## Abstract

Over the past three decades, a number of field experiments have suggested that land-cover heterogeneity (LCH) impacts Monin and Obukhov (M-O) scaling, when applied to second-order statistics of temperature (T), water vapor (q), and CO_{2} (c) fluctuations. To further explore how LCH modifies M-O scaling for second-order statistics, 2 years of atmospheric surface layer (ASL) measurements, conducted above a Mediterranean ecosystem in Sardinia, Italy were analyzed. During wet soil moisture states, when grass and trees dominate the ecosystem, M-O scaling was well recovered. For dry soil moisture states, when bare soil and trees dominate the ecosystem, M-O similarity theory predictions significantly underestimated all three scalar variance measurements, consistent with several recent studies. Among the three scalars, q was poorly predicted by M-O scaling despite its ground source/sink similarity with c. A plausible explanation for the de-correlation between q and c is the dissimilarities originating from the top of the boundary layer via entrainment processes. To establish necessary (but not sufficient) conditions that diagnose departures from M-O scaling, the statistical structure of LCH as quantified by its integral length scale (L_{x}), computed using the NDVI obtained from QuickBird imagery, was employed. When the ecosystem was dominated by grass and trees (wet soil moisture states), L_{x} ∼ 100 m, and when the ecosystem was dominated by soil and trees (dry soil moisture states), L_{x} ∼ 10 m. Using the scalar variance budget equation, two canonical time scales connected with the advection-distortion and relaxation time scales were introduced in the absence of flux-transport terms. We showed that M-O scaling is recovered when relaxation time scales of turbulent eddies are much smaller than the advection-distortion time scale by the mean flow (whose length scale was set to L_{x}). Converting these time scales to approximate length scales, we found that a necessary but not sufficient condition for MOST to be applicable to second-order scalar statistics is whenmL_{x} ≫ κ z_{m} (over(u, ̄) / u_{*}), where κ is the von-Karman constant, z_{m} is the measurement height, over(u, ̄) is the mean wind speed, and u_{*} is the friction velocity. The term κ z_{m} (over(u, ̄) / u_{*}) did not vary considerably between the two seasons. Its value (on average 20 m) was comparable to L_{x} for the tree-soil system but an order of magnitude smaller than L_{x} for the tree-grass system.

Original language | English (US) |
---|---|

Pages (from-to) | 902-916 |

Number of pages | 15 |

Journal | Agricultural and Forest Meteorology |

Volume | 148 |

Issue number | 6-7 |

DOIs | |

State | Published - Jun 30 2008 |

Externally published | Yes |

## All Science Journal Classification (ASJC) codes

- Forestry
- Global and Planetary Change
- Agronomy and Crop Science
- Atmospheric Science

## Keywords

- Flux-variance
- Footprint
- Higher-order scalar statistics
- Mediterranean patchy vegetation
- Similarity theory