Turbulent fluxes from Helipod flights above quasi-homogeneous patches within the LITFASS area

Turbulent fluxes of sensible and latent heat were measured with the helicopter- borne turbulence probe Helipod over a heterogeneous landscape around the Meteorological Observatory Lindenberg during the STINHO-2 and LITFASS-2003 field experiments. Besides the determination of area-averaged heat fluxes, the analysis focused on different aspects of the response of the turbulent structure of the convective boundary layer (CBL) on the surface heterogeneity. A special flight pattern was designed to study flux profiles both over quasi-homogeneous sub-areas of the study region (representing the major land use types—forest, farmland, water) and over a typical mixture of the different surfaces. Significant differences were found between the heat fluxes over the individual surfaces along flight legs at about 80m above ground level, in agreement with large-aperture scintillometer measurements. This flux separation was still present during some flights at levels near the middle of the CBL. Different scales for the blending height and horizontal heterogeneity were calculated, but none of them could be identified as a reliable indicator of the mixing state of the lower CBL. With the exception of the flights over water, the latent heat flux measurements generally showed a larger statistical error when compared with the sensible heat flux. Correlation coefficients and integral length scales were used to characterise the interplay between the vertical transport of sensible and latent heat, which was found to vary between ‘fairly correlated’ and ‘decoupled’, also depending on the soil moisture conditions

Net precipitation over the Baltic Sea for one year using several methods

Precipitation and evaporation over the Baltic Sea are calculated for a one-year period from September 1998 to August 1999 by four different tools, the two atmospheric regional models HIRLAM and REMO, the oceanographic model PROBE-Baltic in combination with the SMHI (1 × 1)° database and Interpolated Fields, based essentially on ship measurements. The investigated period is slightly warmer and wetter than the climatological mean. Correlation coefficients of the differently calculated latent heat fluxes vary between 0.81 (HIRLAM and REMO) and 0.56 (SMHI/PROBE-Baltic and Interpolated Fields), while the correlation coefficients between model fluxes and measured fluxes range from 0.61 and 0.78. Deviations of simulated and interpolated monthly precipitation over the Baltic Sea are less than ±5 mm in the southern Baltic and up to 20 mm near the Finnish coast for the one-year period. The methods simulate the annual cycle of precipitation and evaporation of the Baltic Proper in a similar manner with a broad maximum of net precipitation in spring and early summer and a minimum in late summer. The annual averages of net precipitation of the Baltic Proper range from 57 mm (REMO) to 262 mm (HIRLAM) and for the Baltic Sea from 96 mm (SMHI/PROBE-Baltic) to 209 mm (HIRLAM). This range is considered to give the uncertainty of present-day determination of the net precipitation over the Baltic Sea

One year measurement and simulation of turbulent surface heat fluxes over the Baltic Sea

One of the aims of the "Pilot Study of Evaporation and Precipitation in the Baltic Sea" (PEP in BALTEX) is the improvement of the parameterisation of evaporation over the Baltic Sea in models. The atmospheric regional climate model REMO is used here to simulate evaporation over the Baltic Sea for one year. The turbulent surface fluxes of latent and sensible heat are determined by a bulk formula using prescribed sea surface temperature (SST) values from a coarser grid. Comparison with measurements at four coastal or island sites which can be considered to be marine sites during onshore winds turns out to be problematic: Deviations of the prescribed SST from measured SST are great at certain times and systematic due to seasonal heating or cooling of the water near coasts. A test with realistic SSTs at one site for a 5-days period strongly reduces the heat fluxes. Comparisons with measured fluxes during the periods with strongest evaporation - cold air outbreak with easterly wind in autumn - were not possible due to the specific location of the measuring sites. Three parameterisation schemes are tested in REMO and the effect of e.g. the wind speed dependence of the transfer coefficient C-HN is not as great as expected from tests with artificial data. For future comparisons of regional model results with single- point measurements, measured data in rather homogeneous areas, which are representative for the gridbox area, should be taken. In inhomogeneous areas, mesoscale models may serve as a bridge between the gridbox of the regional model and the measuring point. Application of a fully coupled atmospheric-oceanographic model should improve the SST resolution both in space and time

Mixing layer height derived from radiosoundings and ground-based lidar - comparison and assessment

The inversion on top of the atmospheric boundary layer is a strong barrier for the transport of heat, momentum and matter from or to the earth's surface. Regarding aerosols and gaseous constituents like water vapour which originate from the surface, the concentration of those parameters within the boundary layer strongly depends on the height of the layer, the mixing height. During daytime the mixing height over land increases and reaches a maximum value in situations with constant synoptic conditions. In many applications, e.g. the comparison of model output with observations the mixing height is taken from radiosoundings. Often this value is - due to lack of other measurements - also taken as the height of the fully developed convective boundary layer. Since the mixing height is strongly varying both in time and space an observation along a single line like a radiosonde track represents only an estimate of the mixing height. Quasi-continuous measurements of the backscatter signal with a ground-based lidar from several field campains offer the opportunity to estimate the error associated with a mixing-height determination from radiosoundings. A method to determine the mixing height from the backscattered signal is presented. Data from several field campaigns are used, namely the Nauru99 campaign in the tropical western Pacific in June/July 1999, three campaigns at the ARM-site in Oklahoma (USA) in September/October 1999, September/October 2000 and November/December 2000 and three campaigns in the frame of the German EVA-GRIPS (Regional Evaporation at Grid/Pixel Scale) project near Lindenberg (Brandenburg, Germany) in September 2002, April 2003 and May/June 2003 (LITFASS-2003). From these campaigns measurements simultaneous to approximatly 50 radiosoundings exist and allow a statistical analysis of the results. The comparison of radiosonde mixing heights with lidar mixing heights over 10 min time intervalls reveal a good agreement, the better the shorter the distance between radiosonde launch point and lidar location. Lidar mixing heights averaged over 1 h, which are more representative for an area, may deviate up to 300 m from radiosonde mixing heights. The standard deviation within the averaging interval fairly represents the variability of the mixing layer height. The maximum mixing height which is given by an afternoon radiosounding is also compared with lidar measurements

