As such observations and the conceptual framework introduced by Ek and Holtslag (2004, J. Hydrometeorology, 5, p. 86-99) is utilised. The latter framework is based on a mixed layer model, coupled to the land surface via the surface energy budget. Typically in a regime of relatively high soil moisture, the tendency of RHtop increases if the evaporative fraction at the land surface increases (confirming intuition). The latter supports boundary-layer moistening but only when boundary-layer growth is limited by atmospheric factors, such as dry air entrainment and limited boundary-layer growth. This regime was confirmed earlier by Cabauw observations over relatively moist surface conditions. Here we provide the first observational evidence that the tendency of RHtop can also increase (favouring cloud formation) when the surface becomes more dry. The latter is consistent with a second regime of the conceptual framework. The observations are taken from the AMMA intensive observational campaign near Niamey, Niger, June 20-25, 2006. We also evaluate whether various versions of the WRF single-column model confirm the different regimes of the conceptual framework for typical days in the AMMA campaign. It appears that the model also confirms that dryer soils can support cloud formation.
Original languageEnglish
Title of host publication20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction, American Meteorological Society (9-13 July 2012, Boston, MA)
Place of PublicationBoston
PublisherAmerican Meteorological Society
Pages12A.3
Publication statusPublished - 2012
Event20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction - Boston, United States
Duration: 9 Jul 201213 Jul 2012

Conference

Conference20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction
CountryUnited States
CityBoston
Period9/07/1213/07/12

Fingerprint

cumulus
conceptual framework
boundary layer
land surface
hydrometeorology
convective boundary layer
soil
surface energy
energy budget
semiarid region
entrainment
mixed layer
relative humidity
soil moisture
air

Cite this

Westra, D., Steeneveld, G. J., & Holtslag, A. A. M. (2012). Some observational and model evidence for dry soils supporting cumulus onset. In 20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction, American Meteorological Society (9-13 July 2012, Boston, MA) (pp. 12A.3). Boston: American Meteorological Society.
Westra, D. ; Steeneveld, G.J. ; Holtslag, A.A.M. / Some observational and model evidence for dry soils supporting cumulus onset. 20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction, American Meteorological Society (9-13 July 2012, Boston, MA). Boston : American Meteorological Society, 2012. pp. 12A.3
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abstract = "The tendency of relative humidity at the top of a clear convective boundary layer (RHtop) is studied as indicator of cumulus cloud formation over a semi-arid region. As such observations and the conceptual framework introduced by Ek and Holtslag (2004, J. Hydrometeorology, 5, p. 86-99) is utilised. The latter framework is based on a mixed layer model, coupled to the land surface via the surface energy budget. Typically in a regime of relatively high soil moisture, the tendency of RHtop increases if the evaporative fraction at the land surface increases (confirming intuition). The latter supports boundary-layer moistening but only when boundary-layer growth is limited by atmospheric factors, such as dry air entrainment and limited boundary-layer growth. This regime was confirmed earlier by Cabauw observations over relatively moist surface conditions. Here we provide the first observational evidence that the tendency of RHtop can also increase (favouring cloud formation) when the surface becomes more dry. The latter is consistent with a second regime of the conceptual framework. The observations are taken from the AMMA intensive observational campaign near Niamey, Niger, June 20-25, 2006. We also evaluate whether various versions of the WRF single-column model confirm the different regimes of the conceptual framework for typical days in the AMMA campaign. It appears that the model also confirms that dryer soils can support cloud formation.",
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Westra, D, Steeneveld, GJ & Holtslag, AAM 2012, Some observational and model evidence for dry soils supporting cumulus onset. in 20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction, American Meteorological Society (9-13 July 2012, Boston, MA). American Meteorological Society, Boston, pp. 12A.3, 20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction, Boston, United States, 9/07/12.

Some observational and model evidence for dry soils supporting cumulus onset. / Westra, D.; Steeneveld, G.J.; Holtslag, A.A.M.

20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction, American Meteorological Society (9-13 July 2012, Boston, MA). Boston : American Meteorological Society, 2012. p. 12A.3.

Research output: Chapter in Book/Report/Conference proceedingConference paperAcademic

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T1 - Some observational and model evidence for dry soils supporting cumulus onset

AU - Westra, D.

AU - Steeneveld, G.J.

AU - Holtslag, A.A.M.

N1 - recorded presentation: https://ams.confex.com/ams/20BLT18AirSea/flvgateway.cgi/id/21907?recordingid=21907

PY - 2012

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N2 - The tendency of relative humidity at the top of a clear convective boundary layer (RHtop) is studied as indicator of cumulus cloud formation over a semi-arid region. As such observations and the conceptual framework introduced by Ek and Holtslag (2004, J. Hydrometeorology, 5, p. 86-99) is utilised. The latter framework is based on a mixed layer model, coupled to the land surface via the surface energy budget. Typically in a regime of relatively high soil moisture, the tendency of RHtop increases if the evaporative fraction at the land surface increases (confirming intuition). The latter supports boundary-layer moistening but only when boundary-layer growth is limited by atmospheric factors, such as dry air entrainment and limited boundary-layer growth. This regime was confirmed earlier by Cabauw observations over relatively moist surface conditions. Here we provide the first observational evidence that the tendency of RHtop can also increase (favouring cloud formation) when the surface becomes more dry. The latter is consistent with a second regime of the conceptual framework. The observations are taken from the AMMA intensive observational campaign near Niamey, Niger, June 20-25, 2006. We also evaluate whether various versions of the WRF single-column model confirm the different regimes of the conceptual framework for typical days in the AMMA campaign. It appears that the model also confirms that dryer soils can support cloud formation.

AB - The tendency of relative humidity at the top of a clear convective boundary layer (RHtop) is studied as indicator of cumulus cloud formation over a semi-arid region. As such observations and the conceptual framework introduced by Ek and Holtslag (2004, J. Hydrometeorology, 5, p. 86-99) is utilised. The latter framework is based on a mixed layer model, coupled to the land surface via the surface energy budget. Typically in a regime of relatively high soil moisture, the tendency of RHtop increases if the evaporative fraction at the land surface increases (confirming intuition). The latter supports boundary-layer moistening but only when boundary-layer growth is limited by atmospheric factors, such as dry air entrainment and limited boundary-layer growth. This regime was confirmed earlier by Cabauw observations over relatively moist surface conditions. Here we provide the first observational evidence that the tendency of RHtop can also increase (favouring cloud formation) when the surface becomes more dry. The latter is consistent with a second regime of the conceptual framework. The observations are taken from the AMMA intensive observational campaign near Niamey, Niger, June 20-25, 2006. We also evaluate whether various versions of the WRF single-column model confirm the different regimes of the conceptual framework for typical days in the AMMA campaign. It appears that the model also confirms that dryer soils can support cloud formation.

M3 - Conference paper

SP - 12A.3

BT - 20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction, American Meteorological Society (9-13 July 2012, Boston, MA)

PB - American Meteorological Society

CY - Boston

ER -

Westra D, Steeneveld GJ, Holtslag AAM. Some observational and model evidence for dry soils supporting cumulus onset. In 20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction, American Meteorological Society (9-13 July 2012, Boston, MA). Boston: American Meteorological Society. 2012. p. 12A.3