The humidity structure of the convective boundary layer - six weeks measurements with a ground-based Differential Absorption Lidar (DIAL)

The earth's surface is the source of water vapour in the climate system. Water vapour is transported upwards by turbulent and convective eddies, but the strength of the temperature inversion on top of the atmospheric boundary layer (ABL) controlls the transport into the "free atmosphere". Water vapour can in the first approximation be treated as a passive scalar, and thus reflects the temperature- driven mixing processes. Entrainment at the top of ABL in most cases is a sink for the humidity budget of the ABL - contrary to the temperature budget. These processes result in a different humidity than temperature stratification. During three measuring campaigns in the frame of the project EVA-GRIPS (EVAporation at GRId/Pixel Scale) at an agricultural site in eastern Germany measurements of the absolute humidity have been performed with two Differential Absorption Lidar Systems (DIAL). Time-height sections from 6 to 18 UT and up to 3000 m asl of 25 days under different synoptic situations illustrate the variability of the the humidity ABL appearance. Plots of the backscattered signal - namely, the vertical gradient of the range-corrected backscatter - give some more information on motions in the atmosphere. Radiosonde and surface data complete the data set. Part of the observations fell into an untypically dry period. - In most cases the evolving ABL is clearly present in the morning, but synoptic- scale or mesoscale features often disturb the 'stationary' daytime ABL. - Low-level cloud bases are lifted with the growing ABL. - The variability of the top of ABL due to single convective structures may be as large as 300 m with a time scale of several minutes, depending on the mean wind speed. - Radiosondes launched at 04:45 UT, 10:45 Ut and 16:45 UT therefore do not always give the exact ABL height. - Nearly in all cases the high humidity values in the growing ABL decreases before the top height is reached: the supply of water by evapotranspiration is not sufficient for mixing up to the top of ABL. This effect is clearest in situations when the well-mixed residual layer is high and the ABL grows 'explosively' fast. - Entrainment of dry air from above the ABL results in a humidity gradient in the upper BL. - Low-level convergence - e.g. during a frontal passage - leads to upwinds which break through the ABL inversion and transport humidity into higher levels. - During the growing of ABL the layered features in the stably stratified lower kilometers subside. The features of the humidity ABL may differ strongly from those of the temperature ABL, in particular over land surfaces with low soil moisture. Some observed features - like the last-named - need further investigation in order to explain and assess the participating processe

Double-layer structure in the boundary layer over the Baltic Sea

Double-layered structures found over the Baltic Sea are investigated using radiosoundings and lidar measurements. Situations with double-layer structures are also simulated with the regional model REMO in a realistic manner. The double layer consists of two adjacent well-mixed layers, with a sharp inversion in between. Results from radiosoundings show that the double-layer structure over the Baltic Sea mainly occurs during the autumn with thermally unstable strati. cation near the surface. The structure is present in about 50% of the radiosoundings performed during autumn. The presence of the double-layer structure cannot be related to any specific wind direction, wind speed or sea surface temperature. The lidar measurements give a more continuous picture of the time evolution of the double-layer structure, and show that the top of the lower layer is not a rigid lid for vertical transport. Two possible explanations of the double-layer structure are given, (i) the structure is caused by 'advection' of land boundary-layer air over the convective marine boundary layer or, (ii) by development of Sc clouds in weak frontal zones connected to low pressure systems. Also the forming of Cu clouds is found to be important for the development of a double-layer structur

Double-layer structure in the boundary layer over the Baltic Sea

Double-layered structures found over the Baltic Sea are investigated using radiosoundings and lidar measurements. Situations with double-layer structures are also simulated with the regional model REMO in a realistic manner. The double layer consists of two adjacent well-mixed layers, with a sharp inversion in between. Results from radiosoundings show that the double-layer structure over the Baltic Sea mainly occurs during the autumn with thermally unstable strati. cation near the surface. The structure is present in about 50% of the radiosoundings performed during autumn. The presence of the double-layer structure cannot be related to any specific wind direction, wind speed or sea surface temperature. The lidar measurements give a more continuous picture of the time evolution of the double-layer structure, and show that the top of the lower layer is not a rigid lid for vertical transport. Two possible explanations of the double-layer structure are given, (i) the structure is caused by 'advection' of land boundary-layer air over the convective marine boundary layer or, (ii) by development of Sc clouds in weak frontal zones connected to low pressure systems. Also the forming of Cu clouds is found to be important for the development of a double-layer structur